From: bsegall@google.com
To: Xunlei Pang <xlpang@linux.alibaba.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>,
LKML <linux-kernel@vger.kernel.org>,
Ben Segall <bsegall@google.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime()
Date: Tue, 31 Jul 2018 10:13:56 -0700 [thread overview]
Message-ID: <xm26ftzz8h0r.fsf@bsegall-linux.svl.corp.google.com> (raw)
In-Reply-To: <fad5ebe6-ec49-c67b-7b4f-78a413723aa8@linux.alibaba.com> (Xunlei Pang's message of "Tue, 31 Jul 2018 22:58:49 +0800")
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?
Nothing /important/ goes wrong because distribute_cfs_runtime only fills
runtime_remaining up to 1, not a real amount.
next prev parent reply other threads:[~2018-07-31 17:14 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-28 0:24 [PATCH] sched/fair: sync expires_seq in distribute_cfs_runtime() Cong Wang
2018-07-30 5:28 ` Xunlei Pang
2018-07-30 17:32 ` bsegall
2018-07-30 17:55 ` Cong Wang
2018-07-31 14:58 ` Xunlei Pang
2018-07-31 17:13 ` bsegall [this message]
2018-07-31 20:55 ` Cong Wang
2018-08-01 3:24 ` Xunlei Pang
2018-08-03 18:57 ` Cong Wang
2018-08-01 17:17 ` bsegall
2018-08-03 21:56 ` Cong Wang
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