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

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Date:	Fri, 28 Sep 2012 11:33:23 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	pjt@...gle.com, paul.mckenney@...aro.org, tglx@...utronix.de,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	suresh.b.siddha@...el.com, venki@...gle.com, mingo@...hat.com,
	peterz@...radead.org, robin.randhawa@....com,
	Steve.Bannister@....com, Arvind.Chauhan@....com,
	amit.kucheria@...aro.org, vincent.guittot@...aro.org,
	linaro-dev@...ts.linaro.org, patches@...aro.org,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH V2 3/3] workqueue: Schedule work on non-idle cpu instead
 of current one

On 27 September 2012 14:34, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> Workqueues queues work on current cpu, if the caller haven't passed a preferred
> cpu. This may wake up an idle CPU, which is actually not required.
>
> This work can be processed by any CPU and so we must select a non-idle CPU here.
> This patch adds in support in workqueue framework to get preferred CPU details
> from the scheduler, instead of using current CPU.
>
> Most of the time when a work is queued, the current cpu isn't idle and so we
> will choose it only. There are cases when a cpu is idle when it queues some
> work. For example, consider following scenario:
> - A cpu has programmed a timer and is IDLE now.
> - CPU gets into interrupt handler due to timer and queues a work. As the CPU is
>   currently IDLE, we can queue this work to some other CPU.
>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
>  kernel/workqueue.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 143fd8c..5fa4ba4 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1238,7 +1238,7 @@ static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,
>                 struct global_cwq *last_gcwq;
>
>                 if (cpu == WORK_CPU_UNBOUND)
> -                       cpu = raw_smp_processor_id();
> +                       cpu = sched_select_cpu(0);

Hi Tejun,

Following change is also required with this patch:


--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1383,7 +1383,7 @@ static void __queue_delayed_work(int cpu, struct
workqueue_struct *wq,
                if (gcwq)
                        lcpu = gcwq->cpu;
                if (lcpu == WORK_CPU_UNBOUND)
-                       lcpu = raw_smp_processor_id();
+                       lcpu = sched_select_cpu(0);
        } else {
                lcpu = WORK_CPU_UNBOUND;
        }

Otherwise, we will end up running the re-reentrancy check, which we are trying
to skip for delayed work.

--
viresh
--
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