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

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Date:   Wed, 29 Aug 2018 20:06:56 +0200
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Eelco Chaudron <echaudro@...hat.com>,
        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 Wed, 29 Aug 2018 12:23:15 +0200, Paolo Abeni wrote:
> 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.

I think in our tests we tried with and without pinning relevant
processing to one core, and both results shown improvement.  I don't
have the actual samples any more, just perf script dump without CPU
IDs to confirm things were pinned correctly.. :(