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

Gregory Haskins
2008-06-18T12:08:50Z

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

> * Marin Mitov <mitov@issp.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