Is it ok for deferrable timer wakeup the idle cpu?

On 23 January 2014 11:11, Lei Wen <adrian.wenl@gmail.com> wrote:
> On Wed, Jan 22, 2014 at 10:07 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>> On Wed, 22 Jan 2014, Lei Wen wrote:
>>> Recently I want to do the experiment for cpu isolation over 3.10 kernel.
>>> But I find the isolated one is periodically waken up by IPI interrupt.
>>>
>>> By checking the trace, I find those IPI is generated by add_timer_on,
>>> which would calls wake_up_nohz_cpu, and wake up the already idle cpu.
>>>
>>> With further checking, I find this timer is added by on_demand governor of
>>> cpufreq. It would periodically check each cores' state.
>>> The problem I see here is cpufreq_governor using INIT_DEFERRABLE_WORK
>>> as the tool, while timer is made as deferrable anyway.
>>> And what is more that cpufreq checking is very frequent. In my case, the
>>> isolated cpu is wakenup by IPI every 5ms.
>>>
>>> So why kernel need to wake the remote processor when mount the deferrable
>>> timer? As per my understanding, we'd better keep cpu as idle when use
>>> the deferrable timer.
>>
>> Indeed, we can avoid the wakeup of the remote cpu when the timer is
>> deferrable.
>
> Glad to hear that we could fix this unwanted wakeup.
> Do you have related patches already?
>
>>
>> Though you really want to figure out why the cpufreq governor is
>> arming timers on other cores every 5ms. That smells like an utterly
>> stupid approach.
>
> Not sure why cpufreq choose such frequent profiling over each cpu.
> As my understanding, since kernel is smp, launching profiler over one cpu
> would be enough...


Hi Guys,

So the first question is why cpufreq needs it and is it really stupid?
Yes, it is stupid but that's how its implemented since a long time. It does
so to get data about the load on CPUs, so that freq can be scaled up/down.

Though there is a solution in discussion currently, which will take
inputs from scheduler and so these background timers would go away.
But we need to wait until that time.

Now, why do we need that for every cpu, while that for a single cpu might
be enough? The answer is cpuidle here: What if the cpu responsible for
running timer goes to sleep? Who will evaluate the load then? And if we
make this timer run on one cpu in non-deferrable mode then that cpu
would be waken up again and again from idle. So, it was decided to have
a per-cpu deferrable timer. Though to improve efficiency, once it is fired
on any cpu, timer for all other CPUs are rescheduled, so that they don't
fire before 5ms (sampling time)..

I think below diff might get this fixed for you, though I am not sure if it
breaks something else. Probably Thomas/Frederic can answer here.
If this looks fine I will send it formally again:

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On 23 January 2014 19:05, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> On Thu, Jan 23, 2014 at 11:22:32AM +0530, Viresh Kumar wrote:
>> I think below diff might get this fixed for you, though I am not sure if it
>> breaks something else. Probably Thomas/Frederic can answer here.
>> If this looks fine I will send it formally again:
>>
>> diff --git a/kernel/timer.c b/kernel/timer.c
>> index accfd24..3a2c7fa 100644
>> --- a/kernel/timer.c
>> +++ b/kernel/timer.c
>> @@ -940,7 +940,8 @@ void add_timer_on(struct timer_list *timer, int cpu)
>>          * makes sure that a CPU on the way to stop its tick can not
>>          * evaluate the timer wheel.
>>          */
>> -       wake_up_nohz_cpu(cpu);
>> +       if (!tbase_get_deferrable(timer->base))
>> +               wake_up_nohz_cpu(cpu);

Wait, I got the wrong code here. That's wasn't my initial intention.
I actually wanted to write something like this:

 -       wake_up_nohz_cpu(cpu);
 +       if (!tbase_get_deferrable(timer->base) || idle_cpu(cpu))
 +               wake_up_nohz_cpu(cpu);

Will that work?

> So you simply rely on the next tick to see the new timer. This should work with
> CONFIG_NO_HZ_IDLE but not with CONFIG_NO_HZ_FULL since the target may be running
> without the tick.
>
> Basically, in the case of a deferrable timer you need to manage to call
> wake_up_full_nohz_cpu() but not wake_up_idle_cpu().
>
> It should be even possible to spare the IPI in a full dynticks CPU if it is
> running idle. But that's an optional bonus because it require some deep
> care on complicated races against the call to tick_nohz_idle_exit().
>
> I also realize than when we enqueue a timer on a full nohz CPU, we should set_need_resched()
> the target before sending the IPI if it is idle like does wake_up_idle_cpu(). Otherwise the
> IPI will be ignored without exiting the idle loop nor reevaluating the tick on irq exit.
> If you can fix that along the way, that will be much appreciated.

I haven't thought much about this currently as I have limited knowledge of
these routines. Though the problem we were facing wasn't related to
NO_HZ_FULL. It was just about waking up an idle cpu.
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Sorry was away for short vacation.

On 28 January 2014 19:20, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> On Thu, Jan 23, 2014 at 07:50:40PM +0530, Viresh Kumar wrote:
>> Wait, I got the wrong code here. That's wasn't my initial intention.
>> I actually wanted to write something like this:
>>
>>  -       wake_up_nohz_cpu(cpu);
>>  +       if (!tbase_get_deferrable(timer->base) || idle_cpu(cpu))
>>  +               wake_up_nohz_cpu(cpu);
>>
>> Will that work?

Something is seriously wrong with me, again wrote rubbish code.
Let me phrase what I wanted to write :)

"don't send IPI to a idle CPU for a deferrable timer."

Probably I code it correctly this time atleast.

-       wake_up_nohz_cpu(cpu);
+       if (!(tbase_get_deferrable(timer->base) && idle_cpu(cpu)))
+               wake_up_nohz_cpu(cpu);

> Well, this is going to wake up the target from its idle state, which is
> what we want to avoid if the timer is deferrable, right?

Yeah, sorry for doing it for second time :(

> The simplest thing we want is:
>
>            if (!tbase_get_deferrable(timer->base) || tick_nohz_full_cpu(cpu))
>                wake_up_nohz_cpu(cpu);
>
> This spares the IPI for the common case where the timer is deferrable and we run
> in periodic or dynticks-idle mode (which should be 99.99% of the existing workloads).

I wasn't looking at this problem with NO_HZ_FULL in mind. As I thought its
only about if the CPU is idle or not. And so the solution I was
talking about was:

"don't send IPI to a idle CPU for a deferrable timer."

But I see that still failing with the code you wrote. For normal cases where we
don't enable NO_HZ_FULL, we will still end up waking up idle CPUs which
is what Lei Wen reported initially.

Also if a CPU is marked for NO_HZ_FULL and is not idle currently then we
wouldn't send a IPI for a deferrable timer. But we actually need that, so that
we can reevaluate the timers order again?

> Then we can later optimize that and spare the IPI on full dynticks CPUs when they run
> idle, but that require some special care about subtle races which can't be dealt
> with a simple test on "idle_cpu(target)". And power consumption in full dynticks
> is already very suboptimized anyway.
>
> So I suggest we start simple with the above test, and a big fat comment which explains
> what we are doing and what needs to be done in the future.
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On 29 January 2014 10:57, Preeti Murthy <preeti.lkml@gmail.com> wrote:
> How about simplifying this design by doing the below?
>
> 1. Since anyway cpufreq governors monitor load on the cpu once every
> 5ms, *tie it with tick_sched_timer*, which also gets deferred when the cpu
> enters nohz_idle.

Its configurable. We can change sampling time to whatever we want. Some
might be selecting it as 30 ms.

> 2. To overcome the problem of running this job of monitoring the load
> on every cpu, have the *time keeping* cpu do it for you.
>
> The time keeping cpu has the property that if it has to go to idle, it will do
> so and let the next cpu that runs the periodic timer become the time keeper.
> Hence no cpu is prevented from entering nohz_idle and the cpu that is busy
> and first executes periodic timer will take over as the time keeper.
>
> The result would be:
>
> 1. One cpu at any point in time will be monitoring cpu load, at every sched tick
> as long as its busy. If it goes to sleep, then it gives up this duty
> and enters idle.
> The next cpu that runs the periodic timer becomes the cpu to monitor the load
> and will continue to do so as long as its busy. Hence we do not miss monitoring
> the cpu load.
>
> 2. This will avoid an additional timer for cpufreq.
>
> 3. It avoids sending IPIs each time this timer gets modified since there is just
> one CPU doing the monitoring.
>
> 4. The downside to this could be that we are stretching the functions of the
>  periodic timer into the power management domain which does not seem like
> the right thing to do.

Looks good, but AFAIK timerkeeper is still to be implemented? Also the best
solution is to get rid of this timer completely by getting inputs from
scheduler.
Probably some ARM/Linaro folks are working on it.
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On 12 February 2014 20:36, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> The change I'm applying is strongly inspired from the above. Can I use your Signed-off-by?

Sure :)
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