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

Ingo Molnar
2008-06-18T07:56:12Z

* 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