Messages in this thread | | | Date | Thu, 02 Aug 2007 23:05:32 +0200 | From | Martin Roehricht <> | Subject | Re: Scheduling the highest priority task |
| |
On 02.08.2007 21:48, Ingo Molnar wrote: > * Martin Roehricht <ml@felicis.org> wrote: > >> On 08/02/2007 05:19 PM, Ingo Molnar wrote: >> >* Martin Roehricht <ml@felicis.org> wrote: >> > >> >>That's fine with me, that within the same priority-queue any task can >> >>be chosen. But assume two tasks with highly different priorities, such >> >>as 105 and 135 are scheduled on the same processor and one of them is >> >>now to be migrated -- shouldn't be the queue with task P=105 >> >>considered first for migration by this code? Both tasks would use >> >>different queues with their own linked lists, right? >> > >> >yes. What makes you believe that the lower priority one (prio 135) is >> >chosen? [ as i said before, that will only be chosen if all tasks in the >> >higher-priority queue (prio 105) are either already running on a CPU or >> >have recently run so that the cache-hot logic skips them. ] >> >> This believe is primarily based on my observations of multiple >> benchmark runs and also on your statement earlier: »in the SMP >> migration code, the 'old scheduler' indeed picks the lowest priority >> one«. > > oh, sorry, that was meant to be the 'highest priority one' :-/ > > so i think you got it all right, i just typoed that first sentence.
Okay, now I think I understood this part of the code correctly. The reason why I observe a continous migration of the _lower_ priority tasks is most probably due to the fact that the higher priority one is currently running, according to: can_migrate_task() in move_tasks(), and therein:
if (task_running(rq, p)) return 0;
I tracked down via an extended /proc/schedstats that my tasks fall frequently into this pitfall. I basically solved it by making use of the more active push-strategy which is called later by load_balance() once the move_tasks() function did not succeed. So in case I need the higher priority tasks, I return immediately from move_tasks().
Thanks for your help, Martin - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |