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

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Date:	Wed, 18 Jun 2008 09:55:18 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Marin Mitov <mitov@...p.bas.bg>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-rt-users <linux-rt-users@...r.kernel.org>, akpm@...l.org,
	Clark Williams <clark.williams@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Luis Claudio R. Goncalves" <lclaudio@...g.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi-suse@...stfloor.org>
Subject: Re: [PATCH][resubmit] x86: enable preemption in delay


* 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
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