linux-kernel - Re: [PATCH 14/31] sched_ext: Implement BPF extensible scheduler class

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Message-ID: <Y5i3MeVNe6gAJp85@slm.duckdns.org>
Date:   Tue, 13 Dec 2022 07:32:33 -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 14/31] sched_ext: Implement BPF extensible scheduler class

Hello,

On Tue, Dec 13, 2022 at 11:57:12AM +0100, Peter Zijlstra wrote:
> > Given that UP doesn't need to transfer tasks across, it might be possible to
> > move the whole dispatch operation into ->pick_next_task() but the current
> > state would be different, so it's more complicated and will likely be more
> > brittle.
> 
> That sounds like something is amiss, you fundamentally hold all the
> right locks, there is only one.

Yeah, locking is not the problem on UP. It's more that balance() is before
put_prev_task() and pick_next_task() after, so the %current task state is
different between the two points. I'll see if I can sqaure that from SCX
side. I don't see a reason why it wouldn't be possible but it's likely more
complicated than adding a call in the same spot for UP.

Thanks.

-- 
tejun