linux-kernel - Re: [PATCH] sched/core: fix pick_next

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Date:   Wed, 18 Aug 2021 12:35:13 +0800
From:   Tao Zhou <tao.zhou@...ux.dev>
To:     Josh Don <joshdon@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Vineeth Pillai <vineethrp@...il.com>,
        linux-kernel@...r.kernel.org, tao.zhou@...ux.dev
Subject: Re: [PATCH] sched/core: fix pick_next_task 'max' tracking

Hi Josh,

On Tue, Aug 17, 2021 at 05:56:15PM -0700, Josh Don wrote:
> For core-sched, pick_next_task will update the 'max' task if there is a
> cookie mismatch (since in this case the new task must be of higher
> priority than the current max). However, we fail to update 'max' if
> we've found a task with a matching cookie and higher priority than
> 'max'.
> 
> This can result in extra iterations on SMT-X machines, where X > 2.
> 
> As an example, on a machine with SMT=3, on core 0, SMT-0 might pick
> the following, in order:
> 
> - SMT-0: p1, with cookie A, and priority 10 (max = p1)
> - SMT-1: p2, with cookie A, and priority 30 (max not updated here)

Thanks for your illustration. Good catch.
The guilty is 'cookie_equals(class_pick, cookie))' condition in pick_task()

> - SMT-2: p3, with cookie B, and priority 20 (max = p2)
> 	> invalidate the other picks and retry
> 
> Here, we should have instead updated 'max' when picking for SMT-1. Note
> that this code would eventually have righted itself, since the retry
> loop would re-pick p2, and update 'max' accordingly. However, this patch
> avoids the extra round-trip.

This is correct then may increase the chance to retry. That means it is
more possible to filter the max first(not sure).

> Signed-off-by: Josh Don <joshdon@...gle.com>
> ---
>  kernel/sched/core.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3431939699dc..110ea7582a33 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5623,6 +5623,8 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  					occ = 1;
>  					goto again;
>  				}
> +			} else if (prio_less(max, p, fi_before)) {
> +				max = p;
>  			}
>  		}
>  	}
> -- 
> 2.33.0.rc1.237.g0d66db33f3-goog
> 


Thanks,
Tao