linux-kernel - Re: [PATCH]mmap: add alignment for some variables

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Date:	Wed, 30 Mar 2011 09:54:01 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-mm <linux-mm@...ck.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH]mmap: add alignment for some variables

On Wed, 2011-03-30 at 09:41 +0800, Andrew Morton wrote:
> On Wed, 30 Mar 2011 09:36:40 +0800 Shaohua Li <shaohua.li@...el.com> wrote:
> 
> > > how is it that this improves things?
> > Hmm, it actually is:
> > struct percpu_counter {
> >  	spinlock_t lock;
> >  	s64 count;
> >  #ifdef CONFIG_HOTPLUG_CPU
> >  	struct list_head list;	/* All percpu_counters are on a list */
> >  #endif
> >  	s32 __percpu *counters;
> >  } __attribute__((__aligned__(1 << (INTERNODE_CACHE_SHIFT))))
> > so lock and count are in one cache line.
> 
> ____cacheline_aligned_in_smp would achieve that?
____cacheline_aligned_in_smp can't guarantee the cache alignment for
multiple nodes, because the variable can be updated by multiple
nodes/cpus.

Thanks,
Shaohua


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