Messages in this thread | | | Date | Wed, 18 Jun 2008 06:08:38 -0600 | From | "Gregory Haskins" <> | Subject | Re: [PATCH][resubmit] x86: enable preemption in delay |
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>>> 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
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