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

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Message-ID: <20140506222118.GB23991@suse.de>
Date:	Tue, 6 May 2014 23:21:18 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
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 Tue, May 06, 2014 at 10:23:50PM +0200, Peter Zijlstra wrote:
> 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:
> 

Because number_of_cpusets is used to size a kmalloc(). Potentially I could
abuse the internals of static keys and use the value of key->enabled but
that felt like abuse of the API.

-- 
Mel Gorman
SUSE Labs
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