hi Mike,
* Mike Kravetz <[EMAIL PROTECTED]> wrote:
> 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.
I believe you are right - nice catch of this very nontrivial bug! The
patch below is against .23-rc - do you think this fix (of moving the rt
wakeup sequence to after the activate_task()) is adequate?
Ingo
Index: linux-rt-rebase.q/kernel/sched.c
===================================================================
--- linux-rt-rebase.q.orig/kernel/sched.c
+++ linux-rt-rebase.q/kernel/sched.c
@@ -1819,6 +1819,13 @@ out_set_cpu:
cpu = task_cpu(p);
}
+out_activate:
+#endif /* CONFIG_SMP */
+
+ activate_task(rq, p, 1);
+
+ trace_start_sched_wakeup(p, rq);
+
/*
* If a newly woken up RT task cannot preempt the
* current (RT) task (on a target runqueue) then try
@@ -1849,28 +1856,21 @@ out_set_cpu:
smp_send_reschedule_allbutself_cpumask(p->cpus_allowed);
schedstat_inc(this_rq, rto_wakeup);
- }
-
-out_activate:
-#endif /* CONFIG_SMP */
-
- activate_task(rq, p, 1);
-
- trace_start_sched_wakeup(p, rq);
-
- /*
- * Sync wakeups (i.e. those types of wakeups where the waker
- * has indicated that it will leave the CPU in short order)
- * don't trigger a preemption, if the woken up task will run on
- * this cpu. (in this case the 'I will reschedule' promise of
- * the waker guarantees that the freshly woken up task is going
- * to be considered on this CPU.)
- */
- if (!sync || cpu != this_cpu)
- check_preempt_curr(rq, p);
- else {
- if (TASK_PREEMPTS_CURR(p, rq))
- set_tsk_need_resched_delayed(rq->curr);
+ } else {
+ /*
+ * Sync wakeups (i.e. those types of wakeups where the waker
+ * has indicated that it will leave the CPU in short order)
+ * don't trigger a preemption, if the woken up task will run on
+ * this cpu. (in this case the 'I will reschedule' promise of
+ * the waker guarantees that the freshly woken up task is going
+ * to be considered on this CPU.)
+ */
+ if (!sync || cpu != this_cpu)
+ check_preempt_curr(rq, p);
+ else {
+ if (TASK_PREEMPTS_CURR(p, rq))
+ set_tsk_need_resched_delayed(rq->curr);
+ }
}
if (rq->curr && p && rq && _need_resched())
trace_special_pid(p->pid, PRIO(p), PRIO(rq->curr));
-
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