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

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Message-Id: <20140115.121856.712581760900022040.davem@davemloft.net>
Date:	Wed, 15 Jan 2014 12:18:56 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	christoph.paasch@...ouvain.be, netdev@...r.kernel.org
Subject: Re: [RFC net] tcp: metrics: Avoid duplicate entries with the same
 destination-IP

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.
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