linux-kernel - Re: [PATCH v2] sched/eevdf: Fix wakeup-preempt by checking cfs_rq->nr

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Message-ID: <20240930075045.GB5594@noisy.programming.kicks-ass.net>
Date: Mon, 30 Sep 2024 09:50:45 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Chen Yu <yu.c.chen@...el.com>
Cc: 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>,
	Chunxin Zang <zangchunxin@...iang.com>,
	linux-kernel@...r.kernel.org, Chen Yu <yu.chen.surf@...il.com>,
	kernel test robot <oliver.sang@...el.com>,
	K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [PATCH v2] sched/eevdf: Fix wakeup-preempt by checking
 cfs_rq->nr_running

On Wed, Sep 25, 2024 at 04:54:40PM +0800, Chen Yu wrote:
> Commit 85e511df3cec ("sched/eevdf: Allow shorter slices to wakeup-preempt")
> introduced a mechanism that a wakee with shorter slice could preempt
> the current running task. It also lower the bar for the current task
> to be preempted, by checking the rq->nr_running instead of cfs_rq->nr_running
> when the current task has ran out of time slice. But there is a scenario
> that is problematic. Say, if there is 1 cfs task and 1 rt task, before
> 85e511df3cec, update_deadline() will not trigger a reschedule, and after
> 85e511df3cec, since rq->nr_running is 2 and resched is true, a resched_curr()
> would happen.
> 

So I'm applying the patch (that change was not indended), but I'm a
little confused by the above description. If we already have TIF_RESCHED
set, then doing resched_curr() should be a NO-OP.

At the same time, how can we have an RT task and not get TIF_RESCHED
set.