linux-kernel - Re: [PATCH 2/5] cpumask: make task_struct.cpus_allowed a cpumask_var

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Message-Id: <201006281901.27517.rusty@rustcorp.com.au>
Date:	Mon, 28 Jun 2010 19:01:26 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Russell King <rmk@....linux.org.uk>,
	Bryan Wu <cooloney@...nel.org>,
	Tony Luck <tony.luck@...el.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mundt <lethal@...ux-sh.org>,
	"David S. Miller" <davem@...emloft.net>,
	Arnd Bergmann <arnd@...db.de>, anton@...ba.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mike Travis <travis@....com>
Subject: Re: [PATCH 2/5] cpumask: make task_struct.cpus_allowed a cpumask_var_t

On Sat, 26 Jun 2010 01:30:25 am Christoph Lameter wrote:
> On Fri, 25 Jun 2010, Rusty Russell wrote:
> 
> > Even better would be to point to the fixed "one cpu" and "all cpus"
> > masks where possible, and make a copy when setting it to something
> > else.  But you'd have to track down those naughty places which frob it
> > directly...
> 
> Put it into a read only segment?

Well, I was thinking of an audit as the first step :)

> What is the estimated performance impact of adding a level of indirection
> (and a potential cache miss) to a number of critical code paths?

Unknown.  Originally, I moved the mask to the end of the struct (so we could
under-allocate for the CPUMASK_OFFSTACK nr_cpu_ids < NR_CPUS case), but Ingo
didn't want to change the struct for the non-CPUMASK_OFFSTACK case.

Measurements welcome...
Rusty.
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