linux-kernel - Re: [PATCH] sched/fair: no sync wakeup from interrupt context

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Message-ID: <Ys/9e52SDRp8Kpnn@worktop.programming.kicks-ass.net>
Date:   Thu, 14 Jul 2022 13:26:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Libo Chen <libo.chen@...cle.com>
Cc:     vincent.guittot@...aro.org, mgorman@...e.de,
        tim.c.chen@...ux.intel.com, 21cnbao@...il.com,
        dietmar.eggemann@....com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de
Subject: Re: [PATCH] sched/fair: no sync wakeup from interrupt context

On Mon, Jul 11, 2022 at 03:47:04PM -0700, Libo Chen wrote:
> Barry Song first pointed out that replacing sync wakeup with regular wakeup
> seems to reduce overeager wakeup pulling and shows noticeable performance
> improvement.[1]
> 
> This patch argues that allowing sync for wakeups from interrupt context
> is a bug 

Yes.

> The
> assumption is built into wake_affine() where it discounts the waker's presence
> from the runqueue when sync is true. The random waker from interrupts bears no
> relation to the wakee and don't usually go to sleep immediately afterwards
> unless wakeup granularity is reached. 

Exactly that.

> Signed-off-by: Libo Chen <libo.chen@...cle.com>
> ---
>  kernel/sched/fair.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 794c2cb945f8..59b210d2cdb5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6704,7 +6704,9 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>  static int
>  select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  {
> -	int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
> +	/* Don't set sync for wakeup from irq/soft ctx */
> +	int sync = in_task() && (wake_flags & WF_SYNC)
> +		   && !(current->flags & PF_EXITING);

That's not a coding style you'll find anywhere near this code though.
I'll fix it up.

>  	struct sched_domain *tmp, *sd = NULL;
>  	int cpu = smp_processor_id();
>  	int new_cpu = prev_cpu;
> --
> 2.31.1
>