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Date: Mon, 1 Oct 2007 15:15:19 -0700 From: Mike Kravetz <kravetz@...ibm.com> To: Ingo Molnar <mingo@...e.hu> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: RT scheduling: wakeup bug? I've been trying to track down some unexpected realtime latencies and believe one source is a bug in the wakeup code. Specifically, this is within the try_to_wake_up() routine. Within this routine there is the following code segment: /* * If a newly woken up RT task cannot preempt the * current (RT) task (on a target runqueue) then try * to find another CPU it can preempt: */ if (rt_task(p) && !TASK_PREEMPTS_CURR(p, rq)) { struct rq *this_rq = cpu_rq(this_cpu); /* * Special-case: the task on this CPU can be * preempted. In that case there's no need to * trigger reschedules on other CPUs, we can * mark the current task for reschedule. * * (Note that it's safe to access this_rq without * extra locking in this particular case, because * we are on the current CPU.) */ if (TASK_PREEMPTS_CURR(p, this_rq)) set_tsk_need_resched(this_rq->curr); else /* * Neither the intended target runqueue * nor the current CPU can take this task. * Trigger a reschedule on all other CPUs * nevertheless, maybe one of them can take * this task: */ smp_send_reschedule_allbutself_cpumask(p->cpus_allowed); schedstat_inc(this_rq, rto_wakeup); } This logic seems appropriate. But, the task 'p' is most likely not on the runqueue when sending the IPI. It gets added to the runqueue a little later in the routine. As a result, the 'rt_overload' global may not be set (based on the count of RT tasks on the runqueue) and other CPUs may 'pass over' the runqueue when doing RT load balancing. My observations/debugging/conclusions are based on an earlier version of the code. It appears the same code/issue still exists in the most version. But, I have not not done any work with the latest version. -- Mike - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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