| Message ID | 1518622006-16089-2-git-send-email-vincent.guittot@linaro.org |
|---|---|
| State | New |
| Headers | show |
| Series | sched: Update blocked load | expand |
Hi Vincent, here are some possible small code improvement suggestions from my side. Cheers Patrick On 14-Feb 16:26, Vincent Guittot wrote: > > Stopped the periodic update of blocked load when all idle CPUs have fully > > decayed. We introduce a new nohz.has_blocked that reflect if some idle > > CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked > > is set everytime that a Idle CPU can have blocked load and it is then clear > > when no more blocked load has been detected during an update. We don't need > > atomic operation but only to make cure of the right ordering when updating > > nohz.idle_cpus_mask and nohz.has_blocked. > > > > Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > > --- > > kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++--------- > > kernel/sched/sched.h | 1 + > > 2 files changed, 102 insertions(+), 21 deletions(-) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 7af1fa9..5a6835e 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -5383,8 +5383,9 @@ decay_load_missed(unsigned long load, unsigned long missed_updates, int idx) > > static struct { > > cpumask_var_t idle_cpus_mask; > > atomic_t nr_cpus; > > + int has_blocked; /* Idle CPUS has blocked load */ Why not "bool"? What about "has_blocked_load", which makes it more clear the meaning and is also more consistent with the RQ flags "aggregated" by this one... as well as some other local variables you already name like that? > unsigned long next_balance; /* in jiffy units */ > - unsigned long next_stats; > + unsigned long next_blocked; /* Next update of blocked load in jiffies */ > } nohz ____cacheline_aligned; > > #endif /* CONFIG_NO_HZ_COMMON */ > @@ -6951,6 +6952,7 @@ enum fbq_type { regular, remote, all }; > #define LBF_DST_PINNED 0x04 > #define LBF_SOME_PINNED 0x08 > #define LBF_NOHZ_STATS 0x10 > +#define LBF_NOHZ_AGAIN 0x20 > > struct lb_env { > struct sched_domain *sd; > @@ -7335,8 +7337,6 @@ static void attach_tasks(struct lb_env *env) > rq_unlock(env->dst_rq, &rf); > } > > -#ifdef CONFIG_FAIR_GROUP_SCHED > - > static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq) > { > if (cfs_rq->load.weight) > @@ -7354,11 +7354,14 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq) > return true; > } > > +#ifdef CONFIG_FAIR_GROUP_SCHED > + > static void update_blocked_averages(int cpu) > { > struct rq *rq = cpu_rq(cpu); > struct cfs_rq *cfs_rq, *pos; > struct rq_flags rf; > + bool done = true; > > rq_lock_irqsave(rq, &rf); > update_rq_clock(rq); > @@ -7388,10 +7391,14 @@ static void update_blocked_averages(int cpu) > */ > if (cfs_rq_is_decayed(cfs_rq)) > list_del_leaf_cfs_rq(cfs_rq); > + else > + done = false; > } > > #ifdef CONFIG_NO_HZ_COMMON > rq->last_blocked_load_update_tick = jiffies; > + if (done) > + rq->has_blocked_load = 0; > #endif > rq_unlock_irqrestore(rq, &rf); > } > @@ -7454,6 +7461,8 @@ static inline void update_blocked_averages(int cpu) > update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq); > #ifdef CONFIG_NO_HZ_COMMON > rq->last_blocked_load_update_tick = jiffies; > + if (cfs_rq_is_decayed(cfs_rq)) > + rq->has_blocked_load = 0; > #endif > rq_unlock_irqrestore(rq, &rf); > } > @@ -7789,18 +7798,25 @@ group_type group_classify(struct sched_group *group, > return group_other; > } > > -static void update_nohz_stats(struct rq *rq) > +static bool update_nohz_stats(struct rq *rq) > { > #ifdef CONFIG_NO_HZ_COMMON > unsigned int cpu = rq->cpu; > > + if (!rq->has_blocked_load) > + return false; > + > if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask)) > - return; > + return false; > > if (!time_after(jiffies, rq->last_blocked_load_update_tick)) > - return; > + return true; > > update_blocked_averages(cpu); > + > + return rq->has_blocked_load; > +#else > + return false; > #endif > } > > @@ -7826,8 +7842,8 @@ static inline void update_sg_lb_stats(struct lb_env *env, > for_each_cpu_and(i, sched_group_span(group), env->cpus) { > struct rq *rq = cpu_rq(i); > > - if (env->flags & LBF_NOHZ_STATS) > - update_nohz_stats(rq); > + if ((env->flags & LBF_NOHZ_STATS) && update_nohz_stats(rq)) > + env->flags |= LBF_NOHZ_AGAIN; > > /* Bias balancing toward cpus of our domain */ > if (local_group) > @@ -7979,18 +7995,17 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd > struct sg_lb_stats *local = &sds->local_stat; > struct sg_lb_stats tmp_sgs; > int load_idx, prefer_sibling = 0; > +#ifdef CONFIG_NO_HZ_COMMON > + int has_blocked = READ_ONCE(nohz.has_blocked); > +#endif > bool overload = false; > > if (child && child->flags & SD_PREFER_SIBLING) > prefer_sibling = 1; > > #ifdef CONFIG_NO_HZ_COMMON > - if (env->idle == CPU_NEWLY_IDLE) { > + if (env->idle == CPU_NEWLY_IDLE && has_blocked) Why using a local "has_blocked" here? Is not better to inline directly the READ_ONCE, we avoid the above idfed and also (maybe?) save the load if the previous checks should fail? > env->flags |= LBF_NOHZ_STATS; > - > - if (cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) > - nohz.next_stats = jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD); > - } > #endif > > load_idx = get_sd_load_idx(env->sd, env->idle); > @@ -8046,6 +8061,15 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd > sg = sg->next; > } while (sg != env->sd->groups); > > +#ifdef CONFIG_NO_HZ_COMMON > + if ((env->flags & LBF_NOHZ_AGAIN) && > + cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) { > + > + WRITE_ONCE(nohz.next_blocked, > + jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD)); > + } > +#endif > + Moreover, the above two ifdef maybe can be better moved into compile time inline functions to keep update_sg_lb_stats more streamlined, e.g. something like: ---8<--- #ifdef CONFIG_NO_HZ_COMMON static inline void nohz_update_next_stats(struct lb_env *env) { if (env->idle != CPU_NEWLY_IDLE || READ_ONCE(nohz.has_blocked)) return; eenv->flags |= LBF_NOHZ_STATS; etc... } static inline void nohz_update_next_blocked(struct lb_env *env) { if (!(env->flags & LBF_NOHZ_AGAIN) return if (!cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) return WRITE_ONCE(); } #else #define nohz_update_next_stats(env) do {} while(false); #define nohz_update_blocked(env) do {} while(false); #endif ---8<--- > if (env->sd->flags & SD_NUMA) > env->fbq_type = fbq_classify_group(&sds->busiest_stat); > > @@ -9069,6 +9093,8 @@ static void nohz_balancer_kick(struct rq *rq) > struct sched_domain *sd; > int nr_busy, i, cpu = rq->cpu; > unsigned int flags = 0; > + unsigned long has_blocked = READ_ONCE(nohz.has_blocked); > + unsigned long next_blocked = READ_ONCE(nohz.next_blocked); > > if (unlikely(rq->idle_balance)) > return; > @@ -9086,7 +9112,7 @@ static void nohz_balancer_kick(struct rq *rq) > if (likely(!atomic_read(&nohz.nr_cpus))) > return; > > - if (time_after(now, nohz.next_stats)) > + if (time_after(now, next_blocked) && has_blocked) Why not using READ_ONCE right here, these are the only two usages in this function, isn't it? This would allow also to skip (some of) the loads if some of the checks should fails. Ins't it? > flags = NOHZ_STATS_KICK; > > if (time_before(now, nohz.next_balance)) > @@ -9207,13 +9233,26 @@ void nohz_balance_enter_idle(int cpu) > if (!housekeeping_cpu(cpu, HK_FLAG_SCHED)) > return; > > + /* > + * Can be set safely without rq->lock held > + * If a clear happens, it will have evaluated last additions because > + * rq->lock is held during the check and the clear > + */ > + rq->has_blocked_load = 1; > + > + /* > + * The tick is still stopped but load could have been added in the > + * meantime. We set the nohz.has_blocked flag to trig a check of the > + * *_avg. The CPU is already part of nohz.idle_cpus_mask so the clear > + * of nohz.has_blocked can only happen after checking the new load > + */ > if (rq->nohz_tick_stopped) > - return; > + goto out; > > /* > * If we're a completely isolated CPU, we don't play. > */ > - if (on_null_domain(cpu_rq(cpu))) > + if (on_null_domain(rq)) > return; > > rq->nohz_tick_stopped = 1; > @@ -9221,7 +9260,21 @@ void nohz_balance_enter_idle(int cpu) > cpumask_set_cpu(cpu, nohz.idle_cpus_mask); > atomic_inc(&nohz.nr_cpus); > > + /* > + * Ensures that if nohz_idle_balance() fails to observe our > + * @idle_cpus_mask store, it must observe the @has_blocked > + * store. > + */ > + smp_mb__after_atomic(); > + > set_cpu_sd_state_idle(cpu); > + > +out: > + /* > + * Each time a cpu enter idle, we assume that it has blocked load and > + * enable the periodic update of the load of idle cpus > + */ > + WRITE_ONCE(nohz.has_blocked, 1); > } > #else > static inline void nohz_balancer_kick(struct rq *rq) { } > @@ -9355,7 +9408,7 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > /* Earliest time when we have to do rebalance again */ > unsigned long now = jiffies; > unsigned long next_balance = now + 60*HZ; > - unsigned long next_stats = now + msecs_to_jiffies(LOAD_AVG_PERIOD); > + bool has_blocked_load = false; > int update_next_balance = 0; > int this_cpu = this_rq->cpu; > unsigned int flags; > @@ -9374,6 +9427,22 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > > SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK); > > + /* > + * We assume there will be no idle load after this update and clear > + * the has_blocked flag. If a cpu enters idle in the mean time, it will > + * set the has_blocked flag and trig another update of idle load. > + * Because a cpu that becomes idle, is added to idle_cpus_mask before > + * setting the flag, we are sure to not clear the state and not > + * check the load of an idle cpu. > + */ > + WRITE_ONCE(nohz.has_blocked, 0); > + > + /* > + * Ensures that if we miss the CPU, we must see the has_blocked > + * store from nohz_balance_enter_idle(). > + */ > + smp_mb(); > + > for_each_cpu(balance_cpu, nohz.idle_cpus_mask) { > if (balance_cpu == this_cpu || !idle_cpu(balance_cpu)) > continue; > @@ -9383,11 +9452,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > * work being done for other cpus. Next load > * balancing owner will pick it up. > */ > - if (need_resched()) > - break; > + if (need_resched()) { > + has_blocked_load = true; > + goto abort; > + } > > rq = cpu_rq(balance_cpu); > > + update_blocked_averages(rq->cpu); > + has_blocked_load |= rq->has_blocked_load; > + > /* > * If time for next balance is due, > * do the balance. > @@ -9400,7 +9474,6 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > cpu_load_update_idle(rq); > rq_unlock_irq(rq, &rf); > > - update_blocked_averages(rq->cpu); > if (flags & NOHZ_BALANCE_KICK) > rebalance_domains(rq, CPU_IDLE); > } > @@ -9415,7 +9488,13 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > if (flags & NOHZ_BALANCE_KICK) > rebalance_domains(this_rq, CPU_IDLE); > > - nohz.next_stats = next_stats; > + WRITE_ONCE(nohz.next_blocked, > + now + msecs_to_jiffies(LOAD_AVG_PERIOD)); Is it not slightly more clear to still use a local? I.e. next_stats = now + msecs_to_jiffies(LOAD_AVG_PERIOD); WRITE_ONCE(nohz.next_blocked, next_stats); > + > +abort: > + /* There is still blocked load, enable periodic update */ > + if (has_blocked_load) > + WRITE_ONCE(nohz.has_blocked, 1); > > /* > * next_balance will be updated only when there is a need. > @@ -10046,6 +10125,7 @@ __init void init_sched_fair_class(void) > > #ifdef CONFIG_NO_HZ_COMMON > nohz.next_balance = jiffies; > + nohz.next_blocked = jiffies; > zalloc_cpumask_var(&nohz.idle_cpus_mask, GFP_NOWAIT); > #endif > #endif /* SMP */ > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index e200045..ad9b929 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -723,6 +723,7 @@ struct rq { > #ifdef CONFIG_SMP > unsigned long last_load_update_tick; > unsigned long last_blocked_load_update_tick; > + unsigned int has_blocked_load; > #endif /* CONFIG_SMP */ > unsigned int nohz_tick_stopped; > atomic_t nohz_flags; > > -- > > 2.7.4 > > -- #include <best/regards.h> Patrick Bellasi
On Thu, Feb 15, 2018 at 03:22:47PM +0000, Patrick Bellasi wrote: > > > static struct { > > > cpumask_var_t idle_cpus_mask; > > > atomic_t nr_cpus; > > > + int has_blocked; /* Idle CPUS has blocked load */ > > Why not "bool"? > Because then he gets me yelling that sizeof(_Bool) is undefined and the whole structure layout goes out the window :-) Never use bool in a composite type.
On 15-Feb 17:50, Peter Zijlstra wrote: > On Thu, Feb 15, 2018 at 03:22:47PM +0000, Patrick Bellasi wrote: > > > > static struct { > > > > cpumask_var_t idle_cpus_mask; > > > > atomic_t nr_cpus; > > > > + int has_blocked; /* Idle CPUS has blocked load */ > > > > Why not "bool"? > > > Because then he gets me yelling that sizeof(_Bool) is undefined and the > whole structure layout goes out the window :-) Now I know... > Never use bool in a composite type. ... got a note ;-) -- #include <best/regards.h> Patrick Bellasi
Hi Patrick, On 15 February 2018 at 16:22, Patrick Bellasi <patrick.bellasi@arm.com> wrote: > Hi Vincent, > here are some possible small code improvement suggestions from my side. > > Cheers Patrick > > On 14-Feb 16:26, Vincent Guittot wrote: >> > Stopped the periodic update of blocked load when all idle CPUs have fully >> > decayed. We introduce a new nohz.has_blocked that reflect if some idle >> > CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked >> > is set everytime that a Idle CPU can have blocked load and it is then clear >> > when no more blocked load has been detected during an update. We don't need >> > atomic operation but only to make cure of the right ordering when updating >> > nohz.idle_cpus_mask and nohz.has_blocked. >> > >> > Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> >> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> >> > --- >> > kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++--------- >> > kernel/sched/sched.h | 1 + >> > 2 files changed, 102 insertions(+), 21 deletions(-) >> > >> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> > index 7af1fa9..5a6835e 100644 >> > --- a/kernel/sched/fair.c >> > +++ b/kernel/sched/fair.c >> > @@ -5383,8 +5383,9 @@ decay_load_missed(unsigned long load, unsigned long missed_updates, int idx) >> > static struct { >> > cpumask_var_t idle_cpus_mask; >> > atomic_t nr_cpus; >> > + int has_blocked; /* Idle CPUS has blocked load */ > > Why not "bool"? > > What about "has_blocked_load", which makes it more clear the meaning > and is also more consistent with the RQ flags "aggregated" by this > one... as well as some other local variables you already name like > that? > >> unsigned long next_balance; /* in jiffy units */ >> - unsigned long next_stats; >> + unsigned long next_blocked; /* Next update of blocked load in jiffies */ >> } nohz ____cacheline_aligned; >> >> #endif /* CONFIG_NO_HZ_COMMON */ >> @@ -6951,6 +6952,7 @@ enum fbq_type { regular, remote, all }; >> #define LBF_DST_PINNED 0x04 >> #define LBF_SOME_PINNED 0x08 >> #define LBF_NOHZ_STATS 0x10 >> +#define LBF_NOHZ_AGAIN 0x20 >> >> struct lb_env { >> struct sched_domain *sd; >> @@ -7335,8 +7337,6 @@ static void attach_tasks(struct lb_env *env) >> rq_unlock(env->dst_rq, &rf); >> } >> >> -#ifdef CONFIG_FAIR_GROUP_SCHED >> - >> static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq) >> { >> if (cfs_rq->load.weight) >> @@ -7354,11 +7354,14 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq) >> return true; >> } >> >> +#ifdef CONFIG_FAIR_GROUP_SCHED >> + >> static void update_blocked_averages(int cpu) >> { >> struct rq *rq = cpu_rq(cpu); >> struct cfs_rq *cfs_rq, *pos; >> struct rq_flags rf; >> + bool done = true; >> >> rq_lock_irqsave(rq, &rf); >> update_rq_clock(rq); >> @@ -7388,10 +7391,14 @@ static void update_blocked_averages(int cpu) >> */ >> if (cfs_rq_is_decayed(cfs_rq)) >> list_del_leaf_cfs_rq(cfs_rq); >> + else >> + done = false; >> } >> >> #ifdef CONFIG_NO_HZ_COMMON >> rq->last_blocked_load_update_tick = jiffies; >> + if (done) >> + rq->has_blocked_load = 0; >> #endif >> rq_unlock_irqrestore(rq, &rf); >> } >> @@ -7454,6 +7461,8 @@ static inline void update_blocked_averages(int cpu) >> update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq); >> #ifdef CONFIG_NO_HZ_COMMON >> rq->last_blocked_load_update_tick = jiffies; >> + if (cfs_rq_is_decayed(cfs_rq)) >> + rq->has_blocked_load = 0; >> #endif >> rq_unlock_irqrestore(rq, &rf); >> } >> @@ -7789,18 +7798,25 @@ group_type group_classify(struct sched_group *group, >> return group_other; >> } >> >> -static void update_nohz_stats(struct rq *rq) >> +static bool update_nohz_stats(struct rq *rq) >> { >> #ifdef CONFIG_NO_HZ_COMMON >> unsigned int cpu = rq->cpu; >> >> + if (!rq->has_blocked_load) >> + return false; >> + >> if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask)) >> - return; >> + return false; >> >> if (!time_after(jiffies, rq->last_blocked_load_update_tick)) >> - return; >> + return true; >> >> update_blocked_averages(cpu); >> + >> + return rq->has_blocked_load; >> +#else >> + return false; >> #endif >> } >> >> @@ -7826,8 +7842,8 @@ static inline void update_sg_lb_stats(struct lb_env *env, >> for_each_cpu_and(i, sched_group_span(group), env->cpus) { >> struct rq *rq = cpu_rq(i); >> >> - if (env->flags & LBF_NOHZ_STATS) >> - update_nohz_stats(rq); >> + if ((env->flags & LBF_NOHZ_STATS) && update_nohz_stats(rq)) >> + env->flags |= LBF_NOHZ_AGAIN; >> >> /* Bias balancing toward cpus of our domain */ >> if (local_group) >> @@ -7979,18 +7995,17 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd >> struct sg_lb_stats *local = &sds->local_stat; >> struct sg_lb_stats tmp_sgs; >> int load_idx, prefer_sibling = 0; >> +#ifdef CONFIG_NO_HZ_COMMON >> + int has_blocked = READ_ONCE(nohz.has_blocked); >> +#endif >> bool overload = false; >> >> if (child && child->flags & SD_PREFER_SIBLING) >> prefer_sibling = 1; >> >> #ifdef CONFIG_NO_HZ_COMMON >> - if (env->idle == CPU_NEWLY_IDLE) { >> + if (env->idle == CPU_NEWLY_IDLE && has_blocked) > > Why using a local "has_blocked" here? > > Is not better to inline directly the READ_ONCE, we avoid the above > idfed and also (maybe?) save the load if the previous checks should fail? I agree that saving ifdef could be interesting although it make the condition less readable but that's just an opinion About saving the load, I expect the compiler to optimize that correctly and i'm not sure that inlining the READ_ONCE will prevent any reordering and will really save the load > >> env->flags |= LBF_NOHZ_STATS; >> - >> - if (cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) >> - nohz.next_stats = jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD); >> - } >> #endif >> >> load_idx = get_sd_load_idx(env->sd, env->idle); >> @@ -8046,6 +8061,15 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd >> sg = sg->next; >> } while (sg != env->sd->groups); >> >> +#ifdef CONFIG_NO_HZ_COMMON >> + if ((env->flags & LBF_NOHZ_AGAIN) && >> + cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) { >> + >> + WRITE_ONCE(nohz.next_blocked, >> + jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD)); >> + } >> +#endif >> + > > Moreover, the above two ifdef maybe can be better moved into compile > time inline functions to keep update_sg_lb_stats more streamlined, > e.g. something like: > > ---8<--- > #ifdef CONFIG_NO_HZ_COMMON > static inline void nohz_update_next_stats(struct lb_env *env) > { > if (env->idle != CPU_NEWLY_IDLE || READ_ONCE(nohz.has_blocked)) > return; > eenv->flags |= LBF_NOHZ_STATS; > etc... > } IMHO, Having inline function for 2 lines of code doesn't make it more readable as we then have to looks for the content of the function to understand what it does > > static inline void nohz_update_next_blocked(struct lb_env *env) > { > if (!(env->flags & LBF_NOHZ_AGAIN) > return > if (!cpumask_subset(nohz.idle_cpus_mask, > sched_domain_span(env->sd))) > return > WRITE_ONCE(); ditto > } > #else > #define nohz_update_next_stats(env) do {} while(false); > #define nohz_update_blocked(env) do {} while(false); > #endif > ---8<--- > >> if (env->sd->flags & SD_NUMA) >> env->fbq_type = fbq_classify_group(&sds->busiest_stat); >> >> @@ -9069,6 +9093,8 @@ static void nohz_balancer_kick(struct rq *rq) >> struct sched_domain *sd; >> int nr_busy, i, cpu = rq->cpu; >> unsigned int flags = 0; >> + unsigned long has_blocked = READ_ONCE(nohz.has_blocked); >> + unsigned long next_blocked = READ_ONCE(nohz.next_blocked); >> >> if (unlikely(rq->idle_balance)) >> return; >> @@ -9086,7 +9112,7 @@ static void nohz_balancer_kick(struct rq *rq) >> if (likely(!atomic_read(&nohz.nr_cpus))) >> return; >> >> - if (time_after(now, nohz.next_stats)) >> + if (time_after(now, next_blocked) && has_blocked) > > Why not using READ_ONCE right here, these are the only two usages in > this function, isn't it? It' s mainly a matter of keeping the condition on 1 line to make it more readable. > This would allow also to skip (some of) the loads if some of the > checks should fails. Ins't it? As mentioned previously I expect the compiler to re order all data access to the most optimized way > >> flags = NOHZ_STATS_KICK; >> >> if (time_before(now, nohz.next_balance)) >> @@ -9207,13 +9233,26 @@ void nohz_balance_enter_idle(int cpu) >> if (!housekeeping_cpu(cpu, HK_FLAG_SCHED)) >> return; >> >> + /* >> + * Can be set safely without rq->lock held >> + * If a clear happens, it will have evaluated last additions because >> + * rq->lock is held during the check and the clear >> + */ >> + rq->has_blocked_load = 1; >> + >> + /* >> + * The tick is still stopped but load could have been added in the >> + * meantime. We set the nohz.has_blocked flag to trig a check of the >> + * *_avg. The CPU is already part of nohz.idle_cpus_mask so the clear >> + * of nohz.has_blocked can only happen after checking the new load >> + */ >> if (rq->nohz_tick_stopped) >> - return; >> + goto out; >> >> /* >> * If we're a completely isolated CPU, we don't play. >> */ >> - if (on_null_domain(cpu_rq(cpu))) >> + if (on_null_domain(rq)) >> return; >> >> rq->nohz_tick_stopped = 1; >> @@ -9221,7 +9260,21 @@ void nohz_balance_enter_idle(int cpu) >> cpumask_set_cpu(cpu, nohz.idle_cpus_mask); >> atomic_inc(&nohz.nr_cpus); >> >> + /* >> + * Ensures that if nohz_idle_balance() fails to observe our >> + * @idle_cpus_mask store, it must observe the @has_blocked >> + * store. >> + */ >> + smp_mb__after_atomic(); >> + >> set_cpu_sd_state_idle(cpu); >> + >> +out: >> + /* >> + * Each time a cpu enter idle, we assume that it has blocked load and >> + * enable the periodic update of the load of idle cpus >> + */ >> + WRITE_ONCE(nohz.has_blocked, 1); >> } >> #else >> static inline void nohz_balancer_kick(struct rq *rq) { } >> @@ -9355,7 +9408,7 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) >> /* Earliest time when we have to do rebalance again */ >> unsigned long now = jiffies; >> unsigned long next_balance = now + 60*HZ; >> - unsigned long next_stats = now + msecs_to_jiffies(LOAD_AVG_PERIOD); >> + bool has_blocked_load = false; >> int update_next_balance = 0; >> int this_cpu = this_rq->cpu; >> unsigned int flags; >> @@ -9374,6 +9427,22 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) >> >> SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK); >> >> + /* >> + * We assume there will be no idle load after this update and clear >> + * the has_blocked flag. If a cpu enters idle in the mean time, it will >> + * set the has_blocked flag and trig another update of idle load. >> + * Because a cpu that becomes idle, is added to idle_cpus_mask before >> + * setting the flag, we are sure to not clear the state and not >> + * check the load of an idle cpu. >> + */ >> + WRITE_ONCE(nohz.has_blocked, 0); >> + >> + /* >> + * Ensures that if we miss the CPU, we must see the has_blocked >> + * store from nohz_balance_enter_idle(). >> + */ >> + smp_mb(); >> + >> for_each_cpu(balance_cpu, nohz.idle_cpus_mask) { >> if (balance_cpu == this_cpu || !idle_cpu(balance_cpu)) >> continue; >> @@ -9383,11 +9452,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) >> * work being done for other cpus. Next load >> * balancing owner will pick it up. >> */ >> - if (need_resched()) >> - break; >> + if (need_resched()) { >> + has_blocked_load = true; >> + goto abort; >> + } >> >> rq = cpu_rq(balance_cpu); >> >> + update_blocked_averages(rq->cpu); >> + has_blocked_load |= rq->has_blocked_load; >> + >> /* >> * If time for next balance is due, >> * do the balance. >> @@ -9400,7 +9474,6 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) >> cpu_load_update_idle(rq); >> rq_unlock_irq(rq, &rf); >> >> - update_blocked_averages(rq->cpu); >> if (flags & NOHZ_BALANCE_KICK) >> rebalance_domains(rq, CPU_IDLE); >> } >> @@ -9415,7 +9488,13 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) >> if (flags & NOHZ_BALANCE_KICK) >> rebalance_domains(this_rq, CPU_IDLE); >> >> - nohz.next_stats = next_stats; >> + WRITE_ONCE(nohz.next_blocked, >> + now + msecs_to_jiffies(LOAD_AVG_PERIOD)); > > Is it not slightly more clear to still use a local? I.e. > > next_stats = now + msecs_to_jiffies(LOAD_AVG_PERIOD); next_stats doesn't exist anymore with this patch > WRITE_ONCE(nohz.next_blocked, next_stats); > >> + >> +abort: >> + /* There is still blocked load, enable periodic update */ >> + if (has_blocked_load) >> + WRITE_ONCE(nohz.has_blocked, 1); >> >> /* >> * next_balance will be updated only when there is a need. >> @@ -10046,6 +10125,7 @@ __init void init_sched_fair_class(void) >> >> #ifdef CONFIG_NO_HZ_COMMON >> nohz.next_balance = jiffies; >> + nohz.next_blocked = jiffies; >> zalloc_cpumask_var(&nohz.idle_cpus_mask, GFP_NOWAIT); >> #endif >> #endif /* SMP */ >> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h >> index e200045..ad9b929 100644 >> --- a/kernel/sched/sched.h >> +++ b/kernel/sched/sched.h >> @@ -723,6 +723,7 @@ struct rq { >> #ifdef CONFIG_SMP >> unsigned long last_load_update_tick; >> unsigned long last_blocked_load_update_tick; >> + unsigned int has_blocked_load; >> #endif /* CONFIG_SMP */ >> unsigned int nohz_tick_stopped; >> atomic_t nohz_flags; >> > -- >> > 2.7.4 >> > > > -- > #include <best/regards.h> > > Patrick Bellasi
On 02/14/2018 03:26 PM, Vincent Guittot wrote: > Stopped the periodic update of blocked load when all idle CPUs have fully > decayed. We introduce a new nohz.has_blocked that reflect if some idle > CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked > is set everytime that a Idle CPU can have blocked load and it is then clear > when no more blocked load has been detected during an update. We don't need > atomic operation but only to make cure of the right ordering when updating > nohz.idle_cpus_mask and nohz.has_blocked. > > Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++--------- > kernel/sched/sched.h | 1 + > 2 files changed, 102 insertions(+), 21 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 7af1fa9..5a6835e 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > > [...] >> @@ -9374,6 +9427,22 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > > SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK); > > + /* > + * We assume there will be no idle load after this update and clear > + * the has_blocked flag. If a cpu enters idle in the mean time, it will > + * set the has_blocked flag and trig another update of idle load. > + * Because a cpu that becomes idle, is added to idle_cpus_mask before > + * setting the flag, we are sure to not clear the state and not > + * check the load of an idle cpu. > + */ > + WRITE_ONCE(nohz.has_blocked, 0); > + > + /* > + * Ensures that if we miss the CPU, we must see the has_blocked > + * store from nohz_balance_enter_idle(). > + */ > + smp_mb(); > + > for_each_cpu(balance_cpu, nohz.idle_cpus_mask) { > if (balance_cpu == this_cpu || !idle_cpu(balance_cpu)) > continue; > @@ -9383,11 +9452,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > * work being done for other cpus. Next load > * balancing owner will pick it up. > */ > - if (need_resched()) > - break; > + if (need_resched()) { > + has_blocked_load = true; > + goto abort; > + } > > rq = cpu_rq(balance_cpu); > I'd say it's safe to do the following here. The flag is raised in nohz_balance_enter_idle() before the smp_mb(), so we won't skip a CPU that just got added to nohz.idle_cpus_mask. /* * This cpu doesn't have any remaining blocked load, skip it. * It's sane to do this because this flag is raised in * nohz_balance_enter_idle() */ if ((flags & NOHZ_KICK_MASK) == NOHZ_STATS_KICK && !rq->has_blocked_load) continue; > + update_blocked_averages(rq->cpu); > + has_blocked_load |= rq->has_blocked_load; > + > /* > * If time for next balance is due, > * do the balance. > @@ -9400,7 +9474,6 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > cpu_load_update_idle(rq); > rq_unlock_irq(rq, &rf); > > - update_blocked_averages(rq->cpu); > if (flags & NOHZ_BALANCE_KICK) > rebalance_domains(rq, CPU_IDLE); > } > @@ -9415,7 +9488,13 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > if (flags & NOHZ_BALANCE_KICK) > rebalance_domains(this_rq, CPU_IDLE); > > - nohz.next_stats = next_stats; > + WRITE_ONCE(nohz.next_blocked, > + now + msecs_to_jiffies(LOAD_AVG_PERIOD)); > + > +abort: > + /* There is still blocked load, enable periodic update */ > + if (has_blocked_load) > + WRITE_ONCE(nohz.has_blocked, 1); > > /* > * next_balance will be updated only when there is a need.
On 02/14/2018 03:26 PM, Vincent Guittot wrote: > Stopped the periodic update of blocked load when all idle CPUs have fully > decayed. We introduce a new nohz.has_blocked that reflect if some idle > CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked > is set everytime that a Idle CPU can have blocked load and it is then clear > when no more blocked load has been detected during an update. We don't need > atomic operation but only to make cure of the right ordering when updating > nohz.idle_cpus_mask and nohz.has_blocked. > > Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++--------- > kernel/sched/sched.h | 1 + > 2 files changed, 102 insertions(+), 21 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 7af1fa9..5a6835e 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > > [...] > > -static void update_nohz_stats(struct rq *rq) > +static bool update_nohz_stats(struct rq *rq) > { > #ifdef CONFIG_NO_HZ_COMMON > unsigned int cpu = rq->cpu; > > + if (!rq->has_blocked_load) > + return false; > + > if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask)) > - return; > + return false; > > if (!time_after(jiffies, rq->last_blocked_load_update_tick)) > - return; > + return true; > > update_blocked_averages(cpu); > + > + return rq->has_blocked_load; > +#else > + return false; > #endif > } > (Wrongly thought that this bit was in a different patch, comment should have been squashed in previous reply...) I've been thinking about this, and it's a messy one if we want to skip CPUs in idle_balance() / clear the nohz.has_blocked_flag. AFAICT, the rq->has_blocked_load flag can be wrongly cleared: if we're calling update_nohz_stats() for CPU A, but CPU A got out/in of idle really quickly in that same timeframe, I'm not sure you can guarantee the clearing of rq->has_blocked_load done in update_blocked_averages() will always end up in memory before the setting of the flag in nohz_balance_enter_idle(). I was going to say we don't have this problem in _nohz_idle_balance() but actually I think we do. We have the checking of nohz.idle_cpus_mask after the smp_mb(), which makes sure the clear of nohz.has_blocked will never overwrite the set in nohz_balance_enter_idle(), but it doesn't guarantee the same for the rq flag. So we can have nohz CPUs with blocked load but with rq->has_blocked_load set to false. Which isn't a problem now but it is if we want to use the flag to skip CPUs. Am I correct or am I going crazy ? There's a comment about this in nohz_balance_enter_idle() but I'm confused now: /* * Can be set safely without rq->lock held * If a clear happens, it will have evaluated last additions because * rq->lock is held during the check and the clear */ rq->has_blocked_load = 1;
On 16 February 2018 at 13:13, Valentin Schneider <valentin.schneider@arm.com> wrote: > On 02/14/2018 03:26 PM, Vincent Guittot wrote: >> Stopped the periodic update of blocked load when all idle CPUs have fully >> decayed. We introduce a new nohz.has_blocked that reflect if some idle >> CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked >> is set everytime that a Idle CPU can have blocked load and it is then clear >> when no more blocked load has been detected during an update. We don't need >> atomic operation but only to make cure of the right ordering when updating >> nohz.idle_cpus_mask and nohz.has_blocked. >> >> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> >> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> >> --- >> kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++--------- >> kernel/sched/sched.h | 1 + >> 2 files changed, 102 insertions(+), 21 deletions(-) >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index 7af1fa9..5a6835e 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> >> [...] >>> @@ -9374,6 +9427,22 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) >> >> SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK); >> >> + /* >> + * We assume there will be no idle load after this update and clear >> + * the has_blocked flag. If a cpu enters idle in the mean time, it will >> + * set the has_blocked flag and trig another update of idle load. >> + * Because a cpu that becomes idle, is added to idle_cpus_mask before >> + * setting the flag, we are sure to not clear the state and not >> + * check the load of an idle cpu. >> + */ >> + WRITE_ONCE(nohz.has_blocked, 0); >> + >> + /* >> + * Ensures that if we miss the CPU, we must see the has_blocked >> + * store from nohz_balance_enter_idle(). >> + */ >> + smp_mb(); >> + >> for_each_cpu(balance_cpu, nohz.idle_cpus_mask) { >> if (balance_cpu == this_cpu || !idle_cpu(balance_cpu)) >> continue; >> @@ -9383,11 +9452,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) >> * work being done for other cpus. Next load >> * balancing owner will pick it up. >> */ >> - if (need_resched()) >> - break; >> + if (need_resched()) { >> + has_blocked_load = true; >> + goto abort; >> + } >> >> rq = cpu_rq(balance_cpu); >> > > I'd say it's safe to do the following here. The flag is raised in > nohz_balance_enter_idle() before the smp_mb(), so we won't skip a CPU > that just got added to nohz.idle_cpus_mask. rq->has_blocked_load will be set before the barrier only if nohz_tick_stopped is not already set, Otherwise, we skip cpumask update and the barrier in nohz_balance_enter_idle > > /* > * This cpu doesn't have any remaining blocked load, skip it. > * It's sane to do this because this flag is raised in > * nohz_balance_enter_idle() > */ > if ((flags & NOHZ_KICK_MASK) == NOHZ_STATS_KICK && > !rq->has_blocked_load) > continue; > >> + update_blocked_averages(rq->cpu); >> + has_blocked_load |= rq->has_blocked_load; >> + >> /* >> * If time for next balance is due, >> * do the balance. >> @@ -9400,7 +9474,6 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) >> cpu_load_update_idle(rq); >> rq_unlock_irq(rq, &rf); >> >> - update_blocked_averages(rq->cpu); >> if (flags & NOHZ_BALANCE_KICK) >> rebalance_domains(rq, CPU_IDLE); >> } >> @@ -9415,7 +9488,13 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) >> if (flags & NOHZ_BALANCE_KICK) >> rebalance_domains(this_rq, CPU_IDLE); >> >> - nohz.next_stats = next_stats; >> + WRITE_ONCE(nohz.next_blocked, >> + now + msecs_to_jiffies(LOAD_AVG_PERIOD)); >> + >> +abort: >> + /* There is still blocked load, enable periodic update */ >> + if (has_blocked_load) >> + WRITE_ONCE(nohz.has_blocked, 1); >> >> /* >> * next_balance will be updated only when there is a need.
On 16 February 2018 at 13:53, Valentin Schneider <valentin.schneider@arm.com> wrote: > On 02/14/2018 03:26 PM, Vincent Guittot wrote: >> Stopped the periodic update of blocked load when all idle CPUs have fully >> decayed. We introduce a new nohz.has_blocked that reflect if some idle >> CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked >> is set everytime that a Idle CPU can have blocked load and it is then clear >> when no more blocked load has been detected during an update. We don't need >> atomic operation but only to make cure of the right ordering when updating >> nohz.idle_cpus_mask and nohz.has_blocked. >> >> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> >> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> >> --- >> kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++--------- >> kernel/sched/sched.h | 1 + >> 2 files changed, 102 insertions(+), 21 deletions(-) >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index 7af1fa9..5a6835e 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> >> [...] >> >> -static void update_nohz_stats(struct rq *rq) >> +static bool update_nohz_stats(struct rq *rq) >> { >> #ifdef CONFIG_NO_HZ_COMMON >> unsigned int cpu = rq->cpu; >> >> + if (!rq->has_blocked_load) >> + return false; >> + >> if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask)) >> - return; >> + return false; >> >> if (!time_after(jiffies, rq->last_blocked_load_update_tick)) >> - return; >> + return true; >> >> update_blocked_averages(cpu); >> + >> + return rq->has_blocked_load; >> +#else >> + return false; >> #endif >> } >> > > (Wrongly thought that this bit was in a different patch, comment should have > been squashed in previous reply...) > > I've been thinking about this, and it's a messy one if we want to > skip CPUs in idle_balance() / clear the nohz.has_blocked_flag. > > AFAICT, the rq->has_blocked_load flag can be wrongly cleared: if we're > calling update_nohz_stats() for CPU A, but CPU A got out/in of > idle really quickly in that same timeframe, I'm not sure you can guarantee > the clearing of rq->has_blocked_load done in update_blocked_averages() will > always end up in memory before the setting of the flag in > nohz_balance_enter_idle(). Not sure it's a problem in this case because the clear done in update_blocked_averages() only happens if there is no load on the rq and new load can't be added in the mean time > > I was going to say we don't have this problem in _nohz_idle_balance() but > actually I think we do. We have the checking of nohz.idle_cpus_mask after the > smp_mb(), which makes sure the clear of nohz.has_blocked will never > overwrite the set in nohz_balance_enter_idle(), but it doesn't > guarantee the same for the rq flag. So we can have nohz CPUs with blocked > load but with rq->has_blocked_load set to false. Which isn't a problem now I don't think that we can have this case. Or at least I can't find a sequence leading to this state. Have you got a particular sequence in mind ? > but it is if we want to use the flag to skip CPUs. > > Am I correct or am I going crazy ? There's a comment about this in > nohz_balance_enter_idle() but I'm confused now: > > /* > * Can be set safely without rq->lock held > * If a clear happens, it will have evaluated last additions because > * rq->lock is held during the check and the clear > */ > rq->has_blocked_load = 1;
On 02/16/2018 05:02 PM, Vincent Guittot wrote: > On 16 February 2018 at 13:53, Valentin Schneider > <valentin.schneider@arm.com> wrote: >> On 02/14/2018 03:26 PM, Vincent Guittot wrote: >>> Stopped the periodic update of blocked load when all idle CPUs have fully >>> decayed. We introduce a new nohz.has_blocked that reflect if some idle >>> CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked >>> is set everytime that a Idle CPU can have blocked load and it is then clear >>> when no more blocked load has been detected during an update. We don't need >>> atomic operation but only to make cure of the right ordering when updating >>> nohz.idle_cpus_mask and nohz.has_blocked. >>> >>> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> >>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> >>> --- >>> kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++--------- >>> kernel/sched/sched.h | 1 + >>> 2 files changed, 102 insertions(+), 21 deletions(-) >>> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>> index 7af1fa9..5a6835e 100644 >>> --- a/kernel/sched/fair.c >>> +++ b/kernel/sched/fair.c >>> >>> [...] >>> >>> -static void update_nohz_stats(struct rq *rq) >>> +static bool update_nohz_stats(struct rq *rq) >>> { >>> #ifdef CONFIG_NO_HZ_COMMON >>> unsigned int cpu = rq->cpu; >>> >>> + if (!rq->has_blocked_load) >>> + return false; >>> + >>> if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask)) >>> - return; >>> + return false; >>> >>> if (!time_after(jiffies, rq->last_blocked_load_update_tick)) >>> - return; >>> + return true; >>> >>> update_blocked_averages(cpu); >>> + >>> + return rq->has_blocked_load; >>> +#else >>> + return false; >>> #endif >>> } >>> >> >> (Wrongly thought that this bit was in a different patch, comment should have >> been squashed in previous reply...) >> >> I've been thinking about this, and it's a messy one if we want to >> skip CPUs in idle_balance() / clear the nohz.has_blocked_flag. >> >> AFAICT, the rq->has_blocked_load flag can be wrongly cleared: if we're >> calling update_nohz_stats() for CPU A, but CPU A got out/in of >> idle really quickly in that same timeframe, I'm not sure you can guarantee >> the clearing of rq->has_blocked_load done in update_blocked_averages() will >> always end up in memory before the setting of the flag in >> nohz_balance_enter_idle(). > > Not sure it's a problem in this case because the clear done in > update_blocked_averages() only happens if there is no load on the rq > and new load can't be added in the mean time > You're right, and that's why there's that comment: >> /* >> * Can be set safely without rq->lock held >> * If a clear happens, it will have evaluated last additions because >> * rq->lock is held during the check and the clear >> */ >> rq->has_blocked_load = 1; Even though it's clearly written there my brain wouldn't process the fact that the flag is cleared with the rq lock held. So yeah, we can't wrongly clear rq->has_blocked_load. The only mishap that can happen is that it is re-raised even though we just went though an update_nohz_stats(), which would lead to a useless stats update in the future, but that's not as bad.
On 02/16/2018 01:44 PM, Vincent Guittot wrote: > On 16 February 2018 at 13:13, Valentin Schneider > <valentin.schneider@arm.com> wrote: >> On 02/14/2018 03:26 PM, Vincent Guittot wrote: >>> Stopped the periodic update of blocked load when all idle CPUs have fully >>> decayed. We introduce a new nohz.has_blocked that reflect if some idle >>> CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked >>> is set everytime that a Idle CPU can have blocked load and it is then clear >>> when no more blocked load has been detected during an update. We don't need >>> atomic operation but only to make cure of the right ordering when updating >>> nohz.idle_cpus_mask and nohz.has_blocked. >>> >>> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> >>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> >>> --- >>> kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++--------- >>> kernel/sched/sched.h | 1 + >>> 2 files changed, 102 insertions(+), 21 deletions(-) >>> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>> index 7af1fa9..5a6835e 100644 >>> --- a/kernel/sched/fair.c >>> +++ b/kernel/sched/fair.c >>> >>> [...] >>> @@ -9383,11 +9452,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) >>> * work being done for other cpus. Next load >>> * balancing owner will pick it up. >>> */ >>> - if (need_resched()) >>> - break; >>> + if (need_resched()) { >>> + has_blocked_load = true; >>> + goto abort; >>> + } >>> >>> rq = cpu_rq(balance_cpu); >>> >> >> I'd say it's safe to do the following here. The flag is raised in >> nohz_balance_enter_idle() before the smp_mb(), so we won't skip a CPU >> that just got added to nohz.idle_cpus_mask. > > rq->has_blocked_load will be set before the barrier only if > nohz_tick_stopped is not already set, > Otherwise, we skip cpumask update and the barrier in nohz_balance_enter_idle > Right, forgot about that bit. I think it's still fine because: - nohz_balance_enter_idle() can't be called before the last running task is dequeued, which requires the rq lock. - update_blocked_averages takes the rq lock and clears rq->has_blocked_load with the lock held. So even though we could have some very unlikely scenario where a CPU quickly goes out/in of idle after nohz.idle_cpus_mask has been read, the blocked load itself is safe so rq->has_blocked_load will end up being set correctly. (Took me a while to see it that way) BTW, with the current set on Peter's sched/testing, update_nohz_stats() is called here, which doesn't do the update if !rq->has_blocked_load (Although that check is done without lock/barrier, so maybe we could not see a CPU that just went idle ?) I have one more question on that bit: has_blocked_load |= update_nohz_stats(rq, true); /* * If time for next balance is due, * do the balance. */ if (time_after_eq(jiffies, rq->next_balance)) { struct rq_flags rf; rq_lock_irqsave(rq, &rf); update_rq_clock(rq); cpu_load_update_idle(rq); rq_unlock_irqrestore(rq, &rf); if (flags & NOHZ_BALANCE_KICK) rebalance_domains(rq, CPU_IDLE); } if (time_after(next_balance, rq->next_balance)) { next_balance = rq->next_balance; update_next_balance = 1; } Now that I think about it, shouldn't we always have a 'continue' after the blocked load update if (flags & NOHZ_KICK_MASK) == NOHZ_STATS_KICK ? AFAICT we don't want to push the next_balance forward, only the next_blocked. That would also take care of not doing the load balance. >> >> /* >> * This cpu doesn't have any remaining blocked load, skip it. >> * It's sane to do this because this flag is raised in >> * nohz_balance_enter_idle() >> */ >> if ((flags & NOHZ_KICK_MASK) == NOHZ_STATS_KICK && >> !rq->has_blocked_load) >> continue; >> >>> + update_blocked_averages(rq->cpu); >>> + has_blocked_load |= rq->has_blocked_load; >>> +
On 02/21/2018 01:13 PM, Valentin Schneider wrote: > On 02/16/2018 01:44 PM, Vincent Guittot wrote: >> On 16 February 2018 at 13:13, Valentin Schneider >> <valentin.schneider@arm.com> wrote: >>> On 02/14/2018 03:26 PM, Vincent Guittot wrote: > BTW, with the current set on Peter's sched/testing, update_nohz_stats() > is called here, which doesn't do the update if !rq->has_blocked_load > (Although that check is done without lock/barrier, so maybe we could not see > a CPU that just went idle ?) Ignore that, that's another case of me being overly paranoid after reading too much of Documentation/memory-barriers.txt
On 21 February 2018 at 14:13, Valentin Schneider <valentin.schneider@arm.com> wrote: > On 02/16/2018 01:44 PM, Vincent Guittot wrote: >> On 16 February 2018 at 13:13, Valentin Schneider >> <valentin.schneider@arm.com> wrote: >>> On 02/14/2018 03:26 PM, Vincent Guittot wrote: >>>> Stopped the periodic update of blocked load when all idle CPUs have fully >>>> decayed. We introduce a new nohz.has_blocked that reflect if some idle >>>> CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked >>>> is set everytime that a Idle CPU can have blocked load and it is then clear >>>> when no more blocked load has been detected during an update. We don't need >>>> atomic operation but only to make cure of the right ordering when updating >>>> nohz.idle_cpus_mask and nohz.has_blocked. >>>> >>>> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> >>>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> >>>> --- >>>> kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++--------- >>>> kernel/sched/sched.h | 1 + >>>> 2 files changed, 102 insertions(+), 21 deletions(-) >>>> >>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>>> index 7af1fa9..5a6835e 100644 >>>> --- a/kernel/sched/fair.c >>>> +++ b/kernel/sched/fair.c >>>> >>>> [...] >>>> @@ -9383,11 +9452,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) >>>> * work being done for other cpus. Next load >>>> * balancing owner will pick it up. >>>> */ >>>> - if (need_resched()) >>>> - break; >>>> + if (need_resched()) { >>>> + has_blocked_load = true; >>>> + goto abort; >>>> + } >>>> >>>> rq = cpu_rq(balance_cpu); >>>> >>> >>> I'd say it's safe to do the following here. The flag is raised in >>> nohz_balance_enter_idle() before the smp_mb(), so we won't skip a CPU >>> that just got added to nohz.idle_cpus_mask. >> >> rq->has_blocked_load will be set before the barrier only if >> nohz_tick_stopped is not already set, >> Otherwise, we skip cpumask update and the barrier in nohz_balance_enter_idle >> > > Right, forgot about that bit. I think it's still fine because: > - nohz_balance_enter_idle() can't be called before the last running task is > dequeued, which requires the rq lock. > - update_blocked_averages takes the rq lock and clears rq->has_blocked_load > with the lock held. > > So even though we could have some very unlikely scenario where a CPU quickly > goes out/in of idle after nohz.idle_cpus_mask has been read, the blocked load > itself is safe so rq->has_blocked_load will end up being set correctly. > (Took me a while to see it that way) > > > BTW, with the current set on Peter's sched/testing, update_nohz_stats() > is called here, which doesn't do the update if !rq->has_blocked_load > (Although that check is done without lock/barrier, so maybe we could not see > a CPU that just went idle ?) > > I have one more question on that bit: > > > has_blocked_load |= update_nohz_stats(rq, true); > > /* > * If time for next balance is due, > * do the balance. > */ > if (time_after_eq(jiffies, rq->next_balance)) { > struct rq_flags rf; > > rq_lock_irqsave(rq, &rf); > update_rq_clock(rq); > cpu_load_update_idle(rq); > rq_unlock_irqrestore(rq, &rf); > > if (flags & NOHZ_BALANCE_KICK) > rebalance_domains(rq, CPU_IDLE); > } > > if (time_after(next_balance, rq->next_balance)) { > next_balance = rq->next_balance; > update_next_balance = 1; > } > > > Now that I think about it, shouldn't we always have a 'continue' after > the blocked load update if (flags & NOHZ_KICK_MASK) == NOHZ_STATS_KICK ? > AFAICT we don't want to push the next_balance forward, only the next_blocked. But we don't push next_balance forward. It just get the shortest next_balance and update nohz.next_balance exactly like what is done in full idle load balance > That would also take care of not doing the load balance. >>> >>> /* >>> * This cpu doesn't have any remaining blocked load, skip it. >>> * It's sane to do this because this flag is raised in >>> * nohz_balance_enter_idle() >>> */ >>> if ((flags & NOHZ_KICK_MASK) == NOHZ_STATS_KICK && >>> !rq->has_blocked_load) >>> continue; Then, it's worth keeping the call to cpu_load_update_idle(rq); which update the cpu_load[] array which is still used at some level >>> >>>> + update_blocked_averages(rq->cpu); >>>> + has_blocked_load |= rq->has_blocked_load; >>>> +
On 02/22/2018 08:37 AM, Vincent Guittot wrote: > On 21 February 2018 at 14:13, Valentin Schneider > <valentin.schneider@arm.com> wrote: >> On 02/16/2018 01:44 PM, Vincent Guittot wrote: >>> On 16 February 2018 at 13:13, Valentin Schneider >>> <valentin.schneider@arm.com> wrote: >>>> On 02/14/2018 03:26 PM, Vincent Guittot wrote: >>>>> Stopped the periodic update of blocked load when all idle CPUs have fully >>>>> decayed. We introduce a new nohz.has_blocked that reflect if some idle >>>>> CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked >>>>> is set everytime that a Idle CPU can have blocked load and it is then clear >>>>> when no more blocked load has been detected during an update. We don't need >>>>> atomic operation but only to make cure of the right ordering when updating >>>>> nohz.idle_cpus_mask and nohz.has_blocked. >>>>> >>>>> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> >>>>> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> >>>>> --- >>>>> kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++--------- >>>>> kernel/sched/sched.h | 1 + >>>>> 2 files changed, 102 insertions(+), 21 deletions(-) >>>>> >>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>>>> index 7af1fa9..5a6835e 100644 >>>>> --- a/kernel/sched/fair.c >>>>> +++ b/kernel/sched/fair.c >>>>> >>>>> [...] >> >> I have one more question on that bit: >> >> >> has_blocked_load |= update_nohz_stats(rq, true); >> >> /* >> * If time for next balance is due, >> * do the balance. >> */ >> if (time_after_eq(jiffies, rq->next_balance)) { >> struct rq_flags rf; >> >> rq_lock_irqsave(rq, &rf); >> update_rq_clock(rq); >> cpu_load_update_idle(rq); >> rq_unlock_irqrestore(rq, &rf); >> >> if (flags & NOHZ_BALANCE_KICK) >> rebalance_domains(rq, CPU_IDLE); >> } >> >> if (time_after(next_balance, rq->next_balance)) { >> next_balance = rq->next_balance; >> update_next_balance = 1; >> } >> >> >> Now that I think about it, shouldn't we always have a 'continue' after >> the blocked load update if (flags & NOHZ_KICK_MASK) == NOHZ_STATS_KICK ? >> AFAICT we don't want to push the next_balance forward, only the next_blocked. > > But we don't push next_balance forward. It just get the shortest > next_balance and update nohz.next_balance exactly like what is done in > full idle load balance > Sorry, that was a poor choice of words - I probably should've just gone with "update". What I meant by that is that if we have (flags & NOHZ_KICK_MASK) == NOHZ_STATS_KICK then we're not going to do the load balance. Then, in this case, I thought that we should not be going through any condition that uses nohz.next_balance (since we're not doing any balancing). Arguably *updating* nohz.next_balance still makes sense in this scenario. In short, my comment was mostly about "cleanly" separating stats update vs load balance. >> That would also take care of not doing the load balance. >>>> >>>> /* >>>> * This cpu doesn't have any remaining blocked load, skip it. >>>> * It's sane to do this because this flag is raised in >>>> * nohz_balance_enter_idle() >>>> */ >>>> if ((flags & NOHZ_KICK_MASK) == NOHZ_STATS_KICK && >>>> !rq->has_blocked_load) >>>> continue; > > Then, it's worth keeping the call to cpu_load_update_idle(rq); which > update the cpu_load[] array which is still used at some level > Is that something we would want to have in update_nohz_stats() to also cover the idle_balance -> load_balance update scenario ? From a quick glance I would've said it shouldn't be needed since the CPU doing the updates wouldn't have been nohz previously, but we're currently calling it when going through nohz_newidle_balance() so I might have gotten that wrong. >>>> >>>>> + update_blocked_averages(rq->cpu); >>>>> + has_blocked_load |= rq->has_blocked_load; >>>>> +
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 7af1fa9..5a6835e 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5383,8 +5383,9 @@ decay_load_missed(unsigned long load, unsigned long missed_updates, int idx) static struct { cpumask_var_t idle_cpus_mask; atomic_t nr_cpus; + int has_blocked; /* Idle CPUS has blocked load */ unsigned long next_balance; /* in jiffy units */ - unsigned long next_stats; + unsigned long next_blocked; /* Next update of blocked load in jiffies */ } nohz ____cacheline_aligned; #endif /* CONFIG_NO_HZ_COMMON */ @@ -6951,6 +6952,7 @@ enum fbq_type { regular, remote, all }; #define LBF_DST_PINNED 0x04 #define LBF_SOME_PINNED 0x08 #define LBF_NOHZ_STATS 0x10 +#define LBF_NOHZ_AGAIN 0x20 struct lb_env { struct sched_domain *sd; @@ -7335,8 +7337,6 @@ static void attach_tasks(struct lb_env *env) rq_unlock(env->dst_rq, &rf); } -#ifdef CONFIG_FAIR_GROUP_SCHED - static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq) { if (cfs_rq->load.weight) @@ -7354,11 +7354,14 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq) return true; } +#ifdef CONFIG_FAIR_GROUP_SCHED + static void update_blocked_averages(int cpu) { struct rq *rq = cpu_rq(cpu); struct cfs_rq *cfs_rq, *pos; struct rq_flags rf; + bool done = true; rq_lock_irqsave(rq, &rf); update_rq_clock(rq); @@ -7388,10 +7391,14 @@ static void update_blocked_averages(int cpu) */ if (cfs_rq_is_decayed(cfs_rq)) list_del_leaf_cfs_rq(cfs_rq); + else + done = false; } #ifdef CONFIG_NO_HZ_COMMON rq->last_blocked_load_update_tick = jiffies; + if (done) + rq->has_blocked_load = 0; #endif rq_unlock_irqrestore(rq, &rf); } @@ -7454,6 +7461,8 @@ static inline void update_blocked_averages(int cpu) update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq); #ifdef CONFIG_NO_HZ_COMMON rq->last_blocked_load_update_tick = jiffies; + if (cfs_rq_is_decayed(cfs_rq)) + rq->has_blocked_load = 0; #endif rq_unlock_irqrestore(rq, &rf); } @@ -7789,18 +7798,25 @@ group_type group_classify(struct sched_group *group, return group_other; } -static void update_nohz_stats(struct rq *rq) +static bool update_nohz_stats(struct rq *rq) { #ifdef CONFIG_NO_HZ_COMMON unsigned int cpu = rq->cpu; + if (!rq->has_blocked_load) + return false; + if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask)) - return; + return false; if (!time_after(jiffies, rq->last_blocked_load_update_tick)) - return; + return true; update_blocked_averages(cpu); + + return rq->has_blocked_load; +#else + return false; #endif } @@ -7826,8 +7842,8 @@ static inline void update_sg_lb_stats(struct lb_env *env, for_each_cpu_and(i, sched_group_span(group), env->cpus) { struct rq *rq = cpu_rq(i); - if (env->flags & LBF_NOHZ_STATS) - update_nohz_stats(rq); + if ((env->flags & LBF_NOHZ_STATS) && update_nohz_stats(rq)) + env->flags |= LBF_NOHZ_AGAIN; /* Bias balancing toward cpus of our domain */ if (local_group) @@ -7979,18 +7995,17 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd struct sg_lb_stats *local = &sds->local_stat; struct sg_lb_stats tmp_sgs; int load_idx, prefer_sibling = 0; +#ifdef CONFIG_NO_HZ_COMMON + int has_blocked = READ_ONCE(nohz.has_blocked); +#endif bool overload = false; if (child && child->flags & SD_PREFER_SIBLING) prefer_sibling = 1; #ifdef CONFIG_NO_HZ_COMMON - if (env->idle == CPU_NEWLY_IDLE) { + if (env->idle == CPU_NEWLY_IDLE && has_blocked) env->flags |= LBF_NOHZ_STATS; - - if (cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) - nohz.next_stats = jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD); - } #endif load_idx = get_sd_load_idx(env->sd, env->idle); @@ -8046,6 +8061,15 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd sg = sg->next; } while (sg != env->sd->groups); +#ifdef CONFIG_NO_HZ_COMMON + if ((env->flags & LBF_NOHZ_AGAIN) && + cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd))) { + + WRITE_ONCE(nohz.next_blocked, + jiffies + msecs_to_jiffies(LOAD_AVG_PERIOD)); + } +#endif + if (env->sd->flags & SD_NUMA) env->fbq_type = fbq_classify_group(&sds->busiest_stat); @@ -9069,6 +9093,8 @@ static void nohz_balancer_kick(struct rq *rq) struct sched_domain *sd; int nr_busy, i, cpu = rq->cpu; unsigned int flags = 0; + unsigned long has_blocked = READ_ONCE(nohz.has_blocked); + unsigned long next_blocked = READ_ONCE(nohz.next_blocked); if (unlikely(rq->idle_balance)) return; @@ -9086,7 +9112,7 @@ static void nohz_balancer_kick(struct rq *rq) if (likely(!atomic_read(&nohz.nr_cpus))) return; - if (time_after(now, nohz.next_stats)) + if (time_after(now, next_blocked) && has_blocked) flags = NOHZ_STATS_KICK; if (time_before(now, nohz.next_balance)) @@ -9207,13 +9233,26 @@ void nohz_balance_enter_idle(int cpu) if (!housekeeping_cpu(cpu, HK_FLAG_SCHED)) return; + /* + * Can be set safely without rq->lock held + * If a clear happens, it will have evaluated last additions because + * rq->lock is held during the check and the clear + */ + rq->has_blocked_load = 1; + + /* + * The tick is still stopped but load could have been added in the + * meantime. We set the nohz.has_blocked flag to trig a check of the + * *_avg. The CPU is already part of nohz.idle_cpus_mask so the clear + * of nohz.has_blocked can only happen after checking the new load + */ if (rq->nohz_tick_stopped) - return; + goto out; /* * If we're a completely isolated CPU, we don't play. */ - if (on_null_domain(cpu_rq(cpu))) + if (on_null_domain(rq)) return; rq->nohz_tick_stopped = 1; @@ -9221,7 +9260,21 @@ void nohz_balance_enter_idle(int cpu) cpumask_set_cpu(cpu, nohz.idle_cpus_mask); atomic_inc(&nohz.nr_cpus); + /* + * Ensures that if nohz_idle_balance() fails to observe our + * @idle_cpus_mask store, it must observe the @has_blocked + * store. + */ + smp_mb__after_atomic(); + set_cpu_sd_state_idle(cpu); + +out: + /* + * Each time a cpu enter idle, we assume that it has blocked load and + * enable the periodic update of the load of idle cpus + */ + WRITE_ONCE(nohz.has_blocked, 1); } #else static inline void nohz_balancer_kick(struct rq *rq) { } @@ -9355,7 +9408,7 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) /* Earliest time when we have to do rebalance again */ unsigned long now = jiffies; unsigned long next_balance = now + 60*HZ; - unsigned long next_stats = now + msecs_to_jiffies(LOAD_AVG_PERIOD); + bool has_blocked_load = false; int update_next_balance = 0; int this_cpu = this_rq->cpu; unsigned int flags; @@ -9374,6 +9427,22 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK); + /* + * We assume there will be no idle load after this update and clear + * the has_blocked flag. If a cpu enters idle in the mean time, it will + * set the has_blocked flag and trig another update of idle load. + * Because a cpu that becomes idle, is added to idle_cpus_mask before + * setting the flag, we are sure to not clear the state and not + * check the load of an idle cpu. + */ + WRITE_ONCE(nohz.has_blocked, 0); + + /* + * Ensures that if we miss the CPU, we must see the has_blocked + * store from nohz_balance_enter_idle(). + */ + smp_mb(); + for_each_cpu(balance_cpu, nohz.idle_cpus_mask) { if (balance_cpu == this_cpu || !idle_cpu(balance_cpu)) continue; @@ -9383,11 +9452,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) * work being done for other cpus. Next load * balancing owner will pick it up. */ - if (need_resched()) - break; + if (need_resched()) { + has_blocked_load = true; + goto abort; + } rq = cpu_rq(balance_cpu); + update_blocked_averages(rq->cpu); + has_blocked_load |= rq->has_blocked_load; + /* * If time for next balance is due, * do the balance. @@ -9400,7 +9474,6 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) cpu_load_update_idle(rq); rq_unlock_irq(rq, &rf); - update_blocked_averages(rq->cpu); if (flags & NOHZ_BALANCE_KICK) rebalance_domains(rq, CPU_IDLE); } @@ -9415,7 +9488,13 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) if (flags & NOHZ_BALANCE_KICK) rebalance_domains(this_rq, CPU_IDLE); - nohz.next_stats = next_stats; + WRITE_ONCE(nohz.next_blocked, + now + msecs_to_jiffies(LOAD_AVG_PERIOD)); + +abort: + /* There is still blocked load, enable periodic update */ + if (has_blocked_load) + WRITE_ONCE(nohz.has_blocked, 1); /* * next_balance will be updated only when there is a need. @@ -10046,6 +10125,7 @@ __init void init_sched_fair_class(void) #ifdef CONFIG_NO_HZ_COMMON nohz.next_balance = jiffies; + nohz.next_blocked = jiffies; zalloc_cpumask_var(&nohz.idle_cpus_mask, GFP_NOWAIT); #endif #endif /* SMP */ diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index e200045..ad9b929 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -723,6 +723,7 @@ struct rq { #ifdef CONFIG_SMP unsigned long last_load_update_tick; unsigned long last_blocked_load_update_tick; + unsigned int has_blocked_load; #endif /* CONFIG_SMP */ unsigned int nohz_tick_stopped; atomic_t nohz_flags;
Stopped the periodic update of blocked load when all idle CPUs have fully decayed. We introduce a new nohz.has_blocked that reflect if some idle CPUs has blocked load that have to be periodiccally updated. nohz.has_blocked is set everytime that a Idle CPU can have blocked load and it is then clear when no more blocked load has been detected during an update. We don't need atomic operation but only to make cure of the right ordering when updating nohz.idle_cpus_mask and nohz.has_blocked. Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> --- kernel/sched/fair.c | 122 ++++++++++++++++++++++++++++++++++++++++++--------- kernel/sched/sched.h | 1 + 2 files changed, 102 insertions(+), 21 deletions(-) -- 2.7.4