linux-kernel - Re: [PATCH 31/31] sched_ext: Add a rust userspace hybrid example scheduler

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Date:   Mon, 12 Dec 2022 14:18:59 -0800
From:   Josh Don <joshdon@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Tejun Heo <tj@...nel.org>, 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,
        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, Peter Oskolkov <posk@...gle.com>
Subject: Re: [PATCH 31/31] sched_ext: Add a rust userspace hybrid example scheduler

Hey Peter,

On Mon, Dec 12, 2022 at 6:03 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Nov 29, 2022 at 10:23:13PM -1000, Tejun Heo wrote:
> > From: Dan Schatzberg <dschatzberg@...a.com>
> >
> > Atropos is a multi-domain BPF / userspace hybrid scheduler where the BPF
> > part does simple round robin in each domain and the userspace part
> > calculates the load factor of each domain and tells the BPF part how to load
> > balance the domains.
> >
> > This scheduler demonstrates dividing scheduling logic between BPF and
> > userspace and using rust to build the userspace part.
>
> And here I am, speaking neither Rust nor BPF.
>
> But really, having seen some of this I long for the UMCG patches -- that
> at least was somewhat sane and trivially composes, unlike all this
> madness.

I wasn't sure if you were focusing specifically on how the BPF portion
is implemented, or on UMCG vs sched_ext. For the latter, and ignoring
the specifics of this example, the UMCG and sched_ext work are
complementary, but not mutually exclusive. UMCG is about driving
cooperative scheduling within a particular application. UMCG does not
have control over or react to external preemption, nor does it make
thread placement decisions. sched_ext is considering things more at
the system level: arbitrating fairness and preemption between
processes, deciding when and where threads run, etc., and also being
able to take application-specific hints if desired.