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

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Message-ID: <1301447843.3981.48.camel@sli10-conroe>
Date:	Wed, 30 Mar 2011 09:17:23 +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 09:06 +0800, Andrew Morton wrote:
> On Wed, 30 Mar 2011 09:01:22 +0800 Shaohua Li <shaohua.li@...el.com> wrote:
> 
> > +/*
> > + * Make sure vm_committed_as in one cacheline and not cacheline shared with
> > + * other variables. It can be updated by several CPUs frequently.
> > + */
> > +struct percpu_counter vm_committed_as ____cacheline_internodealigned_in_smp;
> 
> The mystery deepens.  The only cross-cpu writeable fields in there are
> percpu_counter.lock and its companion percpu_counter.count.  If CPUs
> are contending for the lock then that itself is a problem - how does
> adding some padding to the struct help anything?
I had another patch trying to address the lock contention (for case
OVERCOMMIT_GUESS), will send out soon. But thought better to have the
correct alignment for OVERCOMMIT_NEVER case.

Thanks,
Shaohua

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