linux-kernel - Re: [PATCH 08/31] sched: Expose css_tg() and __setscheduler

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Date:   Mon, 12 Dec 2022 07:47:21 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     torvalds@...ux-foundation.org, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...nel.org, joshdon@...gle.com, brho@...gle.com,
        pjt@...gle.com, derkling@...gle.com, haoluo@...gle.com,
        dvernet@...a.com, dschatzberg@...a.com, dskarlat@...cmu.edu,
        riel@...riel.com, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH 08/31] sched: Expose css_tg() and __setscheduler_prio()
 in kernel/sched/sched.h

On Mon, Dec 12, 2022 at 12:49:59PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 29, 2022 at 10:22:50PM -1000, Tejun Heo wrote:
> > These will be used by a new sched_class.
> 
> Uhhh.. wtf does a policy need __setscheduler_prio() for?

Whether a task is on sched_ext or not is determined by a few factors - the
chosen sched class of the task, whether the BPF scheduler is loaded and
whether it's using scx_bpf_switch_all(). That selection logic is implemented
in __setscheduler_prio(). When the BPF scheduler gets initialized and
exited, the selection changes and sched_ext needs to walk the tasks
reapplying the selection logic, so the export.

This ties together with the sched_enq_and_set_task() that you replied to in
another thread. Imma continue there.

-- 
tejun