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

[Gregory Haskins]

>>> 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.

-Greg

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