linux-kernel - Re: [PATCH V2 Resend 3/4] workqueue: Schedule work on non-idle cpu instead of current one

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Message-ID: <CAKfTPtD6T8oTHFeB82NFDOt6anAL=23vaShsDws0c7812=GkFQ@mail.gmail.com>
Date:	Tue, 27 Nov 2012 15:55:08 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>, pjt@...gle.com,
	paul.mckenney@...aro.org, tglx@...utronix.de, tj@...nel.org,
	suresh.b.siddha@...el.com, venki@...gle.com, mingo@...hat.com,
	peterz@...radead.org, Arvind.Chauhan@....com,
	linaro-dev@...ts.linaro.org, patches@...aro.org,
	pdsw-power-team@....com, linux-kernel@...r.kernel.org,
	linux-rt-users@...r.kernel.org
Subject: Re: [PATCH V2 Resend 3/4] workqueue: Schedule work on non-idle cpu
 instead of current one

On 27 November 2012 14:59, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 2012-11-27 at 19:18 +0530, Viresh Kumar wrote:
>> On 27 November 2012 18:56, Steven Rostedt <rostedt@...dmis.org> wrote:
>> > A couple of things. The sched_select_cpu() is not cheap. It has a double
>> > loop of domains/cpus looking for a non idle cpu. If we have 1024 CPUs,
>> > and we are CPU 1023 and all other CPUs happen to be idle, we could be
>> > searching 1023 CPUs before we come up with our own.
>>
>> Not sure if you missed the first check sched_select_cpu()
>>
>> +int sched_select_cpu(unsigned int sd_flags)
>> +{
>> +       /* If Current cpu isn't idle, don't migrate anything */
>> +       if (!idle_cpu(cpu))
>> +               return cpu;
>>
>> We aren't going to search if we aren't idle.
>
> OK, we are idle, but CPU 1022 isn't. We still need a large search. But,
> heh we are idle we can spin. But then why go through this in the first
> place ;-)

By migrating it now, it will create its activity and wake up on the
right CPU next time.

If migrating on any CPUs seems a bit risky, we could restrict the
migration on a CPU on the same node. We can pass such contraints on
sched_select_cpu

>
>
>>
>> > Also, I really don't like this as a default behavior. It seems that this
>> > solution is for a very special case, and this can become very intrusive
>> > for the normal case.
>>
>> We tried with an KCONFIG option for it, which Tejun rejected.
>
> Yeah, I saw that. I don't like adding KCONFIG options either. Best is to
> get something working that doesn't add any regressions. If you can get
> this to work without making *any* regressions in the normal case than
> I'm totally fine with that. But if this adds any issues with the normal
> case, then it's a show stopper.
>
>>
>> > To be honest, I'm uncomfortable with this approach. It seems to be
>> > fighting a symptom and not the disease. I'd rather find a way to keep
>> > work from being queued on wrong CPU. If it is a timer, find a way to
>> > move the timer. If it is something else, lets work to fix that. Doing
>> > searches of possibly all CPUs (unlikely, but it is there), just seems
>> > wrong to me.
>>
>> As Vincent pointed out, on big LITTLE systems we just don't want to
>> serve works on big cores. That would be wasting too much of power.
>> Specially if we are going to wake up big cores.
>>
>> It would be difficult to control the source driver (which queues work) to
>> little cores. We thought, if somebody wanted to queue work on current
>> cpu then they must use queue_work_on().
>
> As Tejun has mentioned earlier, is there any assumptions anywhere that
> expects an unbounded work queue to not migrate? Where per cpu variables
> might be used. Tejun had a good idea of forcing this to migrate the work
> *every* time. To not let a work queue run on the same CPU that it was
> queued on. If it can survive that, then it is probably OK. Maybe add a
> config option that forces this? That way, anyone can test that this
> isn't an issue.
>
> -- Steve
>
>
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