Re: [PATCH 04/31] sched: Allow sched_cgroup_fork() to fail and introduce sched_cancel_fork()

[Peter Zijlstra]

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?