linux-kernel - Re: [Q x86-64] on kernel

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Message-ID: <4E134E1E.6040304@zytor.com>
Date:	Tue, 05 Jul 2011 10:47:10 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Brian Gerst <brgerst@...il.com>,
	Jan Beulich <JBeulich@...ell.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [Q x86-64] on kernel_eflags

On 07/05/2011 03:47 AM, Cyrill Gorcunov wrote:
> 
> Should not every cpu has own copy of kernel_eflags? Just
> to be consistent in style? Or this would be space waisting
> and an optimization is done here?
> 

Not specific to this particular case, but in general: a shared variable
that used often but rarely written to will automatically replicate
itself in the caches of multiple processors.  This is the purpose of the
read_mostly segment (writes are permitted but expected to be rare),
which exists to make sure that a frequently written variable doesn't
randomly end up in the cache line next to a read-mostly variable.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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