linux-kernel - Re: [PATCH] sched_ext: introduce cpu tick

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Message-ID: <aEhyZJmjhYJfVe9v@slm.duckdns.org>
Date: Tue, 10 Jun 2025 07:59:00 -1000
From: 'Tejun Heo' <tj@...nel.org>
To: liuwenfang <liuwenfang@...or.com>
Cc: 'David Vernet' <void@...ifault.com>, 'Andrea Righi' <arighi@...dia.com>,
	'Changwoo Min' <changwoo@...lia.com>,
	'Ingo Molnar' <mingo@...hat.com>,
	'Peter Zijlstra' <peterz@...radead.org>,
	'Juri Lelli' <juri.lelli@...hat.com>,
	'Vincent Guittot' <vincent.guittot@...aro.org>,
	'Dietmar Eggemann' <dietmar.eggemann@....com>,
	'Steven Rostedt' <rostedt@...dmis.org>,
	'Ben Segall' <bsegall@...gle.com>, 'Mel Gorman' <mgorman@...e.de>,
	'Valentin Schneider' <vschneid@...hat.com>,
	"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>,
	Kumar Kartikeya Dwivedi <memxor@...il.com>
Subject: Re: [PATCH] sched_ext: introduce cpu tick

Hello,

On Tue, Jun 10, 2025 at 08:59:45AM +0000, liuwenfang wrote:
> Assume one CPU is running one RT task and one runnable scx task on
> its local dsq, the scx task cannot be scheduled until RT task enters
> sleep, if RT task will run for 100ms, the scx task should be migrated
> to other dsqs, then it can have a chance to be scheduled by other CPUs.
> 
> So cpu_tick is added to notitfy BPF scheduler to check long runnable
> scx on its local dsq, related policy can be taken to improve the
> performance.

(cc'ing Kumar as we discussed similar issue recently)

There are some race conditions we need to address but calling
scx_bpf_reenqueue_local() from ops.cpu_release() is the intended way of
handling these situations. I don't think periodically polling from ticks is
a good approach, especially given that ticks can be skipped w/ nohz_full.

Thanks.

-- 
tejun