linux-kernel - Re: [RFC PATCH v15 6/7] sched: Fix proxy/current (push,pull)ability

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Message-ID: <20250317140754.GD36386@noisy.programming.kicks-ass.net>
Date: Mon, 17 Mar 2025 15:07:54 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: John Stultz <jstultz@...gle.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Valentin Schneider <valentin.schneider@....com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Valentin Schneider <vschneid@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Zimuzo Ezeozue <zezeozue@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Metin Kaya <Metin.Kaya@....com>,
	Xuewen Yan <xuewen.yan94@...il.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Suleiman Souhlal <suleiman@...gle.com>, kernel-team@...roid.com,
	Connor O'Brien <connoro@...gle.com>
Subject: Re: [RFC PATCH v15 6/7] sched: Fix proxy/current (push,pull)ability

On Wed, Mar 12, 2025 at 03:11:36PM -0700, John Stultz wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b4f7b14f62a24..3596244f613f8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6722,6 +6722,23 @@ find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
>  }
>  #endif /* SCHED_PROXY_EXEC */
>  
> +static inline void proxy_tag_curr(struct rq *rq, struct task_struct *owner)
> +{
> +	if (!sched_proxy_exec())
> +		return;
> +	/*
> +	 * pick_next_task() calls set_next_task() on the chosen task
> +	 * at some point, which ensures it is not push/pullable.
> +	 * However, the chosen/donor task *and* the mutex owner form an
> +	 * atomic pair wrt push/pull.
> +	 *
> +	 * Make sure owner we run is not pushable. Unfortunately we can
> +	 * only deal with that by means of a dequeue/enqueue cycle. :-/
> +	 */
> +	dequeue_task(rq, owner, DEQUEUE_NOCLOCK | DEQUEUE_SAVE);
> +	enqueue_task(rq, owner, ENQUEUE_NOCLOCK | ENQUEUE_RESTORE);
> +}

So this is probably fine at this point; but we should eventually look at
fixing this.

We can probably look at (ab)using sched_class::set_cpus_allowed() for
this.