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

[bsegall@google.com]

Cong Wang <xiyou.wangcong@gmail.com> writes:

> 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.

Yes, in practice what's actually going to happen is that the
runtime_remaining will be put to 1 by distribute, the cfs_rq will be
unthrottled, and then when it runs it will go negative immediately and
hit the negative check in expires, so expires_seq being wrong will not
actually matter. In addition, the worst thing that will happen if one of
the account_cfs_rq_runtime(cfs_rq, 0) paths is hit first is that it will
lose 1ns of quota, which also doesn't really matter.