linux-kernel - Re: [PATCH 11/20] sched: Handle CPU isolation on last resort fallback rq selection

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Message-ID: <ZvZeKBzOBbVyA-xL@tiehlicka>
Date: Fri, 27 Sep 2024 09:26:32 +0200
From: Michal Hocko <mhocko@...e.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	Uladzislau Rezki <urezki@...il.com>, rcu@...r.kernel.org
Subject: Re: [PATCH 11/20] sched: Handle CPU isolation on last resort
 fallback rq selection

On Fri 27-09-24 00:48:59, Frederic Weisbecker wrote:
> When a kthread or any other task has an affinity mask that is fully
> offline or unallowed, the scheduler reaffines the task to all possible
> CPUs as a last resort.
> 
> This default decision doesn't mix up very well with nohz_full CPUs that
> are part of the possible cpumask but don't want to be disturbed by
> unbound kthreads or even detached pinned user tasks.
> 
> Make the fallback affinity setting aware of nohz_full. This applies to
> all architectures supporting nohz_full except arm32. However this
> architecture that overrides the task possible mask is unlikely to be
> willing to integrate new development.
> 
> Suggested-by: Michal Hocko <mhocko@...e.com>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>

Thanks, this makes sense to me. Up to scheduler maitainers whether this
makes sense in general though.

Thanks for looking into this Frederic!

> ---
>  kernel/sched/core.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 43e453ab7e20..d4b759c1cbf1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3421,6 +3421,21 @@ void kick_process(struct task_struct *p)
>  }
>  EXPORT_SYMBOL_GPL(kick_process);
>  
> +static const struct cpumask *task_cpu_fallback_mask(struct task_struct *t)
> +{
> +	const struct cpumask *mask;
> +
> +	mask = task_cpu_possible_mask(p);
> +	/*
> +	 * Architectures that overrides the task possible mask
> +	 * must handle CPU isolation.
> +	 */
> +	if (mask != cpu_possible_mask)
> +		return mask;
> +	else
> +		return housekeeping_cpumask(HK_TYPE_TICK);
> +}
> +
>  /*
>   * ->cpus_ptr is protected by both rq->lock and p->pi_lock
>   *
> @@ -3489,7 +3504,7 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
>  			 *
>  			 * More yuck to audit.
>  			 */
> -			do_set_cpus_allowed(p, task_cpu_possible_mask(p));
> +			do_set_cpus_allowed(p, task_cpu_fallback_mask(p));
>  			state = fail;
>  			break;
>  		case fail:
> -- 
> 2.46.0

-- 
Michal Hocko
SUSE Labs