linux-kernel - Re: [PATCH] sched/fair: Fix util_est UTIL_AVG

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Message-ID: <20210519160633.GA230499@e120877-lin.cambridge.arm.com>
Date:   Wed, 19 May 2021 17:06:34 +0100
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Xuewen Yan <xuewen.yan94@...il.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Quentin Perret <qperret@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Fix util_est UTIL_AVG_UNCHANGED handling

On Fri, May 14, 2021 at 12:37:48PM +0200, Dietmar Eggemann wrote:
> The util_est internal UTIL_AVG_UNCHANGED flag which is used to prevent
> unnecessary util_est updates uses the LSB of util_est.enqueued. It is
> exposed via _task_util_est() (and task_util_est()).
> 
> Commit 92a801e5d5b7 ("sched/fair: Mask UTIL_AVG_UNCHANGED usages")
> mentions that the LSB is lost for util_est resolution but
> find_energy_efficient_cpu() checks if task_util_est() returns 0 to
> return prev_cpu early.
> 
> _task_util_est() returns the max value of util_est.ewma and
> util_est.enqueued or'ed w/ UTIL_AVG_UNCHANGED.
> So task_util_est() returning the max of task_util() and
> _task_util_est() will never return 0 under the default
> SCHED_FEAT(UTIL_EST, true).
> 
> To fix this use the MSB of util_est.enqueued instead and keep the flag
> util_est internal, i.e. don't export it via _task_util_est().
> 
> The maximal possible util_avg value for a task is 1024 so the MSB of
> 'unsigned int util_est.enqueued' isn't used to store a util value.
> 
> As a caveat the code behind the util_est_se trace point has to filter
> UTIL_AVG_UNCHANGED to see the real util_est.enqueued value which should
> be easy to do.
> 
> This also fixes an issue report by Xuewen Yan that util_est_update()
> only used UTIL_AVG_UNCHANGED for the subtrahend of the equation:
> 
>   last_enqueued_diff = ue.enqueued - (task_util() | UTIL_AVG_UNCHANGED)
> 
> Fixes: b89997aa88f0b sched/pelt: Fix task util_est update filtering
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>

I don't believe this made it through our integration branch testing, so I gave a
try manually on my Hikey with LISA's UtilConvergence test. 20 iterations of that
test passed without any problem.

> ---
>  kernel/sched/fair.c |  5 +++--
>  kernel/sched/pelt.h | 13 +++++++------
>  2 files changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 161b92aa1c79..0150d440b0a2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3856,7 +3856,7 @@ static inline unsigned long _task_util_est(struct task_struct *p)
>  {
>  	struct util_est ue = READ_ONCE(p->se.avg.util_est);
>  
> -	return (max(ue.ewma, ue.enqueued) | UTIL_AVG_UNCHANGED);
> +	return max(ue.ewma, (ue.enqueued & ~UTIL_AVG_UNCHANGED));
>  }


Shall we also update proc_sched_show_task() to avoid reading this flag there?

-- 
Vincent