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Subject: Re: [PATCH 14/31] sched_ext: Implement BPF extensible scheduler
 class
From:   Rik van Riel <riel@surriel.com>
To:     Tejun Heo <tj@kernel.org>, Peter Zijlstra <peterz@infradead.org>
Cc:     torvalds@linux-foundation.org, mingo@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
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        linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
        kernel-team@meta.com
Date:   Tue, 13 Dec 2022 13:40:40 -0500
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On Tue, 2022-12-13 at 08:12 -1000, Tejun Heo wrote:
> Hello,
>=20
> On Tue, Dec 13, 2022 at 11:55:10AM +0100, Peter Zijlstra wrote:
> > On Mon, Dec 12, 2022 at 11:33:12AM -1000, Tejun Heo wrote:
> >=20
> > > Here, the way it's handled is a bit different, SCX has
> > > a watchdog mechanism implemented in "[PATCH 18/31] sched_ext:
> > > Implement
> > > runnable task stall watchdog", so if SCX tasks hang for whatever
> > > reason
> > > including being starved by CFS, it will get aborted and all tasks
> > > will be
> > > handed back to CFS. IOW, it's treated like any other BPF
> > > scheduler errors
> > > that can lead to stalls and recovered the same way.
> >=20
> > That all sounds quite terrible.. :/
>=20
> The main source of difference is that we can't implicitly trust the
> BPF
> scheduler and if it malfunctions or on user request, the system
> should
> always be recoverable, so there are some extra things which are
> inherently
> necessary to support that.
>=20
That makes me wonder whether loading an SCX policy
should just have that policy take over all of the
SCHED_OTHER tasks by default, and have a failure of
the policy just return those tasks to CFS?

Having the two be operative at the same time seems
to be a cause of hard to resolve issues, while simply
running all non-RT tasks under the loadable policy
could simplify both internal kernel interfaces, as
well as externally visible effects?

--=20
All Rights Reversed.

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