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

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Message-ID: <20140507094316.GG23991@suse.de>
Date:	Wed, 7 May 2014 10:43:16 +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 Wed, May 07, 2014 at 11:04:21AM +0200, Peter Zijlstra wrote:
> On Tue, May 06, 2014 at 11:21:18PM +0100, Mel Gorman wrote:
> > On Tue, May 06, 2014 at 10:23:50PM +0200, Peter Zijlstra wrote:
> 
> > > 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.
> 
> But are those ifdefs worth the saving of 4 bytes of .data?
> 
> That said, I see no real problem adding static_key_count().

I thought it would be considered API abuse as I always viewed the labels
as being a enabled/disabled thing with the existence of the ref count
being an internal implementation detail. I'll take this approach.

Thanks.

-- 
Mel Gorman
SUSE Labs
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