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

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Message-ID: <1389631669.31367.221.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Mon, 13 Jan 2014 08:47:49 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Paasch <christoph.paasch@...ouvain.be>
Cc:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [RFC net] tcp: metrics: Avoid duplicate entries with the same
 destination-IP

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.


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