linux-kernel - Re: Possible netns creation and execution performance/scalability regression since v3.8 due to rcu callbacks being offloaded to multiple cpus

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Date:	Mon, 16 Jun 2014 12:01:23 -0300
From:	Rafael Tinoco <rafael.tinoco@...onical.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Dave Chiluk <chiluk@...onical.com>,
	linux-kernel@...r.kernel.org, davem@...emloft.net,
	Christopher Arges <chris.j.arges@...onical.com>,
	Jay Vosburgh <jay.vosburgh@...onical.com>
Subject: Re: Possible netns creation and execution performance/scalability
 regression since v3.8 due to rcu callbacks being offloaded to multiple cpus

...

On Fri, Jun 13, 2014 at 9:02 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Rafael Tinoco <rafael.tinoco@...onical.com> writes:
>
>> Okay,
>>
>> Tests with the same script were done.
>> I'm comparing : master + patch vs 3.15.0-rc5 (last sync'ed rcu commit)
>> and 3.9 last bisect good.
>>
>> Same tests were made. I'm comparing the following versions:
>>
>> 1) master + suggested patch
>> 2) 3.15.0-rc5 (last rcu commit in my clone)
>> 3) 3.9-rc2 (last bisect good)
>
> I am having a hard time making sense of your numbers.
>
> If I have read your email correctly my suggested patch caused:
> "ip netns add" numbers to improve
> 1x "ip netns exec" to improve some
> 2x "ip netns exec" to show no improvement
> "ip link add" to show no effect (after the 2x ip netns exec)

 - "netns add" are as good as they were before this regression.
 - "netns exec" are improved but still 50% of the last good bisect commit.
 - "link add" didn't show difference.

> This is interesting in a lot of ways.
> - This seems to confirm that the only rcu usage in ip netns add
>   was switch_task_namespaces.  Which is convinient as that rules
>   out most of the network stack when looking for performance oddities.
>
> - "ip netns exec" had an expected performance improvement
> - "ip netns exec" is still slow (so something odd is still going on)
> - "ip link add" appears immaterial to the performance problem.
>
> It would be interesting to switch the "ip link add" and "ip netns exec"
> in your test case to confirm that there is nothing interesting/slow
> going on in "ip link add"

 - will do that.

>
> Which leaves me with the question what ip "ip netns exec" remains
> that is using rcu and is slowing all of this down.

 - will check this also.

> Eric

Tks

Rafael
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