Xunlei Pang <[email protected]> writes: > On 7/31/18 1:55 AM, Cong Wang wrote: >> On Sun, Jul 29, 2018 at 10:29 PM Xunlei Pang <[email protected]> >> 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?
Nothing /important/ goes wrong because distribute_cfs_runtime only fills runtime_remaining up to 1, not a real amount.
