On Wed, Apr 27, 2011 at 12:28:37AM +0200, Thomas Gleixner wrote:
> 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
I believe that the above is the script that prints out the RCU debugfs
information periodically. Unless there is something else that begins
with "collectd" instead of just collectdebugfs.sh.
Thanx, Paul
> 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;
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