Re: [PATCH][resubmit] x86: enable preemption in delay

[Gregory Haskins]

>>> On Wed, Jun 18, 2008 at  8:08 AM, in message <4858C286.BA47.005A.0@xxxxxxxxxx>,
"Gregory Haskins" <ghaskins@xxxxxxxxxx> wrote: 
>>>> On Wed, Jun 18, 2008 at  3:55 AM, in message <20080618075518.GD4135@xxxxxxx>,
> Ingo Molnar <mingo@xxxxxxx> wrote: 
> 
>> * Marin Mitov <mitov@xxxxxxxxxxx> 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.

Sorry, should have provided a reference:

http://lkml.org/lkml/2008/2/12/344

> 
> -Greg
> 
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