Messages in this thread | | | Subject | Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime() | From | Xunlei Pang <> | Date | Tue, 31 Jul 2018 22:58:49 +0800 |
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On 7/31/18 1:55 AM, Cong Wang wrote: > On Sun, Jul 29, 2018 at 10:29 PM Xunlei Pang <xlpang@linux.alibaba.com> wrote: >> >> Hi Cong, >> >> On 7/28/18 8:24 AM, Cong Wang wrote: >>> Each time we sync cfs_rq->runtime_expires with cfs_b->runtime_expires, >>> we should sync its ->expires_seq too. However it is missing >>> for distribute_cfs_runtime(), especially the slack timer call path. >> >> I don't think it's a problem, as expires_seq will get synced in >> assign_cfs_rq_runtime(). > > Sure, but there is a small window during which they are not synced. > Why do you want to wait until the next assign_cfs_rq_runtime() when > you already know runtime_expires is synced? > > Also, expire_cfs_rq_runtime() is called before assign_cfs_rq_runtime() > inside __account_cfs_rq_runtime(), which means the check of > cfs_rq->expires_seq is not accurate for unthrottling case if the clock > drift happens soon enough? >
expire_cfs_rq_runtime(): if (cfs_rq->expires_seq == cfs_b->expires_seq) { /* extend local deadline, drift is bounded above by 2 ticks */ cfs_rq->runtime_expires += TICK_NSEC; } else { /* global deadline is ahead, expiration has passed */ cfs_rq->runtime_remaining = 0; }
So if clock drift happens soon, then expires_seq decides the correct thing we should do: if cfs_b->expires_seq advanced, then clear the stale cfs_rq->runtime_remaining from the slack timer of the past period, then assign_cfs_rq_runtime() will refresh them afterwards, otherwise it is a real clock drift. I am still not getting where the race is?
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