linux-kernel - Re: [RFC PATCH 1/3] sched: Make migrate_task

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Date:   Wed, 21 Jun 2023 21:07:17 -0500
From:   David Vernet <void@...ifault.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        rostedt@...dmis.org, dietmar.eggemann@....com, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        joshdon@...gle.com, roman.gushchin@...ux.dev, tj@...nel.org,
        kernel-team@...a.com
Subject: Re: [RFC PATCH 1/3] sched: Make migrate_task_to() take any task

On Wed, Jun 21, 2023 at 03:04:39PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 13, 2023 at 12:20:02AM -0500, David Vernet wrote:
> > The migrate_task_to() function exposed from kernel/sched/core.c migrates
> > the current task, which is silently assumed to also be its first
> > argument, to the specified CPU. The function uses stop_one_cpu() to
> > migrate the task to the target CPU, which won't work if @p is not the
> > current task as the stop_one_cpu() callback isn't invoked on remote
> > CPUs.
> > 
> > While this operation is useful for task_numa_migrate() in fair.c, it
> > would be useful if __migrate_task() in core.c was given external
> > linkage, as it actually can be used to migrate any task to a CPU.
> > 
> > This patch therefore:
> > 1. Renames the existing migrate_task_to() be called
> >    numa_migrate_current_task_to().
> > 2. Renames __migrate_task() to migrate_task_to(), gives it global
> >    linkage, and updates all callers accordingly.
> > 
> > A follow-on patch will call the new migrate_task_to() from fair.c when
> > migrating a task in a shared wakequeue to a remote CPU.
> 
> I would suggest simply exposing move_queued_task(). You can actually do
> is_cpu_allowed() before you do the whole lock dance, or even before
> pull.

Good call, I'll make that improvement for v2. Also, ack on exposing
move_queued_task(). That makes more sense.

> And then you don't have to create silly long function names either.

Yeah, the function name I chose is admittedly crap, but IMO it is pretty
unintuitive that p == current. But yeah, it'd probably be better to just
do something like remove the @p parameter and use current directly. That
whole call chain passes p == current around though, so *shrug*.