Re: [Bug #12650] Strange load average and ksoftirqd behavior with 2.6.29-rc2-git1

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Feb 17, 2009 at 05:34:23AM +0100, Frederic Weisbecker wrote:
> On Mon, Feb 16, 2009 at 02:39:44PM -0800, Paul E. McKenney wrote:
> > On Mon, Feb 16, 2009 at 09:09:23PM +0100, Ingo Molnar wrote:
> > > 
> > > * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> > > 
> > > > Here the calls to rcu_process_callbacks() are only 75 
> > > > microseconds apart, so that this function is consuming more 
> > > > than 10% of a CPU.  The strange thing is that I don't see a 
> > > > raise_softirq() in between, though perhaps it gets inlined or 
> > > > something that makes it invisible to ftrace.
> > > 
> > > look at the latest trace please, that has even the most inline 
> > > raise-softirq method instrumented, so all the raising is 
> > > visible.
> > 
> > Ah, my apologies!  This time looking at:
> > 
> > http://damien.wyart.free.fr/ksoftirqd_pb/trace_tip_2009.02.16_ksoftirqd_pb_abstime_proc.txt.gz
> > 
> > 
> >   799.521187 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.521371 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.521555 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.521738 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.521934 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.522068 |   1)  ksoftir-2324  |               |                rcu_check_callbacks() {
> >   799.522208 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.522392 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.522575 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.522759 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.522956 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.523074 |   1)  ksoftir-2324  |               |                  rcu_check_callbacks() {
> >   799.523214 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.523397 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.523579 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.523762 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.523960 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.524079 |   1)  ksoftir-2324  |               |                  rcu_check_callbacks() {
> >   799.524220 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.524403 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.524587 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> >   799.524770 |   1)    <idle>-0    |               |  rcu_check_callbacks() {
> > [ . . . ]
> > 
> > Yikes!!!
> > 
> > Why is rcu_check_callbacks() being invoked so often?  It should be called
> > but once per jiffy, and here it is called no less than 22 times in about
> > 3.5 milliseconds, meaning one call every 160 microseconds or so.
> > 
> > Hmmm...
> > 
> > Looks like we never return from:
> > 
> >   799.521142 |   1)    <idle>-0    |          | tick_nohz_stop_sched_tick() {
> > 
> > Perhaps we are taking an interrupt immediately after the
> > local_irq_restore()?  And at 799.521209 deciding to exit nohz mode.
> > And then deciding to go back into nohz mode at 799.521326, 117
> > microseconds later, after which we re-invoke rcu_check_callbacks(),
> > which again raises RCU's softirq.
> > 
> > And the reason we are invoking rcu_check_callbacks() so often appears
> > to be in in arch/x86/kernel/process_32.c cpu_idle() near line 107,
> > which explains my failure to reproduce on a 64-bit system:
> > 
> > 	void cpu_idle(void)
> > 	{
> > 		int cpu = smp_processor_id();
> > 
> > 		current_thread_info()->status |= TS_POLLING;
> > 
> > 		/* endless idle loop with no priority at all */
> > 		while (1) {
> > 			tick_nohz_stop_sched_tick(1);
> > 			while (!need_resched()) {
> > 
> > 				check_pgt_cache();
> > 				rmb();
> > 
> > 				if (rcu_pending(cpu))
> > 					rcu_check_callbacks(cpu, 0);
> > 
> > 				if (cpu_is_offline(cpu))
> > 					play_dead();
> > 
> > 				local_irq_disable();
> > 				__get_cpu_var(irq_stat).idle_timestamp = jiffies;
> > 				/* Don't trace irqs off for idle */
> > 				stop_critical_timings();
> > 				pm_idle();
> > 				start_critical_timings();
> > 			}
> > 			tick_nohz_restart_sched_tick();
> > 			preempt_enable_no_resched();
> > 			schedule();
> > 			preempt_disable();
> > 		}
> > 	}
> > 
> > If we go in and out of nohz mode quickly, we will invoke rcu_pending()
> > each time.  I would expect rcu_pending() to return 0 most of the time,
> > but that apparently isn't the case with treercu...
> > 
> > What is the easiest way for me to make it easy to trace the return path
> > from __rcu_pending()?  Make each return path call an empty function
> > located off where the compiler cannot see it, I guess...  Diagnostic
> > patch along these lines below.  Frederic, Damien, could you please give
> > it a go?  (And of course please let me know if something else is
> > needed.)
> 
> 
> No, you don't need that, you can use ftrace_printk, it will generate a C-comment like
> inside the functions, ie:
> 
> __rcu_pending() {
> 	 /* pending_qs */
> }

Ah!!!  So if I were to put ftrace_printk() calls at strategic points
in the RCU code, that would be a good thing?

> I've converted your below patch with ftrace_printks and tested it under an old P2
> with rcu_tree and 1000 Hz. I made a trace during an idle state, and well, looks like I'm
> lucky :-) 
> I guess I successfully reproduced the softirq/rcu overhead.
> Please find the below patch to trace the rcu_pending return path, as well as the trace I made.
> Sorry, the trace is a bit buggy with sometimes flying orphans C like comments.
> When I will have more time, I will fix that.
> 
> The trace is here http://dl.free.fr/uyWGgCbx4
> 
> It looks like it mostly returns 1 because of the waiting for quiescent state:
> 
> $ cat rcutrace | grep "/* pending_none" | wc -l
> 221
> $ cat rcutrace | grep "/* pending_qs" | wc -l
> 248
> $ cat rcutrace | grep "/* pending" | wc -l
> 469

Hmmm...  This looks like normal behavior.  Though I wonder if
rcu_check_callbacks() is recognizing that we are in the idle loop given
the large number of "pending_qs" entries.  To that end, would you be
willing to try the attached patch (on top of your ftrace_printk() patch)?

Add ftrace_printk() to rcu_check_callbacks() to allow ftrace to
determine when RCU has detected a quiescent state due to interrupting
from within it.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

 rcutree.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index b2fd602..fa14a0f 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -966,6 +966,7 @@ void rcu_check_callbacks(int cpu, int user)
 
 		rcu_qsctr_inc(cpu);
 		rcu_bh_qsctr_inc(cpu);
+		ftrace_printk("rcu user/idle");
 
 	} else if (!in_softirq()) {
 
@@ -977,6 +978,7 @@ void rcu_check_callbacks(int cpu, int user)
 		 */
 
 		rcu_bh_qsctr_inc(cpu);
+		ftrace_printk("rcu !softirq");
 	}
 	raise_softirq(RCU_SOFTIRQ);
 }