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

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Message-ID: <1309944338.634.162.camel@zakaz.uk.xensource.com>
Date:	Wed, 06 Jul 2011 10:25:38 +0100
From:	Ian Campbell <ijc@...lion.org.uk>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	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 Tue, 2011-07-05 at 10:47 -0700, H. Peter Anvin wrote:
> 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.

FWIW the variable in this particular case isn't actually marked as
__read_mostly...

Ian.
-- 
Ian Campbell

* Simunye is on a oc3->oc12
 simmy: bite me. :)
 daemon: okay :)

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