Re: [RFC PATCH v7 08/23] sched: Add core wide task selection and scheduling.

[peterz@infradead.org]

On Fri, Aug 28, 2020 at 06:02:25PM -0400, Vineeth Pillai wrote:
> On 8/28/20 4:51 PM, Peter Zijlstra wrote:

> > So where do things go side-ways?

> During hotplug stress test, we have noticed that while a sibling is in
> pick_next_task, another sibling can go offline or come online. What
> we have observed is smt_mask get updated underneath us even if
> we hold the lock. From reading the code, looks like we don't hold the
> rq lock when the mask is updated. This extra logic was to take care of that.

Sure, the mask is updated async, but _where_ is the actual problem with
that?

On Fri, Aug 28, 2020 at 06:23:55PM -0400, Joel Fernandes wrote:
> Thanks Vineeth. Peter, also the "v6+" series (which were some addons on v6)
> detail the individual hotplug changes squashed into this patch:
> https://lore.kernel.org/lkml/20200815031908.1015049-9-joel@joelfernandes.org/
> https://lore.kernel.org/lkml/20200815031908.1015049-11-joel@joelfernandes.org/

That one looks fishy, the pick is core wide, making that pick_seq per rq
just doesn't make sense.

> https://lore.kernel.org/lkml/20200815031908.1015049-12-joel@joelfernandes.org/

This one reads like tinkering, there is no description of the actual
problem just some code that makes a symptom go away.

Sure, on hotplug the smt mask can change, but only for a CPU that isn't
actually scheduling, so who cares.

/me re-reads the hotplug code...

..ooOO is the problem that we clear the cpumasks on take_cpu_down()
instead of play_dead() ?! That should be fixable.

> https://lore.kernel.org/lkml/20200815031908.1015049-13-joel@joelfernandes.org/

This is the only one that makes some sense, it makes rq->core consistent
over hotplug.

> Agreed we can split the patches for the next series, however for final
> upstream merge, I suggest we fix hotplug issues in this patch itself so that
> we don't break bisectability.

Meh, who sodding cares about hotplug :-). Also you can 'fix' such things
by making sure you can't actually enable core-sched until after
everything is in place.