netdev - Re: [PATCH net] net: avoid dependency of net_get_random

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Date:	Wed, 14 May 2014 00:37:55 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	hannes@...essinduktion.org
Cc:	netdev@...r.kernel.org, torvalds@...ux-foundation.org
Subject: Re: [PATCH net] net: avoid dependency of net_get_random_once on
 nop patching

From: Hannes Frederic Sowa <hannes@...essinduktion.org>
Date: Sun, 11 May 2014 22:59:30 +0200

> net_get_random_once depends on the static keys infrastructure to patch up
> the branch to the slow path during boot. This was realized by abusing the
> static keys api and defining a new initializer to not enable the call
> site while still indicating that the branch point should get patched
> up. This was needed to have the fast path considered likely by gcc.
> 
> The static key initialization during boot up normally walks through all
> the registered keys and either patches in ideal nops or enables the jump
> site but omitted that step on x86 if ideal nops where already placed at
> static_key branch points. Thus net_get_random_once branches not always
> became active.
> 
> This patch switches net_get_random_once to the ordinary static_key
> api and thus places the kernel fast path in the - by gcc considered -
> unlikely path.  Microbenchmarks on Intel and AMD x86-64 showed that
> the unlikely path actually beats the likely path in terms of cycle cost
> and that different nop patterns did not make much difference, thus this
> switch should not be noticeable.
> 
> Fixes: a48e42920ff38b ("net: introduce new macro net_get_random_once")
> Reported-by: Tuomas Räsänen <tuomasjjrasanen@...r.fi>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Hannes Frederic Sowa <hannes@...essinduktion.org>

Applied and queued up for -stable, thanks.
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