netdev - Re: [RFC net-next] net: sched: do not acquire qdisc spinlock in qdisc/class stats dump

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Message-ID: <CAM_iQpUj9u4bgG+7Tc06z0_wSRhAWCVSz5KsydrhcmpoJf39tg@mail.gmail.com>
Date:	Thu, 19 May 2016 18:50:29 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev <netdev@...r.kernel.org>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	John Fastabend <john.fastabend@...il.com>,
	Kevin Athey <kda@...gle.com>,
	Xiaotian Pei <xiaotian@...gle.com>
Subject: Re: [RFC net-next] net: sched: do not acquire qdisc spinlock in
 qdisc/class stats dump

On Thu, May 19, 2016 at 5:35 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> These stats are using u64 or u32 fields, so reading integral values
> should not prevent writers from doing concurrent updates if the kernel
> arch is a 64bit one.
>
> Being able to atomically fetch all counters like packets and bytes sent
> at the expense of interfering in fast path (queue and dequeue packets)
> is simply not worth the pain, as the values are generally stale after 1
> usec.

I think one purpose of this lock is to make sure we have an atomic
snapshot of these counters as a whole. IOW, we may need another
lock rather than the qdisc root lock to guarantee this.