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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Mon, 23 Aug 2021 12:09:32 +0200
Message-ID: <CAKfTPtCmhtPWcFYo+ArNBDGOvmgfLJrNm+QbqGEMsQnpj0TJSA@mail.gmail.com>
Subject: Re: [PATCH v3 4/4] sched: adjust sleeper credit for SCHED_IDLE entities
To:     Josh Don <joshdon@google.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Paul Turner <pjt@google.com>,
        Oleg Rombakh <olegrom@google.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Steve Sistare <steven.sistare@oracle.com>,
        Tejun Heo <tj@kernel.org>, Rik van Riel <riel@surriel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Fri, 20 Aug 2021 at 03:04, Josh Don <joshdon@google.com> wrote:
>
> Give reduced sleeper credit to SCHED_IDLE entities. As a result, woken
> SCHED_IDLE entities will take longer to preempt normal entities.
>
> The benefit of this change is to make it less likely that a newly woken
> SCHED_IDLE entity will preempt a short-running normal entity before it
> blocks.
>
> We still give a small sleeper credit to SCHED_IDLE entities, so that
> idle<->idle competition retains some fairness.
>
> Example: With HZ=1000, spawned four threads affined to one cpu, one of
> which was set to SCHED_IDLE. Without this patch, wakeup latency for the
> SCHED_IDLE thread was ~1-2ms, with the patch the wakeup latency was
> ~5ms.
>
> Signed-off-by: Josh Don <joshdon@google.com>

Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>

> ---
>  kernel/sched/fair.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 31f40aa005b9..aa9c046d2aab 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4230,7 +4230,12 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
>
>         /* sleeps up to a single latency don't count. */
>         if (!initial) {
> -               unsigned long thresh = sysctl_sched_latency;
> +               unsigned long thresh;
> +
> +               if (se_is_idle(se))
> +                       thresh = sysctl_sched_min_granularity;
> +               else
> +                       thresh = sysctl_sched_latency;
>
>                 /*
>                  * Halve their sleep time's effect, to allow
> --
> 2.33.0.rc2.250.ged5fa647cd-goog
>