linux-kernel - Re: [PATCH 13/14] sched: Add {DE,EN}QUEUE

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Message-ID: <20250915084856.GC3289052@noisy.programming.kicks-ass.net>
Date: Mon, 15 Sep 2025 10:48:56 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Tejun Heo <tj@...nel.org>
Cc: linux-kernel@...r.kernel.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, longman@...hat.com, hannes@...xchg.org,
	mkoutny@...e.com, void@...ifault.com, arighi@...dia.com,
	changwoo@...lia.com, cgroups@...r.kernel.org,
	sched-ext@...ts.linux.dev, liuwenfang@...or.com, tglx@...utronix.de
Subject: Re: [PATCH 13/14] sched: Add {DE,EN}QUEUE_LOCKED

On Sat, Sep 13, 2025 at 12:32:27PM -1000, Tejun Heo wrote:
> Hello,
> 
> On Fri, Sep 12, 2025 at 06:32:32AM -1000, Tejun Heo wrote:
> > Yeah, or I can make scx_tasks iteration smarter so that it can skip through
> > the list for tasks which aren't runnable. As long as it doesn't do lock ops
> > on every task, it should be fine. I think this is solvable one way or
> > another. Let's continue in the other subthread.
> 
> Thought more about it. There's another use case for this runnable list,
> which is the watchdog. As in the migration synchronization, I think the
> right thing to do here is just adding a nested lock. That doesn't add any
> overhead or complications to other sched classes and from sched_ext POV
> given how expensive migrations can be, if we make that a bit cheaper (and I
> believe we will with changes being discussed), added up, the outcome would
> likely be lower overhead.

I really don't see how you could possibly retain that runnable_list.

pick_next_task() must be able to migrate a task from a shared runqueue
to a local runqueue. It must do this without taking a random other
per-cpu runqueue. Therefore, a task on a DSQ must have no 'local' state.

This very much means the runnable_list cannot be per cpu.

No per-task lock is going to help with that.

The watchdog will have to go iterate DSQs or something like that.