Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

[Cong Wang <xiyou.wangcong@gmail.com>]

On Tue, Jul 31, 2018 at 10:13 AM <bsegall@google.com> wrote:
>
> Xunlei Pang <xlpang@linux.alibaba.com> writes:
>
> > 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?

But expires_seq is supposed to be the same here, after
distribute_cfs_runtime(), therefore runtime_remaining is not supposed
to be cleared.

Which part do I misunderstand? expires_seq should not be same here?
Or you are saying a wrongly clear of runtime_remaning is fine?


>
> 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.

Which means the following check could be passed:

 4655         if (cfs_rq->runtime_remaining < 0)
 4656                 return;

therefore we are reaching the clock drift logic code inside
expire_cfs_rq_runtime()
where expires_seq is supposed to be same as they should be sync'ed.
Therefore without patch, we wrongly clear the runtime_remainng?

Thanks.