netdev - Re: [RFC net] tcp: metrics: Avoid duplicate entries with the same destination-IP

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Date:	Thu, 16 Jan 2014 00:21:33 +0100
From:	Christoph Paasch <christoph.paasch@...ouvain.be>
To:	David Miller <davem@...emloft.net>
Cc:	eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [RFC net] tcp: metrics: Avoid duplicate entries with the same
 destination-IP

On 15/01/14 - 12:18:56, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Mon, 13 Jan 2014 08:47:49 -0800
> 
> > On Mon, 2014-01-13 at 16:25 +0100, Christoph Paasch wrote:
> > 
> >> Another solution might be to leave tcp_get_metrics() as it is, and in
> >> tcpm_new do another call to __tcp_get_metrics() while holding the
> >> spin-lock. We would then check __tcp_get_metrics twice for new entries
> >> but we won't hold the spin-lock needlessly anymore.
> > 
> > This is the only solution if you want to fix this.
> > Cost of lookup are the cache line misses.
> > Avoiding the spinlock is a must.
> > 
> > The second 'lookup' is basically free, as the first one have populated
> > cpu caches.
> 
> Indeed, taking the lock in tcp_get_metrics() is to be avoided at all
> costs.

Yes, I will send the updated patch tomorrow. Didn't yet had the time to send
it.


Christoph

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