netdev - Re: [PATCH/RFC] make unregister_netdev() delete more than 4 interfaces per second

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Message-ID: <20091018161356.GA23395@kvack.org>
Date:	Sun, 18 Oct 2009 12:13:57 -0400
From:	Benjamin LaHaise <bcrl@...et.ca>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH/RFC] make unregister_netdev() delete more than 4 interfaces per second

On Sun, Oct 18, 2009 at 06:26:22AM +0200, Eric Dumazet wrote:
> Unfortunatly this slow down fast path by an order of magnitude.
> 
> atomic_dec() is pretty cheap (and eventually could use a per_cpu thing,
> now we have a new and sexy per_cpu allocator), but atomic_dec_and_test()
> is not that cheap and more important forbids a per_cpu conversion.

dev_put() is not a fast path by any means.  atomic_dec_and_test() costs 
the same as atomic_dec() on any modern CPU -- the cost is in the cacheline 
bouncing and serialisation both require.  The case of the device count 
becoming 0 is quite rare -- any device with a route on it will never hit 
a reference count of 0.

		-ben
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