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

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Date:	Tue, 25 Sep 2012 13:38:49 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.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,
	robin.randhawa@....com, Steve.Bannister@....com,
	Arvind.Chauhan@....com, amit.kucheria@...aro.org,
	linaro-dev@...ts.linaro.org, patches@...aro.org
Subject: Re: [PATCH 3/3] workqueue: Schedule work on non-idle cpu instead of
 current one

On 25 September 2012 13:30, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 25 September 2012 16:52, Peter Zijlstra <peterz@...radead.org> wrote:
>> On Tue, 2012-09-25 at 16:06 +0530, Viresh Kumar wrote:
>>> @@ -1066,8 +1076,9 @@ int queue_work(struct workqueue_struct *wq,
>>> struct work_struct *work)
>>>  {
>>>         int ret;
>>>
>>> -       ret = queue_work_on(get_cpu(), wq, work);
>>> -       put_cpu();
>>> +       preempt_disable();
>>> +       ret = queue_work_on(wq_select_cpu(), wq, work);
>>> +       preempt_enable();
>>>
>>>         return ret;
>>>  }
>>
>> Right, so the problem I see here is that wq_select_cpu() is horridly
>> expensive..
>
> But this is what the initial idea during LPC we had. Any improvements here
> you can suggest?

The main outcome of the LPC was that we should be able to select
another CPU than the local one.
Using the same policy than timer, is a 1st step to consolidate
interface. A next step should be to update the policy of the function

Vincent
>
>>> @@ -1102,7 +1113,7 @@ static void delayed_work_timer_fn(unsigned long
>>> __data)
>>>         struct delayed_work *dwork = (struct delayed_work *)__data;
>>>         struct cpu_workqueue_struct *cwq = get_work_cwq(&dwork->work);
>>>
>>> -       __queue_work(smp_processor_id(), cwq->wq, &dwork->work);
>>> +       __queue_work(wq_select_cpu(), cwq->wq, &dwork->work);
>>>  }
>>
>> Shouldn't timer migration have sorted this one?
>
> Maybe yes. Will investigate more on it.
>
> Thanks for your early feedback.
>
> --
> viresh
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