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

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Date:	Sun, 22 May 2016 12:35:07 -0400
From:	Jamal Hadi Salim <jhs@...atatu.com>
To:	Eric Dumazet <eric.dumazet@...il.com>,
	Cong Wang <xiyou.wangcong@...il.com>
Cc:	netdev <netdev@...r.kernel.org>,
	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 16-05-20 01:50 AM, Eric Dumazet wrote:

> It wont be inaccurate (no drift), as the writers hold the qdisc
> spinlock.
>
> It it the same with say IP/TCP SNMP counters :
>
> When an incoming frame is handled, it might change many SNMP counters,
> but an SNMP agent might fetch the whole set of SNMP values in the middle
> of the changes.
>
> IpInReceives
> IpInDelivers
> TcpInSegs
> IpExtInOctets
> IpExtInNoECTPkts
> ...
>
> The only 'problem' can be a off-by-one (or off-by-bytes-in-the-packet)
> transient error, that all SNMP agents are normally handling just fine.
>


I think qdisc stats being off by a bit are ok - hence the rcu
suggestion. I would have problems with action stats being off
(where they are used for billing).

cheers,
jamal