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

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Date:	Tue, 29 Mar 2011 19:10:07 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Shaohua Li <shaohua.li@...el.com>
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, 30 Mar 2011 09:54:01 +0800 Shaohua Li <shaohua.li@...el.com> wrote:

> 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.

Confused.  If an object is aligned at a mulitple-of-128 address on one
node, it is aligned at a multiple-of-128 address when viewed from other
nodes, surely?

Even if the cache alignment to which you're referring is the internode
cache, can a 34-byte, L1-cache-aligned structure ever span multiple
internode cachelines?


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