[Devel] [PATCH] ms/sched: Guarantee new group-entities always have weight

[Cyrill Gorcunov]

On Fri, Oct 07, 2016 at 11:11:15AM +0300, Kirill Tkhai wrote:
> There is a node with never scheduled cpu cgroup. Its cfs_rq looks like:
> 
> struct cfs_rq {
>   load = {
>     weight = 0,
>     inv_weight = 0
>   },
>   nr_running = 2,
>   throttled = 0,
> 
> while its sched_entity has on_rq = 0.
> 
> The only way, where not-empty cfs_rq may be deqeued,
> is when it was throttled. But now it's not throttled,
> this means that unthrottle_cfs_rq() earlier skipped
> its queueing. This may happen only
> if cfs_rq->load.weight is 0, and we see that.
> 
> cfs_rq.load.weight is assigned by update_load_set(),
> and it's possible in two places:
> 
> update_cfs_shares()
>   shares = calc_cfs_shares(cfs_rq, tg); /* returns shares > 0 */
>   reweight_entity(shares);
>      update_load_set(shares);
> and
> 
> init_tg_cfs_entry()
>   update_load_set(0);
> 
> As it's clearly seen, assignment of 0 is possible in only place,
> and there is a race, which has already fixed in mainstream.
> So, let's backport their patch to fix the problem:
> 
> ms commit 0ac9b1c21874 from Paul Turner <pjt at google.com>
> "sched: Guarantee new group-entities always have weight"
> 
> Currently, group entity load-weights are initialized to zero. This
> admits some races with respect to the first time they are re-weighted in
> earlty use. ( Let g[x] denote the se for "g" on cpu "x". )
> 
> Suppose that we have root->a and that a enters a throttled state,
> immediately followed by a[0]->t1 (the only task running on cpu[0])
> blocking:
> 
>   put_prev_task(group_cfs_rq(a[0]), t1)
>   put_prev_entity(..., t1)
>   check_cfs_rq_runtime(group_cfs_rq(a[0]))
>   throttle_cfs_rq(group_cfs_rq(a[0]))
> 
> Then, before unthrottling occurs, let a[0]->b[0]->t2 wake for the first
> time:
> 
>   enqueue_task_fair(rq[0], t2)
>   enqueue_entity(group_cfs_rq(b[0]), t2)
>   enqueue_entity_load_avg(group_cfs_rq(b[0]), t2)
>   account_entity_enqueue(group_cfs_ra(b[0]), t2)
>   update_cfs_shares(group_cfs_rq(b[0]))
>   < skipped because b is part of a throttled hierarchy >
>   enqueue_entity(group_cfs_rq(a[0]), b[0])
>   ...
> 
> We now have b[0] enqueued, yet group_cfs_rq(a[0])->load.weight == 0
> which violates invariants in several code-paths. Eliminate the
> possibility of this by initializing group entity weight.
> 
> Signed-off-by: Paul Turner <pjt at google.com>
> Signed-off-by: Peter Zijlstra <peterz at infradead.org>
> Link: http://lkml.kernel.org/r/20131016181627.22647.47543.stgit@sword-of-the-dawn.mtv.corp.google.com
> Signed-off-by: Ingo Molnar <mingo at kernel.org>
> 
> https://jira.sw.ru/browse/PSBM-53136
> 
> Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
Acked-by: Cyrill Gorcunov <gorcunov at openvz.org>