Re: [PATCH 06/31] sched: Add sched_class->switching_to() and expose check_class_changing/changed()

[Tejun Heo]

On Mon, Dec 12, 2022 at 12:28:29PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 29, 2022 at 10:22:48PM -1000, Tejun Heo wrote:
> > When a task switches to a new sched_class, the prev and new classes are
> > notified through ->switched_from() and ->switched_to(), respectively, after
> > the switching is done. However, a new sched_class needs to prepare the task
> > state before it is enqueued on the new class for the first time.
> 
> How and why isn't sched_fork() sufficient?

sched_ext has callbacks which allow the BPF scheduler to keep track of
relevant task states (like priority and cpumask). Those callbacks aren't
called while a task isn't on sched_ext. When a task comes back to SCX, we
wanna tell the BPF scheduler the up-to-date state before the task gets
enqueued, so the need for a hook which is called before the switching is
committed.

Thanks.

-- 
tejun