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SubjectRe: [PATCH] sched/fair: initialize throttle_count for new task-groups lazily
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Konstantin Khlebnikov <khlebnikov@yandex-team.ru> writes:

> 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).
>
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> Cc: Stable <stable@vger.kernel.org> # v3.2+
> ---
> kernel/sched/fair.c | 19 +++++++++++++++++++
> kernel/sched/sched.h | 2 +-
> 2 files changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 218f8e83db73..fe809fe169d2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4185,6 +4185,25 @@ static void check_enqueue_throttle(struct cfs_rq *cfs_rq)
> if (!cfs_bandwidth_used())
> return;
>
> + /* synchronize hierarchical throttle counter */
> + if (unlikely(!cfs_rq->throttle_uptodate)) {
> + struct rq *rq = rq_of(cfs_rq);
> + struct cfs_rq *pcfs_rq;
> + struct task_group *tg;
> +
> + cfs_rq->throttle_uptodate = 1;
> + /* get closest uptodate node because leaves goes first */
> + for (tg = cfs_rq->tg->parent; tg; tg = tg->parent) {
> + pcfs_rq = tg->cfs_rq[cpu_of(rq)];
> + if (pcfs_rq->throttle_uptodate)
> + break;
> + }
> + if (tg) {
> + cfs_rq->throttle_count = pcfs_rq->throttle_count;
> + cfs_rq->throttled_clock_task = rq_clock_task(rq);
> + }
> + }
> +

Checking just in enqueue is not sufficient - throttled_lb_pair can check
against a cfs_rq that has never been enqueued (and possibly other
paths).

It might also make sense to go ahead and initialize all the cfs_rqs we
skipped over to avoid some n^2 pathological behavior. You could also use
throttle_count == -1 or < 0. (We had our own version of this patch that
I guess we forgot to push?)


> /* an active group must be handled by the update_curr()->put() path */
> if (!cfs_rq->runtime_enabled || cfs_rq->curr)
> return;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 72f1f3087b04..7cbeb92a1cb9 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -437,7 +437,7 @@ struct cfs_rq {
>
> u64 throttled_clock, throttled_clock_task;
> u64 throttled_clock_task_time;
> - int throttled, throttle_count;
> + int throttled, throttle_count, throttle_uptodate;
> struct list_head throttled_list;
> #endif /* CONFIG_CFS_BANDWIDTH */
> #endif /* CONFIG_FAIR_GROUP_SCHED */

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