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

[Tejun Heo]

On Mon, Dec 12, 2022 at 09:07:11PM +0100, Peter Zijlstra wrote:
> 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?

Because SCX's ops.prepare_enable() wants the cgroup (p->sched_task_group) to
be initialized in case the BPF scheduler wants to perform cgroup related
initializations.

Thanks.

-- 
tejun