linux-kernel - Re: [PATCH] sched, fair: try to prevent migration thread from preempting non-cfs task

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Message-ID: <87mtrrb2jw.mognet@arm.com>
Date:   Tue, 15 Jun 2021 15:55:15 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Yafang Shao <laoar.shao@...il.com>, vincent.guittot@...aro.org,
        mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com
Cc:     linux-kernel@...r.kernel.org, Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH] sched, fair: try to prevent migration thread from preempting non-cfs task


Hi,

On 15/06/21 20:15, Yafang Shao wrote:

> - Prev version
>   https://lore.kernel.org/lkml/CAKfTPtBd349eyDhA5ThCAHFd83cGMQKb_LDxD4QvyP-cJOBjqA@mail.gmail.com/
>
> - Similar discussion
>   https://lore.kernel.org/lkml/CAKfTPtBygNcVewbb0GQOP5xxO96am3YeTZNP5dK9BxKHJJAL-g@mail.gmail.com/

I knew that sounded familiar :-)

> ---
>  kernel/sched/fair.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 3248e24a90b0..597c7a940746 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9797,6 +9797,20 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>                       /* Record that we found at least one task that could run on this_cpu */
>                       env.flags &= ~LBF_ALL_PINNED;
>
> +			/*
> +			 * There may be a race between load balance starting migration
> +			 * thread to pull the cfs running thread and the RT thread
> +			 * waking up and preempting cfs task before migration threads
> +			 * which then preempt the RT thread.
> +			 * We'd better do the last minute check before starting
> +			 * migration thread to avoid preempting latency-sensitive thread.
> +			 */

This can be summarized as in the below, no?

                        /*
                         * Don't cause a higher-than-CFS task to be preempted by
                         * the CPU stopper.
                         */

> +			if (busiest->curr->sched_class != &fair_sched_class) {
> +				raw_spin_unlock_irqrestore(&busiest->lock,
> +							   flags);
> +				goto out;

Since you goto out this could be moved before the

  env.flags &= ~LBF_ALL_PINNED;

above (it only has an impact if you'd goto out_balanced).

> +			}
> +

Other than the above, this looks OK to me.

Back then I had argued that having a >CFS task and holding the remote rq
lock could let us invoke detach_one_task() locally (rather than on the
stopper thread), but realistically if we got to this !ld_moved condition
then the chances of us actually pulling something here are very slim (we'd
depend on enqueues happening between ~detach_tasks() and here).

>                       /*
>                        * ->active_balance synchronizes accesses to
>                        * ->active_balance_work.  Once set, it's cleared
> --
> 2.17.1