On Tue, 26 Apr 2011, Linus Torvalds wrote:
> On Tue, Apr 26, 2011 at 10:09 AM, Bruno Prémont
> <bonbons@xxxxxxxxxxxxxxxxx> wrote:
> >
> > Just in case, /proc/$(pidof rcu_kthread)/status shows ~20k voluntary
> > context switches and exactly one non-voluntary one.
> >
> > In addition when rcu_kthread has stopped doing its work
> > `swapoff $(swapdevice)` seems to block forever (at least normal shutdown
> > blocks on disabling swap device).
> > If I get to do it when I get back home I will manually try to swapoff
> > and take process traces with sysrq-t.
>
> That "exactly one non-voluntary one" sounds like the smoking gun.
>
> Normally SCHED_FIFO runs until it voluntarily gives up the CPU. That's
> kind of the point of SCHED_FIFO. Involuntary context switches happen
> when some higher-priority SCHED_FIFO process becomes runnable (irq
> handlers? You _do_ have CONFIG_IRQ_FORCED_THREADING=y in your config
> too), and maybe there is a bug in the runqueue handling for that case.
The forced irq threading is only effective when you add the command
line parameter "threadirqs". I don't see any irq threads in the ps
outputs, so that's not the problem.
Though the whole ps output is weird. There is only one thread/process
which accumulated CPU time
collectd 1605 0.6 0.7 49924 3748 ? SNLsl 22:14 0:14
All others show 0:00 CPU time - not only kthread_rcu.
Bruno, are you running on real hardware or in a virtual machine?
Can you please enable CONFIG_SCHED_DEBUG and provide the output of
/proc/sched_stat when the problem surfaces and a minute after the
first snapshot?
Also please apply the patch below and check, whether the printk shows
up in your dmesg.
Thanks,
tglx
---
kernel/sched_rt.c | 1 +
1 file changed, 1 insertion(+)
Index: linux-2.6-tip/kernel/sched_rt.c
===================================================================
--- linux-2.6-tip.orig/kernel/sched_rt.c
+++ linux-2.6-tip/kernel/sched_rt.c
@@ -609,6 +609,7 @@ static int sched_rt_runtime_exceeded(str
if (rt_rq->rt_time > runtime) {
rt_rq->rt_throttled = 1;
+ printk_once(KERN_WARNING "sched: RT throttling activated\n");
if (rt_rq_throttled(rt_rq)) {
sched_rt_rq_dequeue(rt_rq);
return 1;