netdev - Re: Scalability of interface creation and deletion

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Message-ID: <1304796297.3207.35.camel@edumazet-laptop>
Date:	Sat, 07 May 2011 21:24:57 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alex Bligh <alex@...x.org.uk>
Cc:	netdev@...r.kernel.org
Subject: Re: Scalability of interface creation and deletion

Le samedi 07 mai 2011 à 19:51 +0100, Alex Bligh a écrit :
> 
> --On 7 May 2011 20:32:54 +0200 Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
> > Well, there is also one rcu_barrier() call that is expensive.
> > (It was changed from one synchronize_rcu() to one rcu_barrier() lately
> > in commit ef885afb , in 2.6.36 kernel)
> 
> I think you are saying it may be waiting in rcu_barrier(). I'll
> instrument that later plus synchronize_sched().
> 
> > http://git2.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commi
> > tdiff;h=ef885afbf8a37689afc1d9d545e2f3e7a8276c17
> 
> OK, so in head, which I am using, rollback_registered_many which
> previously had 2 calls to synchronize_net(), now has one, followed
> by a call to rc_barrier() at the bottom.
> 

each device dismantle needs 2 synchronize_rcu() and one rcu_barrier()


> Right, that's what I patched before (see patch attached to
> message from earlier today) to do an exponential backoff (see
> previous entry), i.e. do a 5ms sleep, then a 10ms, then a 20ms, but
> never more than 250ms. It made no difference.
> 

Oh well. How many time are you going to tell us about this ?

We suggested to wait no more than 1 ms, or even shout asap.

If after synchronize_rcu() and rcu_barrier() calls, they are still
references to the device, then there is a BUG somewhere.

Since these bugs are usually not fatal, we just wait a bit.


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