Messages in this thread | | | From | Cong Wang <> | Date | Fri, 3 Aug 2018 11:57:31 -0700 | Subject | Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime() |
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On Tue, Jul 31, 2018 at 8:24 PM Xunlei Pang <xlpang@linux.alibaba.com> wrote: > > Let's see the unthrottle cases. > 1. for the periodic timer > distribute_cfs_runtime updates the throttled cfs_rq->runtime_expires to > be a new value, so expire_cfs_rq_runtime does nothing because of: > rq_clock(rq_of(cfs_rq)) - cfs_rq->runtime_expires < 0 > > Afterwards assign_cfs_rq_runtime() will sync its expires_seq.
Is there any guarantee rq_clock(cfs_rq) is always ahead of cfs_rq->runtime_expires in this case?
I doubt, because cfs_rq->runtime_expires could be assigned by a sched_clock() on a different CPU running the periodic timer.
Also, rq_clock() is behind sched_clock() on the same CPU too, sometimes it is merely hundreds of nanoseconds, sometimes it is tens of thousands nanoseconds in my environment. (I have a different patch to address this, but still not sure if it is correct.)
> > 2. for the slack timer > the two expires_seq should be the same, so if clock drift happens soon, > expire_cfs_rq_runtime regards it as true clock drift: > cfs_rq->runtime_expires += TICK_NSEC > If it happens that global expires_seq advances, it also doesn't matter, > expire_cfs_rq_runtime will clear the stale expire_cfs_rq_runtime as > expected.
Hmm, looks like due to the runtime_refresh_within() check in slack timer.
> > > > >> > >> Nothing /important/ goes wrong because distribute_cfs_runtime only fills > >> runtime_remaining up to 1, not a real amount. > > > > No, runtime_remaining is updated right before expire_cfs_rq_runtime(): > > > > static void __account_cfs_rq_runtime(struct cfs_rq *cfs_rq, u64 delta_exec) > > { > > /* dock delta_exec before expiring quota (as it could span periods) */ > > cfs_rq->runtime_remaining -= delta_exec; > > expire_cfs_rq_runtime(cfs_rq); > > > > so almost certainly it can't be 1. > > I think Ben means it firstly gets a distributtion of 1 to run after > unthrottling, soon it will have a negative runtime_remaining, and go > to assign_cfs_rq_runtime().
That is obvious, being 1 in distribute_cfs_runtime is not relevant to the discussion here.
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