linux-kernel - Re: [RFC] Repeated rto_push_irq_work

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Message-ID: <20241002150543.IhYy2Q9k@linutronix.de>
Date: Wed, 2 Oct 2024 17:05:43 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
	Ben Segall <bsegall@...gle.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Mel Gorman <mgorman@...e.de>, Peter Zijlstra <peterz@...radead.org>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC] Repeated rto_push_irq_work_func() invocation.

On 2024-10-02 10:37:49 [-0400], Steven Rostedt wrote:
> On Wed, 2 Oct 2024 13:21:05 +0200
> Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
> 
> > Would it make sense to avoid scheduling rto_push_work if rq->curr has
> > NEED_RESCHED set and make the scheduler do push_rt_task()?
> 
> Can we safely check rq->curr without taking any locks on that CPU?
> 
> I guess tasks do not get freed while a CPU has preemption disabled, so it
> may be safe to do:
> 
> 	task = READ_ONCE(rq->curr);
> 	if (test_task_need_resched(task))
> 		/* skip */
> 
> ??

+rcu_read_lock() but yes. This would be one part. You need to check if
the task on pull-list has lower priority than the current task on rq.
This would be need to be moved to somewhere in schedule() probably after
pick_next_task().

> -- Steve

Sebastian