Re: -rt scheduling: wakeup bug?
* Mike Kravetz <[EMAIL PROTECTED]> wrote: > > if (rq->curr && p && rq && _need_resched()) > > trace_special_pid(p->pid, PRIO(p), PRIO(rq->curr)); > > Not an issue with the patch, just that last bit of code pulled in for > context. I don't think it is a bug, but the checking of 'rq' after > checking 'rq->curr' just doesn't look right (or necessary). Could it > just be an artifact from earlier versions of the code? yeah, you are right - and rq shouldnt ever be NULL there anyway. Ingo - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: -rt scheduling: wakeup bug?
On Tue, Oct 02, 2007 at 07:06:32AM +0200, Ingo Molnar wrote: > 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)); Not an issue with the patch, just that last bit of code pulled in for context. I don't think it is a bug, but the checking of 'rq' after checking 'rq->curr' just doesn't look right (or necessary). Could it just be an artifact from earlier versions of the code? -- Mike - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: -rt scheduling: wakeup bug?
-- On Tue, 2 Oct 2007, Mike Kravetz wrote: > On Tue, Oct 02, 2007 at 07:06:32AM +0200, Ingo Molnar wrote: > > * Mike Kravetz <[EMAIL PROTECTED]> wrote: > > > > > > 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? > > Yes, I have been running with a similar patch on a (much) earlier > version of the code. It has helped quite a bit. I would have > put together a patch for a later version, but my test environment > is limited to this earlier version. FYI, I've incorporated Ingo's patch into the lastest -rt release. http://www.kernel.org/pub/linux/kernel/projects/rt/patch-2.6.23-rc9-rt1.bz2 -- Steve - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: -rt scheduling: wakeup bug?
On Tue, Oct 02, 2007 at 07:06:32AM +0200, Ingo Molnar wrote: > * Mike Kravetz <[EMAIL PROTECTED]> wrote: > > > > 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? Yes, I have been running with a similar patch on a (much) earlier version of the code. It has helped quite a bit. I would have put together a patch for a later version, but my test environment is limited to this earlier version. -- Mike - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: -rt scheduling: wakeup bug?
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)); - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at
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 [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/