linux-kernel - Re: [PATCH 2/2] sched/fair: Scale wakeup granularity relative to nr

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Message-ID: <YVwoy30GY61Z4nid@hirez.programming.kicks-ass.net>
Date:   Tue, 5 Oct 2021 12:28:27 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] sched/fair: Scale wakeup granularity relative to
 nr_running

On Wed, Sep 22, 2021 at 08:22:43PM +0200, Vincent Guittot wrote:

> @@ -7161,6 +7162,13 @@ static void check_preempt_wakeup(struct rq *rq,
> struct task_struct *p, int wake_
>                 return;
> 
>         update_curr(cfs_rq_of(se));
> +       delta_exec = se->sum_exec_runtime - se->prev_sum_exec_runtime;
> +       /*
> +        * Ensure that current got a chance to move forward
> +        */
> +       if (delta_exec < sysctl_sched_min_granularity)
> +               return;
> +

I think we tried that at some point; IIRC the problem with this is that
if the interactive task fails to preempt, that preemption is lost. IOW
interactivity suffers.

Basically if you don't want wake-up preemptions, use SCHED_BATCH, then
those tasks will not preempt one another, but the SCHED_NORMAL tasks
will preempt them.