Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs hotplug-rollback

[Vincent Donnefort <vincent.donnefort@arm.com>]

On Thu, Apr 15, 2021 at 03:32:11PM +0100, Valentin Schneider wrote:
> On 15/04/21 10:59, Peter Zijlstra wrote:
> > Can't make sense of what I did.. I've removed that hunk. Patch now looks
> > like this.
> >
> 
> Small nit below, but regardless feel free to apply to the whole lot:
> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
> 
> @VincentD, ISTR you had tested the initial version of this with your fancy
> shmancy hotplug rollback stresser. Feel like doing this

I indeed wrote a test to verify all the rollback cases, up and down.

It seems I encounter an intermitent issue while running several iterations of
that test ... but I need more time to debug and figure-out where it is blocking.

> 
> > So instead, make sure balance_push is enabled between
> > sched_cpu_deactivate() and sched_cpu_activate() (eg. when
> > !cpu_active()), and gate it's utility with cpu_dying().
> 
> I'd word that "is enabled below sched_cpu_activate()", since
> sched_cpu_deactivate() is now out of the picture.
> 
> [...]
> > @@ -7639,6 +7639,9 @@ static DEFINE_PER_CPU(struct cpu_stop_wo
> >
> >  /*
> >   * Ensure we only run per-cpu kthreads once the CPU goes !active.
> > + *
> > + * This is active/set between sched_cpu_deactivate() / sched_cpu_activate().
> 
> Ditto
> 
> > + * But only effective when the hotplug motion is down.
> >   */
> >  static void balance_push(struct rq *rq)
> >  {