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

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Date:	Sat, 28 Jun 2008 12:44:53 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Marin Mitov <mitov@...p.bas.bg>,
	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

On Wed 2008-06-18 09:55:18, Ingo Molnar 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 ...

Time to RLE the bitmap? 

<runs>							Pavel

-- 
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