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

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Message-Id: <20110329180611.a71fe829.akpm@linux-foundation.org>
Date:	Tue, 29 Mar 2011 18:06:11 -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 09:01:22 +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?

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