Re: [PATCH] sched/fair: initialize throttle_count for new task-groups lazily

[Konstantin Khlebnikov]

On 22.06.2016 00:10, Peter Zijlstra wrote:
> On Thu, Jun 16, 2016 at 03:57:01PM +0300, Konstantin Khlebnikov wrote:
> > Cgroup created inside throttled group must inherit current throttle_count.
> > Broken throttle_count allows to nominate throttled entries as a next buddy,
> > later this leads to null pointer dereference in pick_next_task_fair().
> >
> > This patch initialize cfs_rq->throttle_count at first enqueue: laziness
> > allows to skip locking all rq at group creation. Lazy approach also allows
> > to skip full sub-tree scan at throttling hierarchy (not in this patch).
>
> You're talking about taking rq->lock in alloc_fair_sched_group(), right?
>
> We're about to go do that anyway... But I suppose for backports this
> makes sense. Doing it at creation time also avoids the issues Ben
> raised, right?

Yes, all will be fine. But for 8192-cores this will be disaster =)
throttle_count must be initialized after linking tg into lists. obviously.

--
Konstantin