linux-kernel - Re: [PATCH][resubmit] x86: enable preemption in delay

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Message-Id: <4858C286.BA47.005A.0@novell.com>
Date:	Wed, 18 Jun 2008 06:08:38 -0600
From:	"Gregory Haskins" <ghaskins@...ell.com>
To:	"Ingo Molnar" <mingo@...e.hu>, "Marin Mitov" <mitov@...p.bas.bg>
Cc:	"Andi Kleen" <andi-suse@...stfloor.org>,
	"Clark Williams" <clark.williams@...il.com>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>, <akpm@...l.org>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	"LKML" <linux-kernel@...r.kernel.org>,
	"linux-rt-users" <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH][resubmit] x86: enable preemption in delay

>>> On Wed, Jun 18, 2008 at  3:55 AM, in message <20080618075518.GD4135@...e.hu>,
Ingo Molnar <mingo@...e.hu> wrote: 

> * Marin Mitov <mitov@...p.bas.bg> wrote:
> 
>> Why not something like that (do keep in mind I am not an expert :-):
>> 
>> static void delay_tsc(unsigned long loops)
>> {
>> 	get and store the mask of allowed cpus;
>> 	/*     prevent the migration   */
>> 	set the mask of allowed cpus to the current cpu only;
>> 	/*     is it possible? could it be guaranteed?    */
>> 	loop for the delay;
>> 	restore the old mask of allowed cpus;
>> }
>> 
>> You have got the idea. Could it be realized? Is it more expensive than 
>> the current realization? So, comments, please.
> 
> hm, changing/saving/restorig cpus_allowed is really considered a 'heavy' 
> operation compared to preempt_disable(). On a 4096 CPUs box cpus_allowed 
> is 4096 bits which is half a kilobyte ...
> 
> preempt_disable()/enable() on the other hand only touches a single 
> variable, (thread_info->preempt_count which is an u32)
> 
> 	Ingo

FWIW:  I had submitted some "migration disable" patches a while back that would solve this without the cpus_allowed manipulations described here.  Its more expensive than a preempt-disable (but its preemptible), yet its way cheaper (and more correct / less racy) than chaning cpus_allowed.  I could resubmit if there was any interest, though I think Ingo said he didnt like the concept on the first pass.  Anyway, FYI.

-Greg

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