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From:   Josh Don <joshdon@google.com>
Date:   Mon, 12 Dec 2022 14:18:59 -0800
Message-ID: <CABk29Nu5WiCmhNN2jZrTShELbCDOYUziUeW5xojkwB83R+VzEQ@mail.gmail.com>
Subject: Re: [PATCH 31/31] sched_ext: Add a rust userspace hybrid example scheduler
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Tejun Heo <tj@kernel.org>, torvalds@linux-foundation.org,
        mingo@redhat.com, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, vschneid@redhat.com, ast@kernel.org,
        daniel@iogearbox.net, andrii@kernel.org, martin.lau@kernel.org,
        brho@google.com, pjt@google.com, derkling@google.com,
        haoluo@google.com, dvernet@meta.com, dschatzberg@meta.com,
        dskarlat@cs.cmu.edu, riel@surriel.com,
        linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
        kernel-team@meta.com, Peter Oskolkov <posk@google.com>
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Hey Peter,

On Mon, Dec 12, 2022 at 6:03 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Nov 29, 2022 at 10:23:13PM -1000, Tejun Heo wrote:
> > From: Dan Schatzberg <dschatzberg@meta.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.