linux-kernel - Re: [PATCH] sched/eevdf: Fix wakeup-preempt by checking cfs_rq->nr

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Message-ID: <ZvEZ5kDNdtOK8Q49@xsang-OptiPlex-9020>
Date: Mon, 23 Sep 2024 15:33:58 +0800
From: Oliver Sang <oliver.sang@...el.com>
To: Chen Yu <yu.c.chen@...el.com>
CC: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot
	<vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>, Valentin Schneider
	<vschneid@...hat.com>, Chunxin Zang <zangchunxin@...iang.com>,
	<linux-kernel@...r.kernel.org>, Chen Yu <yu.chen.surf@...il.com>,
	<oliver.sang@...el.com>
Subject: Re: [PATCH] sched/eevdf: Fix wakeup-preempt by checking
 cfs_rq->nr_running

hi, Chenyu,

On Mon, Sep 23, 2024 at 03:21:56PM +0800, Chen Yu wrote:
> Commit 85e511df3cec ("sched/eevdf: Allow shorter slices to wakeup-preempt")
> introduced a mechanism that a wakee with shorter slice could preempt
> the current running task. It also lower the bar for the current task
> to be preempted, by checking the rq->nr_running instead of cfs_rq->nr_running
> when the current task has ran out of time slice. Say, if there is 1 cfs
> task and 1 rt task, before 85e511df3cec, update_deadline() will
> not trigger a reschedule, and after 85e511df3cec, since rq->nr_running
> is 2 and resched is true, a resched_curr() would happen.
> 
> Some workloads (like the hackbench reported by lkp) do not like
> over-scheduling. We can see that the preemption rate has been
> increased by 2.2%:
> 
> 1.654e+08            +2.2%   1.69e+08        hackbench.time.involuntary_context_switches
> 
> Restore its previous check criterion.
> 
> Fixes: 85e511df3cec ("sched/eevdf: Allow shorter slices to wakeup-preempt")

we applied this patch upon 85e511df3cec, confirmed the regression is fixed.

Tested-by: kernel test robot <oliver.sang@...el.com>


=========================================================================================
compiler/cpufreq_governor/ipc/iterations/kconfig/mode/nr_threads/rootfs/tbox_group/testcase:
  gcc-12/performance/socket/4/x86_64-rhel-8.3/process/50%/debian-12-x86_64-20240206.cgz/lkp-spr-r02/hackbench

commit:
  82e9d0456e06 ("sched/fair: Avoid re-setting virtual deadline on 'migrations'")
  85e511df3cec ("sched/eevdf: Allow shorter slices to wakeup-preempt")
  1c3386188ad9 ("sched/eevdf: Fix wakeup-preempt by checking cfs_rq->nr_running")

82e9d0456e06cebe 85e511df3cec46021024176672a 1c3386188ad9007c9975470c817
---------------- --------------------------- ---------------------------
         %stddev     %change         %stddev     %change         %stddev
             \          |                \          |                \
    623219           -13.1%     541887            +0.3%     625154        hackbench.throughput


> Reported-by: kernel test robot <oliver.sang@...el.com>
> Closes: https://lore.kernel.org/oe-lkp/202409231416.9403c2e9-oliver.sang@intel.com
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 225b31aaee55..2859fc7e2da2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1025,7 +1025,7 @@ static bool update_deadline(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  	/*
>  	 * The task has consumed its request, reschedule.
>  	 */
> -	return true;
> +	return (cfs_rq->nr_running > 1);
>  }
>  
>  #include "pelt.h"
> -- 
> 2.25.1
>