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

Frederic Weisbecker
2009-02-17T16:01:13Z

On Tue, Feb 17, 2009 at 07:10:46AM -0800, Paul E. McKenney wrote:
> 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?

Only when you are doing some debugging yes. But it is not a good thing to put an ftrace_printk
for code that has to be officially released since it adds a small overhead.
And actually ftrace_printk() is only for casual debugging, IMHO we shoudn't find any ftrace_printk
on the mainline code.

Instead, if you need some constant and defined probe inside your code, it's better to use
tracepoints, since they only add the overhead of a single branch check when they are off.

> > 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.

Ok. I'm just fixing the orphans comments on the function graph tracer (the init_tasks
were not traced) and I test 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);
> }