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@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> --- 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 majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
