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

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Date:   Mon, 30 Jul 2018 10:55:09 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Xunlei Pang <xlpang@...ux.alibaba.com>
Cc:     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()

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?