linux-kernel - Re: [PATCH 17/24] sched/fair: Implement delayed dequeue

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Message-ID: <9af1bc3d9b0ddd5a4d672004cd9182017c320ec0.camel@gmx.de>
Date: Tue, 05 Nov 2024 07:46:16 +0100
From: Mike Galbraith <efault@....de>
To: K Prateek Nayak <kprateek.nayak@....com>, Phil Auld <pauld@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com, 
 juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com,  rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, vschneid@...hat.com,  linux-kernel@...r.kernel.org,
 wuyun.abel@...edance.com,  youssefesmat@...omium.org, tglx@...utronix.de
Subject: Re: [PATCH 17/24] sched/fair: Implement delayed dequeue

On Tue, 2024-11-05 at 09:52 +0530, K Prateek Nayak wrote:
> Hello Mike,

Greetings,

> Would checking "p->nr_cpus_allowed > 1" be enough instead of doing a
> "cpumask_weight(p->cpus_ptr) > 1"?

Yeah (thwap).

> I was thinking, since the task is indeed delayed, there has to be more
> than one task on the runqueue right since a single task by itself cannot
> be ineligible and be marked for delayed dequeue?

But they migrate via LB, and idle balance unlocks the rq.
trace_printk() just verified that they do still both land with
sched_delayed intact and with nr_running = 1.

> The only time we
> encounter a delayed task with "rq->nr_running == 1" is if the other
> tasks have been fully dequeued and pick_next_task() is in the process of
> picking off all the delayed task, but since that is done with the rq
> lock held in schedule(), it is even possible for the
> "rq->nr_running > 1" to be false here?

I don't see how, the rq being looked at is locked.

	-Mike