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Date:   Mon, 12 Dec 2022 21:07:11 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Tejun Heo <tj@kernel.org>
Cc:     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, joshdon@google.com, 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
Subject: Re: [PATCH 04/31] sched: Allow sched_cgroup_fork() to fail and
 introduce sched_cancel_fork()
Message-ID: <Y5eJ75pH87c+ngmo@hirez.programming.kicks-ass.net>
References: <20221130082313.3241517-1-tj@kernel.org>
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On Mon, Dec 12, 2022 at 08:03:24AM -1000, Tejun Heo wrote:
> On Mon, Dec 12, 2022 at 12:13:31PM +0100, Peter Zijlstra wrote:
> > On Tue, Nov 29, 2022 at 10:22:46PM -1000, Tejun Heo wrote:
> > > A new sched_clas needs a bit more control over forking. This patch makes the
> >                  ^
> >                  (insufficient s's)
> 
> Will update.
> 
> > > following changes:
> > > 
> > > * Add sched_cancel_fork() which is called if fork fails after sched_fork()
> > >   succeeds so that the preparation can be undone.
> > > 
> > > * Allow sched_cgroup_fork() to fail.
> > > 
> > > Neither is used yet and this patch shouldn't cause any behavior changes.
> > 
> > Fails to explain why this would be needed and why that would be a good
> > thing. IOW, total lack of justification.
> 
> This is because sched_ext calls out to BPF scheduler's prepare_enable()
> operation to prepare the task. The operation is allowed to fail (e.g. it
> might need to allocate something which can fail), so we need a way back back
> out of it.

sched_fork() can already fail; why isn't that a suitable location to do
what needs doing?