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 <[email protected]> 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 <[email protected]> > --- > > 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); > } -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
