linux-kernel - Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs

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Message-ID: <xm26ftzz8h0r.fsf@bsegall-linux.svl.corp.google.com>
Date:   Tue, 31 Jul 2018 10:13:56 -0700
From:   bsegall@...gle.com
To:     Xunlei Pang <xlpang@...ux.alibaba.com>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Ben Segall <bsegall@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()

Xunlei Pang <xlpang@...ux.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@...ux.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?

Nothing /important/ goes wrong because distribute_cfs_runtime only fills
runtime_remaining up to 1, not a real amount.