linux-kernel - Re: [PATCH] sched/core: Fix reset-on-fork from RT with uclamp

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Message-ID: <20200414162128.GA66453@google.com>
Date:   Tue, 14 Apr 2020 12:21:28 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Quentin Perret <qperret@...gle.com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        ctheegal@...eaurora.org, dianders@...omium.org,
        patrick.bellasi@...bug.net, valentin.schneider@....com,
        qais.yousef@....com, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH] sched/core: Fix reset-on-fork from RT with uclamp

On Tue, Apr 14, 2020 at 05:13:20PM +0100, Quentin Perret wrote:
> uclamp_fork() resets the uclamp values to their default when the
> reset-on-fork flag is set. It also checks whether the task has a RT
> policy, and sets its uclamp.min to 1024 accordingly. However, during
> reset-on-fork, the task's policy is lowered to SCHED_NORMAL right after,
> hence leading to an erroneous uclamp.min setting for the new task if it
> was forked from RT.
> 
> Fix this by removing the unnecessary check on rt_policy() in
> uclamp_fork() as this doesn't make sense if the reset-on-fork flag is
> set.
> 
> Reported-by: Chitti Babu Theegala <ctheegal@...eaurora.org>
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---
>  kernel/sched/core.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3a61a3b8eaa9..9ea3e484eea2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1234,10 +1234,6 @@ static void uclamp_fork(struct task_struct *p)
>  	for_each_clamp_id(clamp_id) {
>  		unsigned int clamp_value = uclamp_none(clamp_id);
>  
> -		/* By default, RT tasks always get 100% boost */
> -		if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> -			clamp_value = uclamp_none(UCLAMP_MAX);
> -

Shouldn't this be conditional on p->sched_reset_on_fork instead of deleting
the code?

thanks,

 - Joel


>  		uclamp_se_set(&p->uclamp_req[clamp_id], clamp_value, false);
>  	}
>  }
> -- 
> 2.26.0.110.g2183baf09c-goog
>