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

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Message-ID: <52c9d084d9852cc7c769dbb76f03a13df014c37f.camel@surriel.com>
Date:   Tue, 13 Dec 2022 13:40:40 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Tejun Heo <tj@...nel.org>, 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,
        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

On Tue, 2022-12-13 at 08:12 -1000, Tejun Heo wrote:
> Hello,
> 
> 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:
> > 
> > > 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.
> > 
> > That all sounds quite terrible.. :/
> 
> 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.
> 
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?

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