linux-kernel - Re: [PATCH 3/3] sched_ext: Allow scx_bpf_reenqueue

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Message-ID: <20251027174953.GB3419281@noisy.programming.kicks-ass.net>
Date: Mon, 27 Oct 2025 18:49:53 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>,
	Andrea Righi <andrea.righi@...ux.dev>,
	Changwoo Min <changwoo@...lia.com>, linux-kernel@...r.kernel.org,
	sched-ext@...ts.linux.dev, Wen-Fang Liu <liuwenfang@...or.com>
Subject: Re: [PATCH 3/3] sched_ext: Allow scx_bpf_reenqueue_local() to be
 called from anywhere

On Mon, Oct 27, 2025 at 06:00:00AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Mon, Oct 27, 2025 at 10:18:22AM +0100, Peter Zijlstra wrote:
> ...
> > > The main use case for cpu_release() was calling scx_bpf_reenqueue_local() when
> > > a CPU gets preempted by a higher priority scheduling class. However, the old
> > > scx_bpf_reenqueue_local() could only be called from cpu_release() context.
> > 
> > I'm a little confused. Isn't this the problem where balance_one()
> > migrates a task to the local rq and we end up having to RETRY_TASK
> > because another (higher) rq gets modified?
> 
> That's what I thought too and the gap between balance() and pick_task() can
> be closed that way. However, while plugging that, I realized there's another
> bigger gap between ttwu() and pick_task() because ttwu() can directly
> dispatch a task into the local DSQ of a CPU. That one, there's no way to
> close without a global hook.

This would've been prime Changelog material. As is the Changelog was so
vague I wasn't even sure it was that particular problem.

Please update the changelog to be clearer.

Also, why is this patch already in a pull request to Linus? what's the
hurry.