linux-kernel - Re: [PATCH 03/17] mm: page_alloc: Use jump labels to avoid checking number_of

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Date:	Tue, 6 May 2014 22:23:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 03/17] mm: page_alloc: Use jump labels to avoid checking
 number_of_cpusets

On Thu, May 01, 2014 at 09:44:34AM +0100, Mel Gorman wrote:
> If cpusets are not in use then we still check a global variable on every
> page allocation. Use jump labels to avoid the overhead.
> 
> Signed-off-by: Mel Gorman <mgorman@...e.de>
> ---
>  include/linux/cpuset.h | 31 +++++++++++++++++++++++++++++++
>  kernel/cpuset.c        |  8 ++++++--
>  mm/page_alloc.c        |  3 ++-
>  3 files changed, 39 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index b19d3dc..2b89e07 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -17,6 +17,35 @@
>  
>  extern int number_of_cpusets;	/* How many cpusets are defined in system? */
>  
> +#ifdef HAVE_JUMP_LABEL
> +extern struct static_key cpusets_enabled_key;
> +static inline bool cpusets_enabled(void)
> +{
> +	return static_key_false(&cpusets_enabled_key);
> +}
> +#else
> +static inline bool cpusets_enabled(void)
> +{
> +	return number_of_cpusets > 1;
> +}
> +#endif
> +
> +static inline void cpuset_inc(void)
> +{
> +	number_of_cpusets++;
> +#ifdef HAVE_JUMP_LABEL
> +	static_key_slow_inc(&cpusets_enabled_key);
> +#endif
> +}
> +
> +static inline void cpuset_dec(void)
> +{
> +	number_of_cpusets--;
> +#ifdef HAVE_JUMP_LABEL
> +	static_key_slow_dec(&cpusets_enabled_key);
> +#endif
> +}

Why the HAVE_JUMP_LABEL and number_of_cpusets thing? When
!HAVE_JUMP_LABEL the static_key thing reverts to an atomic_t and
static_key_false() becomes:

 return unlikely(atomic_read(&key->enabled) > 0);

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