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

[Peter Zijlstra]

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?