| Message ID | 20250410024439.20859-1-sultan@kerneltoast.com |
|---|---|
| State | New |
| Headers | show |
| Series | cpufreq: schedutil: Don't ignore limit changes when util is unchanged | expand |
Hi Sultan, On Thu, Apr 10, 2025 at 10:46 AM Sultan Alsawaf <sultan@kerneltoast.com> wrote: > > From: Sultan Alsawaf <sultan@kerneltoast.com> > > When utilization is unchanged, a policy limits update is ignored unless > CPUFREQ_NEED_UPDATE_LIMITS is set. This occurs because limits_changed > depends on the old broken behavior of need_freq_update to trigger a call > into cpufreq_driver_resolve_freq() to evaluate the changed policy limits. > > After fixing need_freq_update, limit changes are ignored without > CPUFREQ_NEED_UPDATE_LIMITS, at least until utilization changes enough to > make map_util_freq() return something different. > > Fix the ignored limit changes by preserving the value of limits_changed > until get_next_freq() is called, so limits_changed can trigger a call to > cpufreq_driver_resolve_freq(). > > Reported-and-tested-by: Stephan Gerhold <stephan.gerhold@linaro.org> > Link: https://lore.kernel.org/lkml/Z_Tlc6Qs-tYpxWYb@linaro.org > Fixes: 8e461a1cb43d6 ("cpufreq: schedutil: Fix superfluous updates caused by need_freq_update") > Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com> > --- > kernel/sched/cpufreq_schedutil.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c > index 1a19d69b91ed3..f37b999854d52 100644 > --- a/kernel/sched/cpufreq_schedutil.c > +++ b/kernel/sched/cpufreq_schedutil.c > @@ -82,7 +82,6 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time) > return false; > > if (unlikely(sg_policy->limits_changed)) { > - sg_policy->limits_changed = false; > sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS); > return true; > } > @@ -171,9 +170,11 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy, > freq = get_capacity_ref_freq(policy); > freq = map_util_freq(util, freq, max); > > - if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update) > + if (freq == sg_policy->cached_raw_freq && !sg_policy->limits_changed && > + !sg_policy->need_freq_update) As said in:https://lore.kernel.org/all/CAB8ipk8ARRdR8UPgLqJ3EcAzuE4KNEO=cmLNJLk6thTxdBSHWw@mail.gmail.com/ We also should add the limits_changed in the sugov_update_single_freq(). Thanks！ > return sg_policy->next_freq; > > + sg_policy->limits_changed = false; > sg_policy->cached_raw_freq = freq; > return cpufreq_driver_resolve_freq(policy, freq); > } > -- > 2.49.0 > >
On Thu, Apr 10, 2025 at 4:45 AM Sultan Alsawaf <sultan@kerneltoast.com> wrote: > > From: Sultan Alsawaf <sultan@kerneltoast.com> > > When utilization is unchanged, a policy limits update is ignored unless > CPUFREQ_NEED_UPDATE_LIMITS is set. This occurs because limits_changed > depends on the old broken behavior of need_freq_update to trigger a call > into cpufreq_driver_resolve_freq() to evaluate the changed policy limits. > > After fixing need_freq_update, limit changes are ignored without > CPUFREQ_NEED_UPDATE_LIMITS, at least until utilization changes enough to > make map_util_freq() return something different. > > Fix the ignored limit changes by preserving the value of limits_changed > until get_next_freq() is called, so limits_changed can trigger a call to > cpufreq_driver_resolve_freq(). > > Reported-and-tested-by: Stephan Gerhold <stephan.gerhold@linaro.org> > Link: https://lore.kernel.org/lkml/Z_Tlc6Qs-tYpxWYb@linaro.org > Fixes: 8e461a1cb43d6 ("cpufreq: schedutil: Fix superfluous updates caused by need_freq_update") > Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com> > --- > kernel/sched/cpufreq_schedutil.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c > index 1a19d69b91ed3..f37b999854d52 100644 > --- a/kernel/sched/cpufreq_schedutil.c > +++ b/kernel/sched/cpufreq_schedutil.c > @@ -82,7 +82,6 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time) > return false; > > if (unlikely(sg_policy->limits_changed)) { > - sg_policy->limits_changed = false; > sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS); > return true; > } > @@ -171,9 +170,11 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy, > freq = get_capacity_ref_freq(policy); > freq = map_util_freq(util, freq, max); > > - if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update) > + if (freq == sg_policy->cached_raw_freq && !sg_policy->limits_changed && > + !sg_policy->need_freq_update) > return sg_policy->next_freq; > > + sg_policy->limits_changed = false; AFAICS, after this code modification, a limit change may be missed due to a possible race with sugov_limits() which cannot happen if sg_policy->limits_changed is only cleared when it is set before updating sg_policy->need_freq_update. > sg_policy->cached_raw_freq = freq; > return cpufreq_driver_resolve_freq(policy, freq); > } > -- > 2.49.0 >
On Thu, Apr 10, 2025 at 05:34:39PM +0200, Rafael J. Wysocki wrote: > On Thu, Apr 10, 2025 at 4:45 AM Sultan Alsawaf <sultan@kerneltoast.com> wrote: > > > > From: Sultan Alsawaf <sultan@kerneltoast.com> > > > > When utilization is unchanged, a policy limits update is ignored unless > > CPUFREQ_NEED_UPDATE_LIMITS is set. This occurs because limits_changed > > depends on the old broken behavior of need_freq_update to trigger a call > > into cpufreq_driver_resolve_freq() to evaluate the changed policy limits. > > > > After fixing need_freq_update, limit changes are ignored without > > CPUFREQ_NEED_UPDATE_LIMITS, at least until utilization changes enough to > > make map_util_freq() return something different. > > > > Fix the ignored limit changes by preserving the value of limits_changed > > until get_next_freq() is called, so limits_changed can trigger a call to > > cpufreq_driver_resolve_freq(). > > > > Reported-and-tested-by: Stephan Gerhold <stephan.gerhold@linaro.org> > > Link: https://lore.kernel.org/lkml/Z_Tlc6Qs-tYpxWYb@linaro.org > > Fixes: 8e461a1cb43d6 ("cpufreq: schedutil: Fix superfluous updates caused by need_freq_update") > > Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com> > > --- > > kernel/sched/cpufreq_schedutil.c | 5 +++-- > > 1 file changed, 3 insertions(+), 2 deletions(-) > > > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c > > index 1a19d69b91ed3..f37b999854d52 100644 > > --- a/kernel/sched/cpufreq_schedutil.c > > +++ b/kernel/sched/cpufreq_schedutil.c > > @@ -82,7 +82,6 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time) > > return false; > > > > if (unlikely(sg_policy->limits_changed)) { > > - sg_policy->limits_changed = false; > > sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS); > > return true; > > } > > @@ -171,9 +170,11 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy, > > freq = get_capacity_ref_freq(policy); > > freq = map_util_freq(util, freq, max); > > > > - if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update) > > + if (freq == sg_policy->cached_raw_freq && !sg_policy->limits_changed && > > + !sg_policy->need_freq_update) > > return sg_policy->next_freq; > > > > + sg_policy->limits_changed = false; > > AFAICS, after this code modification, a limit change may be missed due > to a possible race with sugov_limits() which cannot happen if > sg_policy->limits_changed is only cleared when it is set before > updating sg_policy->need_freq_update. I don't think that's the case because sg_policy->limits_changed is cleared before the new policy limits are evaluated in cpufreq_driver_resolve_freq(). Granted, if we wanted to be really certain of this, we'd need release semantics. Looking closer at cpufreq.c actually, isn't there already a race on the updated policy limits (policy->min and policy->max) since they can be updated again while schedutil reads them via cpufreq_driver_resolve_freq()? Sultan
On Thu, Apr 10, 2025 at 08:47:38PM +0200, Rafael J. Wysocki wrote: > On Thu, Apr 10, 2025 at 6:03 PM Sultan Alsawaf <sultan@kerneltoast.com> wrote: > > > > On Thu, Apr 10, 2025 at 05:34:39PM +0200, Rafael J. Wysocki wrote: > > > On Thu, Apr 10, 2025 at 4:45 AM Sultan Alsawaf <sultan@kerneltoast.com> wrote: > > > > > > > > From: Sultan Alsawaf <sultan@kerneltoast.com> > > > > > > > > When utilization is unchanged, a policy limits update is ignored unless > > > > CPUFREQ_NEED_UPDATE_LIMITS is set. This occurs because limits_changed > > > > depends on the old broken behavior of need_freq_update to trigger a call > > > > into cpufreq_driver_resolve_freq() to evaluate the changed policy limits. > > > > > > > > After fixing need_freq_update, limit changes are ignored without > > > > CPUFREQ_NEED_UPDATE_LIMITS, at least until utilization changes enough to > > > > make map_util_freq() return something different. > > > > > > > > Fix the ignored limit changes by preserving the value of limits_changed > > > > until get_next_freq() is called, so limits_changed can trigger a call to > > > > cpufreq_driver_resolve_freq(). > > > > > > > > Reported-and-tested-by: Stephan Gerhold <stephan.gerhold@linaro.org> > > > > Link: https://lore.kernel.org/lkml/Z_Tlc6Qs-tYpxWYb@linaro.org > > > > Fixes: 8e461a1cb43d6 ("cpufreq: schedutil: Fix superfluous updates caused by need_freq_update") > > > > Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com> > > > > --- > > > > kernel/sched/cpufreq_schedutil.c | 5 +++-- > > > > 1 file changed, 3 insertions(+), 2 deletions(-) > > > > > > > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c > > > > index 1a19d69b91ed3..f37b999854d52 100644 > > > > --- a/kernel/sched/cpufreq_schedutil.c > > > > +++ b/kernel/sched/cpufreq_schedutil.c > > > > @@ -82,7 +82,6 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time) > > > > return false; > > > > > > > > if (unlikely(sg_policy->limits_changed)) { > > > > - sg_policy->limits_changed = false; > > > > sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS); > > > > return true; > > > > } > > > > @@ -171,9 +170,11 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy, > > > > freq = get_capacity_ref_freq(policy); > > > > freq = map_util_freq(util, freq, max); > > > > > > > > - if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update) > > > > + if (freq == sg_policy->cached_raw_freq && !sg_policy->limits_changed && > > > > + !sg_policy->need_freq_update) > > > > return sg_policy->next_freq; > > > > > > > > + sg_policy->limits_changed = false; > > > > > > AFAICS, after this code modification, a limit change may be missed due > > > to a possible race with sugov_limits() which cannot happen if > > > sg_policy->limits_changed is only cleared when it is set before > > > updating sg_policy->need_freq_update. > > > > I don't think that's the case because sg_policy->limits_changed is cleared > > before the new policy limits are evaluated in cpufreq_driver_resolve_freq(). > > sugov_limits() may be triggered by a scaling_max_freq update, for > instance, so it is asynchronous with respect to the usual governor > flow. It updates sg_policy->limits_changed and assumes that next time > the governor runs, it will call into the driver, for example via > cpufreq_driver_fast_switch(), so the new limits take effect. This is > not about cpufreq_driver_resolve_freq(). OK, though I think there's still a race in cpufreq_driver_resolve_freq(). > sugov_limits() runs after the driver's ->verify() callback has > returned and it is conditional on that callback's return value, so the > driver already knows the new limits when sugov_limits() runs, but it > may still need to tell the hardware what the new limits are and that's > why cpufreq_driver_fast_switch() may need to run. Which is why CPUFREQ_NEED_UPDATE_LIMITS exists, right. > Now, if sugov_should_update_freq() sees sg_policy->limits_changed set, > it will set sg_policy->need_freq_update which (for drivers with > CPUFREQ_NEED_UPDATE_LIMITS set) guarantees that the driver will be > invoked and so sg_policy->limits_changed can be cleared. > > If a new instance of sugov_limits() runs at this point, there are two > possibilities. Either it completes before the > sg_policy->limits_changed update in sugov_should_update_freq(), in > which case the driver already knows the new limits as per the above > and so the subsequent invocation of cpufreq_driver_fast_switch() will > pick them up, or it sets sg_policy->limits_changed again and the > governor will see it set next time it runs. In both cases the new > limits will be picked up unless they are changed again in the > meantime. > > After the above change, sg_policy->limits_changed may be cleared even > if it has not been set before and that's problematic. Namely, say it > is unset when sugov_should_update_freq() runs, after being called by > sugov_update_single_freq() via sugov_update_single_common(), and > returns 'true' without setting sg_policy->need_freq_update. Next, > sugov_update_single_common() returns 'true' and get_next_freq() is > called. It sees that freq != sg_policy->cached_raw_freq, so it clears > sg_policy->limits_changed. If sugov_limits() runs on a different CPU > between the check and the sg_policy->limits_changed update in > get_next_freq(), it may be missed and it is still not guaranteed that > cpufreq_driver_fast_switch() will run because > sg_policy->need_freq_update is unset and sugov_hold_freq() may return > 'true'. > > For this to work, sg_policy->limits_changed needs to be cleared only > when it is set and sg_policy->need_freq_update needs to be updated > when sg_policy->limits_changed is cleared. Ah I see, thank you for the detailed explanation. So if I am understanding it correctly: the problem with my patch is that CPUFREQ_NEED_UPDATE_LIMITS might not be honored after a limits change, because CPUFREQ_NEED_UPDATE_LIMITS is only honored when sg_policy->limits_changed is set. So there is the following race: CPU-A CPU-B sugov_should_update_freq() // sg_policy->limits_changed == false, sg_policy->need_freq_update == false sugov_limits() // sg_policy->limits_changed == true, sg_policy->need_freq_update == false get_next_freq() // sg_policy->limits_changed == false, sg_policy->need_freq_update == false // cpufreq driver won't be invoked for the limits change if: // next_f == sg_policy->next_freq || (sugov_hold_freq() == true && next_f < sg_policy->next_freq) Does that look right? > It looks like you really want to set sg_policy->need_freq_update to > 'true' in sugov_should_update_freq() when sg_policy->limits_changed is > set, but that would render CPUFREQ_NEED_UPDATE_LIMITS unnecessary. > > > Granted, if we wanted to be really certain of this, we'd need release semantics. > > I don't think so, but feel free to prove me wrong. Well, it appears that there really is synchronization missing between cpufreq_set_policy() and schedutil, since cpufreq_set_policy() changes the live policy->min and policy->max, and schedutil may observe either the old values in there or garbage values due to load/store tearing. Sultan
On Thu, Apr 10, 2025 at 9:26 PM Sultan Alsawaf <sultan@kerneltoast.com> wrote: > > On Thu, Apr 10, 2025 at 08:47:38PM +0200, Rafael J. Wysocki wrote: > > On Thu, Apr 10, 2025 at 6:03 PM Sultan Alsawaf <sultan@kerneltoast.com> wrote: > > > > > > On Thu, Apr 10, 2025 at 05:34:39PM +0200, Rafael J. Wysocki wrote: > > > > On Thu, Apr 10, 2025 at 4:45 AM Sultan Alsawaf <sultan@kerneltoast.com> wrote: > > > > > > > > > > From: Sultan Alsawaf <sultan@kerneltoast.com> > > > > > > > > > > When utilization is unchanged, a policy limits update is ignored unless > > > > > CPUFREQ_NEED_UPDATE_LIMITS is set. This occurs because limits_changed > > > > > depends on the old broken behavior of need_freq_update to trigger a call > > > > > into cpufreq_driver_resolve_freq() to evaluate the changed policy limits. > > > > > > > > > > After fixing need_freq_update, limit changes are ignored without > > > > > CPUFREQ_NEED_UPDATE_LIMITS, at least until utilization changes enough to > > > > > make map_util_freq() return something different. > > > > > > > > > > Fix the ignored limit changes by preserving the value of limits_changed > > > > > until get_next_freq() is called, so limits_changed can trigger a call to > > > > > cpufreq_driver_resolve_freq(). > > > > > > > > > > Reported-and-tested-by: Stephan Gerhold <stephan.gerhold@linaro.org> > > > > > Link: https://lore.kernel.org/lkml/Z_Tlc6Qs-tYpxWYb@linaro.org > > > > > Fixes: 8e461a1cb43d6 ("cpufreq: schedutil: Fix superfluous updates caused by need_freq_update") > > > > > Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com> > > > > > --- > > > > > kernel/sched/cpufreq_schedutil.c | 5 +++-- > > > > > 1 file changed, 3 insertions(+), 2 deletions(-) > > > > > > > > > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c > > > > > index 1a19d69b91ed3..f37b999854d52 100644 > > > > > --- a/kernel/sched/cpufreq_schedutil.c > > > > > +++ b/kernel/sched/cpufreq_schedutil.c > > > > > @@ -82,7 +82,6 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time) > > > > > return false; > > > > > > > > > > if (unlikely(sg_policy->limits_changed)) { > > > > > - sg_policy->limits_changed = false; > > > > > sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS); > > > > > return true; > > > > > } > > > > > @@ -171,9 +170,11 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy, > > > > > freq = get_capacity_ref_freq(policy); > > > > > freq = map_util_freq(util, freq, max); > > > > > > > > > > - if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update) > > > > > + if (freq == sg_policy->cached_raw_freq && !sg_policy->limits_changed && > > > > > + !sg_policy->need_freq_update) > > > > > return sg_policy->next_freq; > > > > > > > > > > + sg_policy->limits_changed = false; > > > > > > > > AFAICS, after this code modification, a limit change may be missed due > > > > to a possible race with sugov_limits() which cannot happen if > > > > sg_policy->limits_changed is only cleared when it is set before > > > > updating sg_policy->need_freq_update. > > > > > > I don't think that's the case because sg_policy->limits_changed is cleared > > > before the new policy limits are evaluated in cpufreq_driver_resolve_freq(). > > > > sugov_limits() may be triggered by a scaling_max_freq update, for > > instance, so it is asynchronous with respect to the usual governor > > flow. It updates sg_policy->limits_changed and assumes that next time > > the governor runs, it will call into the driver, for example via > > cpufreq_driver_fast_switch(), so the new limits take effect. This is > > not about cpufreq_driver_resolve_freq(). > > OK, though I think there's still a race in cpufreq_driver_resolve_freq(). > > > sugov_limits() runs after the driver's ->verify() callback has > > returned and it is conditional on that callback's return value, so the > > driver already knows the new limits when sugov_limits() runs, but it > > may still need to tell the hardware what the new limits are and that's > > why cpufreq_driver_fast_switch() may need to run. > > Which is why CPUFREQ_NEED_UPDATE_LIMITS exists, right. > > > Now, if sugov_should_update_freq() sees sg_policy->limits_changed set, > > it will set sg_policy->need_freq_update which (for drivers with > > CPUFREQ_NEED_UPDATE_LIMITS set) guarantees that the driver will be > > invoked and so sg_policy->limits_changed can be cleared. > > > > If a new instance of sugov_limits() runs at this point, there are two > > possibilities. Either it completes before the > > sg_policy->limits_changed update in sugov_should_update_freq(), in > > which case the driver already knows the new limits as per the above > > and so the subsequent invocation of cpufreq_driver_fast_switch() will > > pick them up, or it sets sg_policy->limits_changed again and the > > governor will see it set next time it runs. In both cases the new > > limits will be picked up unless they are changed again in the > > meantime. > > > > After the above change, sg_policy->limits_changed may be cleared even > > if it has not been set before and that's problematic. Namely, say it > > is unset when sugov_should_update_freq() runs, after being called by > > sugov_update_single_freq() via sugov_update_single_common(), and > > returns 'true' without setting sg_policy->need_freq_update. Next, > > sugov_update_single_common() returns 'true' and get_next_freq() is > > called. It sees that freq != sg_policy->cached_raw_freq, so it clears > > sg_policy->limits_changed. If sugov_limits() runs on a different CPU > > between the check and the sg_policy->limits_changed update in > > get_next_freq(), it may be missed and it is still not guaranteed that > > cpufreq_driver_fast_switch() will run because > > sg_policy->need_freq_update is unset and sugov_hold_freq() may return > > 'true'. > > > > For this to work, sg_policy->limits_changed needs to be cleared only > > when it is set and sg_policy->need_freq_update needs to be updated > > when sg_policy->limits_changed is cleared. > > Ah I see, thank you for the detailed explanation. So if I am understanding it > correctly: the problem with my patch is that CPUFREQ_NEED_UPDATE_LIMITS might > not be honored after a limits change, because CPUFREQ_NEED_UPDATE_LIMITS is only > honored when sg_policy->limits_changed is set. So there is the following race: > > CPU-A CPU-B > sugov_should_update_freq() // sg_policy->limits_changed == false, sg_policy->need_freq_update == false > sugov_limits() // sg_policy->limits_changed == true, sg_policy->need_freq_update == false > get_next_freq() // sg_policy->limits_changed == false, sg_policy->need_freq_update == false > > // cpufreq driver won't be invoked for the limits change if: > // next_f == sg_policy->next_freq || (sugov_hold_freq() == true && next_f < sg_policy->next_freq) > > Does that look right? Yes, it does. > > It looks like you really want to set sg_policy->need_freq_update to > > 'true' in sugov_should_update_freq() when sg_policy->limits_changed is > > set, but that would render CPUFREQ_NEED_UPDATE_LIMITS unnecessary. > > > > > Granted, if we wanted to be really certain of this, we'd need release semantics. > > > > I don't think so, but feel free to prove me wrong. > > Well, it appears that there really is synchronization missing between > cpufreq_set_policy() and schedutil, since cpufreq_set_policy() changes the live > policy->min and policy->max, and schedutil may observe either the old values in > there or garbage values due to load/store tearing. schedutil itself doesn't really read policy->min and policy->max. cpufreq_driver_resolve_freq() does this and drivers do, but since policy->min and policy->max are integers, no garbage values will be observed AFAICS. Stale values can be observed, but in that case the new values will be seen next time and limits_changed enforces picking them up. Of course, if the limits change sufficiently often, schedutil will fall behind, but then there is just too much noise anyway. WRITE_ONCE() and READ_ONCE() could be used there, though, because the compiler can do some damage in principle.
On Thu, Apr 10, 2025 at 9:50 PM Rafael J. Wysocki <rafael@kernel.org> wrote: > > On Thu, Apr 10, 2025 at 9:26 PM Sultan Alsawaf <sultan@kerneltoast.com> wrote: > > > > On Thu, Apr 10, 2025 at 08:47:38PM +0200, Rafael J. Wysocki wrote: > > > On Thu, Apr 10, 2025 at 6:03 PM Sultan Alsawaf <sultan@kerneltoast.com> wrote: > > > > > > > > On Thu, Apr 10, 2025 at 05:34:39PM +0200, Rafael J. Wysocki wrote: > > > > > On Thu, Apr 10, 2025 at 4:45 AM Sultan Alsawaf <sultan@kerneltoast.com> wrote: > > > > > > > > > > > > From: Sultan Alsawaf <sultan@kerneltoast.com> > > > > > > > > > > > > When utilization is unchanged, a policy limits update is ignored unless > > > > > > CPUFREQ_NEED_UPDATE_LIMITS is set. This occurs because limits_changed > > > > > > depends on the old broken behavior of need_freq_update to trigger a call > > > > > > into cpufreq_driver_resolve_freq() to evaluate the changed policy limits. > > > > > > > > > > > > After fixing need_freq_update, limit changes are ignored without > > > > > > CPUFREQ_NEED_UPDATE_LIMITS, at least until utilization changes enough to > > > > > > make map_util_freq() return something different. > > > > > > > > > > > > Fix the ignored limit changes by preserving the value of limits_changed > > > > > > until get_next_freq() is called, so limits_changed can trigger a call to > > > > > > cpufreq_driver_resolve_freq(). > > > > > > > > > > > > Reported-and-tested-by: Stephan Gerhold <stephan.gerhold@linaro.org> > > > > > > Link: https://lore.kernel.org/lkml/Z_Tlc6Qs-tYpxWYb@linaro.org > > > > > > Fixes: 8e461a1cb43d6 ("cpufreq: schedutil: Fix superfluous updates caused by need_freq_update") > > > > > > Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com> > > > > > > --- > > > > > > kernel/sched/cpufreq_schedutil.c | 5 +++-- > > > > > > 1 file changed, 3 insertions(+), 2 deletions(-) > > > > > > > > > > > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c > > > > > > index 1a19d69b91ed3..f37b999854d52 100644 > > > > > > --- a/kernel/sched/cpufreq_schedutil.c > > > > > > +++ b/kernel/sched/cpufreq_schedutil.c > > > > > > @@ -82,7 +82,6 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time) > > > > > > return false; > > > > > > > > > > > > if (unlikely(sg_policy->limits_changed)) { > > > > > > - sg_policy->limits_changed = false; > > > > > > sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS); > > > > > > return true; > > > > > > } > > > > > > @@ -171,9 +170,11 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy, > > > > > > freq = get_capacity_ref_freq(policy); > > > > > > freq = map_util_freq(util, freq, max); > > > > > > > > > > > > - if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update) > > > > > > + if (freq == sg_policy->cached_raw_freq && !sg_policy->limits_changed && > > > > > > + !sg_policy->need_freq_update) > > > > > > return sg_policy->next_freq; > > > > > > > > > > > > + sg_policy->limits_changed = false; > > > > > > > > > > AFAICS, after this code modification, a limit change may be missed due > > > > > to a possible race with sugov_limits() which cannot happen if > > > > > sg_policy->limits_changed is only cleared when it is set before > > > > > updating sg_policy->need_freq_update. > > > > > > > > I don't think that's the case because sg_policy->limits_changed is cleared > > > > before the new policy limits are evaluated in cpufreq_driver_resolve_freq(). > > > > > > sugov_limits() may be triggered by a scaling_max_freq update, for > > > instance, so it is asynchronous with respect to the usual governor > > > flow. It updates sg_policy->limits_changed and assumes that next time > > > the governor runs, it will call into the driver, for example via > > > cpufreq_driver_fast_switch(), so the new limits take effect. This is > > > not about cpufreq_driver_resolve_freq(). > > > > OK, though I think there's still a race in cpufreq_driver_resolve_freq(). > > > > > sugov_limits() runs after the driver's ->verify() callback has > > > returned and it is conditional on that callback's return value, so the > > > driver already knows the new limits when sugov_limits() runs, but it > > > may still need to tell the hardware what the new limits are and that's > > > why cpufreq_driver_fast_switch() may need to run. > > > > Which is why CPUFREQ_NEED_UPDATE_LIMITS exists, right. > > > > > Now, if sugov_should_update_freq() sees sg_policy->limits_changed set, > > > it will set sg_policy->need_freq_update which (for drivers with > > > CPUFREQ_NEED_UPDATE_LIMITS set) guarantees that the driver will be > > > invoked and so sg_policy->limits_changed can be cleared. > > > > > > If a new instance of sugov_limits() runs at this point, there are two > > > possibilities. Either it completes before the > > > sg_policy->limits_changed update in sugov_should_update_freq(), in > > > which case the driver already knows the new limits as per the above > > > and so the subsequent invocation of cpufreq_driver_fast_switch() will > > > pick them up, or it sets sg_policy->limits_changed again and the > > > governor will see it set next time it runs. In both cases the new > > > limits will be picked up unless they are changed again in the > > > meantime. > > > > > > After the above change, sg_policy->limits_changed may be cleared even > > > if it has not been set before and that's problematic. Namely, say it > > > is unset when sugov_should_update_freq() runs, after being called by > > > sugov_update_single_freq() via sugov_update_single_common(), and > > > returns 'true' without setting sg_policy->need_freq_update. Next, > > > sugov_update_single_common() returns 'true' and get_next_freq() is > > > called. It sees that freq != sg_policy->cached_raw_freq, so it clears > > > sg_policy->limits_changed. If sugov_limits() runs on a different CPU > > > between the check and the sg_policy->limits_changed update in > > > get_next_freq(), it may be missed and it is still not guaranteed that > > > cpufreq_driver_fast_switch() will run because > > > sg_policy->need_freq_update is unset and sugov_hold_freq() may return > > > 'true'. > > > > > > For this to work, sg_policy->limits_changed needs to be cleared only > > > when it is set and sg_policy->need_freq_update needs to be updated > > > when sg_policy->limits_changed is cleared. > > > > Ah I see, thank you for the detailed explanation. So if I am understanding it > > correctly: the problem with my patch is that CPUFREQ_NEED_UPDATE_LIMITS might > > not be honored after a limits change, because CPUFREQ_NEED_UPDATE_LIMITS is only > > honored when sg_policy->limits_changed is set. So there is the following race: > > > > CPU-A CPU-B > > sugov_should_update_freq() // sg_policy->limits_changed == false, sg_policy->need_freq_update == false > > sugov_limits() // sg_policy->limits_changed == true, sg_policy->need_freq_update == false > > get_next_freq() // sg_policy->limits_changed == false, sg_policy->need_freq_update == false > > > > // cpufreq driver won't be invoked for the limits change if: > > // next_f == sg_policy->next_freq || (sugov_hold_freq() == true && next_f < sg_policy->next_freq) > > > > Does that look right? > > Yes, it does. > > > > It looks like you really want to set sg_policy->need_freq_update to > > > 'true' in sugov_should_update_freq() when sg_policy->limits_changed is > > > set, but that would render CPUFREQ_NEED_UPDATE_LIMITS unnecessary. > > > > > > > Granted, if we wanted to be really certain of this, we'd need release semantics. > > > > > > I don't think so, but feel free to prove me wrong. > > > > Well, it appears that there really is synchronization missing between > > cpufreq_set_policy() and schedutil, since cpufreq_set_policy() changes the live > > policy->min and policy->max, and schedutil may observe either the old values in > > there or garbage values due to load/store tearing. > > schedutil itself doesn't really read policy->min and policy->max. > cpufreq_driver_resolve_freq() does this and drivers do, but since > policy->min and policy->max are integers, no garbage values will be > observed AFAICS. Well, I've just realized that this is not exactly the case because of the way in which __resolve_freq() is used in cpufreq_set_policy(). That needs to be fixed.
... > > AFAICS, after this code modification, a limit change may be missed due > to a possible race with sugov_limits() which cannot happen if > sg_policy->limits_changed is only cleared when it is set before > updating sg_policy->need_freq_update. > could the following patch prevent the race? https://lore.kernel.org/all/CAB8ipk_Ayqmh=Ch2aH2c+i-q+qdiQ317VBH1kOHYN=R9dt6LOw@mail.gmail.com/ Thanks! Regards
On Fri, Apr 11, 2025 at 10:22 AM Xuewen Yan <xuewen.yan94@gmail.com> wrote: > > ... > > > > AFAICS, after this code modification, a limit change may be missed due > > to a possible race with sugov_limits() which cannot happen if > > sg_policy->limits_changed is only cleared when it is set before > > updating sg_policy->need_freq_update. > > > could the following patch prevent the race? > > https://lore.kernel.org/all/CAB8ipk_Ayqmh=Ch2aH2c+i-q+qdiQ317VBH1kOHYN=R9dt6LOw@mail.gmail.com/ The first hunk is essentially a partial revert of the problematic commit, but I'm not sure what you want to achieve with the second one.
diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c index 1a19d69b91ed3..f37b999854d52 100644 --- a/kernel/sched/cpufreq_schedutil.c +++ b/kernel/sched/cpufreq_schedutil.c @@ -82,7 +82,6 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time) return false; if (unlikely(sg_policy->limits_changed)) { - sg_policy->limits_changed = false; sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS); return true; } @@ -171,9 +170,11 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy, freq = get_capacity_ref_freq(policy); freq = map_util_freq(util, freq, max); - if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update) + if (freq == sg_policy->cached_raw_freq && !sg_policy->limits_changed && + !sg_policy->need_freq_update) return sg_policy->next_freq; + sg_policy->limits_changed = false; sg_policy->cached_raw_freq = freq; return cpufreq_driver_resolve_freq(policy, freq); }