netdev - Re: [PATCH 0/2] net/sched: Add hardware specific counters to TC actions

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Message-ID: <3d74699054d901e963bdac39898cac441f00658f.camel@redhat.com>
Date:   Wed, 29 Aug 2018 12:23:15 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Eelco Chaudron <echaudro@...hat.com>
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        jhs@...atatu.com, xiyou.wangcong@...il.com, jiri@...nulli.us,
        simon.horman@...ronome.com,
        Marcelo Ricardo Leitner <mleitner@...hat.com>,
        louis.peens@...ronome.com
Subject: Re: [PATCH 0/2] net/sched: Add hardware specific counters to TC
 actions

On Thu, 2018-08-23 at 20:14 +0200, Jakub Kicinski wrote:
> I asked Louis to run some tests while I'm travelling, and he reports
> that my worry about reporting the extra stats was unfounded.  Update
> function does not show up in traces at all.  It seems under stress
> (generated with stress-ng) the thread dumping the stats in userspace
> (in OvS it would be the revalidator) actually consumes less CPU in
> __gnet_stats_copy_basic (0.4% less for ~2.0% total).
> 
> Would this match with your results?  I'm not sure why dumping would be
> faster with your change..

Wild guess on my side: the relevant patch changes a bit the binary
layout of the 'tc_action' struct, possibly (I still need to check with
pahole) moving the tcf_lock and the stats field on different
cachelines, reducing false sharing that could affect badly such test.

Cheers,

Paolo