diff options
authorJuri Lelli <juri.lelli@gmail.com>2014-02-20 09:19:39 +0100
committerThomas Gleixner <tglx@linutronix.de>2014-02-21 21:27:09 +0100
commit3d5f35bdfdef5fd627afe9b4bf9c4f32d17f4593 (patch)
parentd158fc7f36a25e19791d25a55da5623399a2644f (diff)
sched/deadline: Fix bad accounting of nr_running
Rostedt writes: My test suite was locking up hard when enabling mmiotracer. This was due to the mmiotracer placing all but one CPU offline. I found this out when I was able to reproduce the bug with just my stress-cpu-hotplug test. This bug baffled me because it would not always trigger, and would only trigger on the first run after boot up. The stress-cpu-hotplug test would crash hard the first run, or never crash at all. But a new reboot may cause it to crash on the first run again. I spent all week bisecting this, as I couldn't find a consistent reproducer. I finally narrowed it down to the sched deadline patches, and even more peculiar, to the commit that added the sched deadline boot up self test to the latency tracer. Then it dawned on me to what the bug was. All it took was to run a task under sched deadline to screw up the CPU hot plugging. This explained why it would lock up only on the first run of the stress-cpu-hotplug test. The bug happened when the boot up self test of the schedule latency tracer would test a deadline task. The deadline task would corrupt something that would cause CPU hotplug to fail. If it didn't corrupt it, the stress test would always work (there's no other sched deadline tasks that would run to cause problems). If it did corrupt on boot up, the first test would lockup hard. I proved this theory by running my deadline test program on another box, and then run the stress-cpu-hotplug test, and it would now consistently lock up. I could run stress-cpu-hotplug over and over with no problem, but once I ran the deadline test, the next run of the stress-cpu-hotplug would lock hard. After adding lots of tracing to the code, I found the cause. The function tracer showed that migrate_tasks() was stuck in an infinite loop, where rq->nr_running never equaled 1 to break out of it. When I added a trace_printk() to see what that number was, it was 335 and never decrementing! Looking at the deadline code I found: static void __dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags) { dequeue_dl_entity(&p->dl); dequeue_pushable_dl_task(rq, p); } static void dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags) { update_curr_dl(rq); __dequeue_task_dl(rq, p, flags); dec_nr_running(rq); } And this: if (dl_runtime_exceeded(rq, dl_se)) { __dequeue_task_dl(rq, curr, 0); if (likely(start_dl_timer(dl_se, curr->dl.dl_boosted))) dl_se->dl_throttled = 1; else enqueue_task_dl(rq, curr, ENQUEUE_REPLENISH); if (!is_leftmost(curr, &rq->dl)) resched_task(curr); } Notice how we call __dequeue_task_dl() and in the else case we call enqueue_task_dl()? Also notice that dequeue_task_dl() has underscores where enqueue_task_dl() does not. The enqueue_task_dl() calls inc_nr_running(rq), but __dequeue_task_dl() does not. This is where we get nr_running out of sync. [snip] Another point where nr_running can get out of sync is when the dl_timer fires: dl_se->dl_throttled = 0; if (p->on_rq) { enqueue_task_dl(rq, p, ENQUEUE_REPLENISH); if (task_has_dl_policy(rq->curr)) check_preempt_curr_dl(rq, p, 0); else resched_task(rq->curr); This patch does two things: - correctly accounts for throttled tasks (that are now considered !running); - fixes the bug, updating nr_running from {inc,dec}_dl_tasks(), since we risk to update it twice in some situations (e.g., a task is dequeued while it has exceeded its budget). Cc: mingo@redhat.com Cc: torvalds@linux-foundation.org Cc: akpm@linux-foundation.org Reported-by: Steven Rostedt <rostedt@goodmis.org> Reviewed-by: Steven Rostedt <rostedt@goodmis.org> Tested-by: Steven Rostedt <rostedt@goodmis.org> Signed-off-by: Juri Lelli <juri.lelli@gmail.com> Signed-off-by: Peter Zijlstra <peterz@infradead.org> Link: http://lkml.kernel.org/r/1392884379-13744-1-git-send-email-juri.lelli@gmail.com Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
1 files changed, 2 insertions, 4 deletions
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 0dd5e0971a0..b819577c21d 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -717,6 +717,7 @@ void inc_dl_tasks(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
+ inc_nr_running(rq_of_dl_rq(dl_rq));
inc_dl_deadline(dl_rq, deadline);
inc_dl_migration(dl_se, dl_rq);
@@ -730,6 +731,7 @@ void dec_dl_tasks(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
+ dec_nr_running(rq_of_dl_rq(dl_rq));
dec_dl_deadline(dl_rq, dl_se->deadline);
dec_dl_migration(dl_se, dl_rq);
@@ -836,8 +838,6 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
if (!task_current(rq, p) && p->nr_cpus_allowed > 1)
enqueue_pushable_dl_task(rq, p);
- inc_nr_running(rq);
static void __dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags)
@@ -850,8 +850,6 @@ static void dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags)
__dequeue_task_dl(rq, p, flags);
- dec_nr_running(rq);