linux-kernel - Re: [PATCH v2 1/2] sched/uclamp: Fix initialization of strut uclamp

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Message-ID: <87sgekorfq.derkling@matbug.net>
Date:   Wed, 24 Jun 2020 09:26:17 +0200
From:   Patrick Bellasi <patrick.bellasi@...bug.net>
To:     Qais Yousef <qais.yousef@....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>,
        Chris Redpath <chris.redpath@....com>,
        Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] sched/uclamp: Fix initialization of strut uclamp_rq


Hi Qais,

On Fri, Jun 19, 2020 at 19:20:10 +0200, Qais Yousef <qais.yousef@....com> wrote...

> struct uclamp_rq was zeroed out entirely in assumption that in the first
> call to uclamp_rq_inc() they'd be initialized correctly in accordance to
> default settings.
>
> But when next patch introduces a static key to skip
> uclamp_rq_{inc,dec}() until userspace opts in to use uclamp, schedutil
> will fail to perform any frequency changes because the
> rq->uclamp[UCLAMP_MAX].value is zeroed at init and stays as such. Which
> means all rqs are capped to 0 by default.

Does not this means the problem is more likely with uclamp_rq_util_with(),
which should be guarded?

Otherwise, we will also keep doing useless min/max aggregations each
time schedutil calls that function, thus not completely removing
uclamp overheads while user-space has not opted in.

What about dropping this and add the guard in the following patch, along
with the others?

> Fix it by making sure we do proper initialization at init without

>
> Fix it by making sure we do proper initialization at init without
> relying on uclamp_rq_inc() doing it later.
>
> Fixes: 69842cba9ace ("sched/uclamp: Add CPU's clamp buckets refcounting")
> Signed-off-by: Qais Yousef <qais.yousef@....com>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ben Segall <bsegall@...gle.com>
> Cc: Mel Gorman <mgorman@...e.de>
> CC: Patrick Bellasi <patrick.bellasi@...bug.net>
> Cc: Chris Redpath <chris.redpath@....com>
> Cc: Lukasz Luba <lukasz.luba@....com>
> Cc: linux-kernel@...r.kernel.org
> ---
>  kernel/sched/core.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a43c84c27c6f..4265861e13e9 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1248,6 +1248,22 @@ static void uclamp_fork(struct task_struct *p)
>  	}
>  }
>  
> +static void __init init_uclamp_rq(struct rq *rq)
> +{
> +	enum uclamp_id clamp_id;
> +	struct uclamp_rq *uc_rq = rq->uclamp;
> +
> +	for_each_clamp_id(clamp_id) {
> +		memset(uc_rq[clamp_id].bucket,
> +		       0,
> +		       sizeof(struct uclamp_bucket)*UCLAMP_BUCKETS);
> +
> +		uc_rq[clamp_id].value = uclamp_none(clamp_id);
> +	}
> +
> +	rq->uclamp_flags = 0;
> +}
> +
>  static void __init init_uclamp(void)
>  {
>  	struct uclamp_se uc_max = {};
> @@ -1256,11 +1272,8 @@ static void __init init_uclamp(void)
>  
>  	mutex_init(&uclamp_mutex);
>  
> -	for_each_possible_cpu(cpu) {
> -		memset(&cpu_rq(cpu)->uclamp, 0,
> -				sizeof(struct uclamp_rq)*UCLAMP_CNT);
> -		cpu_rq(cpu)->uclamp_flags = 0;
> -	}
> +	for_each_possible_cpu(cpu)
> +		init_uclamp_rq(cpu_rq(cpu));
>  
>  	for_each_clamp_id(clamp_id) {
>  		uclamp_se_set(&init_task.uclamp_req[clamp_id],