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

Frederic Weisbecker
2009-02-16T19:09:18Z

On Mon, Feb 16, 2009 at 10:56:16AM -0800, Paul E. McKenney wrote:
> On Mon, Feb 16, 2009 at 08:06:13AM -0800, Paul E. McKenney wrote:
> > On Mon, Feb 16, 2009 at 02:21:51PM +0100, Ingo Molnar wrote:
> > >
> > > * Damien Wyart <damien.wyart@free.fr> wrote:
> > >
> > > > * Ingo Molnar <mingo@elte.hu> [090216 13:26]:
> > > > > We do get 0x100 which is 1 << RCU_SOFTIRQ, i.e. the RCU softirq. Paul,
> > > > > this indeed seems to be a CONFIG_TREE_RCU=y bug.
> > > >
> > > > > What is weird is that RCU_SOFTIRQ gets set again and again - but there's
> > > > > no raise_softirq() calls. Could you please do a two-CPU trace too via:
> > > >
> > > > > echo 3 > /debug/tracing/tracing_cpumask
> > > >
> > > > > So that we can see what's happening on the other CPU?
> > > >
> > > > > Also, could you please apply the debug patch below (or update to the
> > > > > very latest -tip tree), so that we get trace entries of softirq triggers
> > > > > too?
> > > >
> > > > Ok, the new trace with these additional modifications is here:
> > > > http://damien.wyart.free.fr/ksoftirqd_pb/trace_tip_2009.02.16_1300_ksoftirqd_pb_abstime_proc_mask3.txt.gz
> > >
> > > thanks.
> > >
> > > This confirms that SOFTIRQ_RCU gets raised here in the timer IRQ:
> > >
> > > 136.255963 | 0) sleep-2345 | | update_process_times() {
> > > 136.255964 | 0) sleep-2345 | | account_process_tick() {
> > > 136.255965 | 0) sleep-2345 | 0.779 us | account_system_time();
> > > 136.255966 | 0) sleep-2345 | 2.262 us | }
> > > 136.255967 | 0) sleep-2345 | | run_local_timers() {
> > > 136.255968 | 0) sleep-2345 | 0.802 us | hrtimer_run_queues();
> > > 136.255969 | 0) sleep-2345 | | raise_softirq() {
> > > 136.255970 | 0) sleep-2345 | | raise_softirq_irqoff() {
> > > 136.255971 | 0) sleep-2345 | | __raise_softirq_irqoff() {
> > > 136.255972 | 0) sleep-2345 | | /* nr: 1 */
> > > 136.255973 | 0) sleep-2345 | 2.194 us | }
> > > 136.255974 | 0) sleep-2345 | 3.832 us | }
> > > 136.255975 | 0) sleep-2345 | 5.491 us | }
> > > 136.255976 | 0) sleep-2345 | 8.667 us | }
> > > 136.255976 | 0) sleep-2345 | 0.792 us | rcu_pending();
> > > 136.255978 | 0) sleep-2345 | | rcu_check_callbacks() {
> > > 136.255979 | 0) sleep-2345 | 0.781 us | idle_cpu();
> > > 136.255981 | 0) sleep-2345 | | raise_softirq() {
> > > 136.255981 | 0) sleep-2345 | | raise_softirq_irqoff() {
> > > 136.255982 | 0) sleep-2345 | | __raise_softirq_irqoff() {
> > > 136.255983 | 0) sleep-2345 | | /* nr: 8 */
> > > 136.255984 | 0) sleep-2345 | 1.555 us | }
> > > 136.255984 | 0) sleep-2345 | 3.059 us | }
> > > 136.255985 | 0) sleep-2345 | 4.594 us | }
> > > 136.255986 | 0) sleep-2345 | 7.800 us | }
> > > 136.255987 | 0) sleep-2345 | 0.737 us | printk_tick();
> > >
> > > again and again.
> >
> > Interesting...
> >
> > I will take a look!
>
> The above sequence is more or less normal behavior -- the RCU softirq
> handler rcu_process_callbacks() is being invoked once per tick, which
> appears to be HZ=1000 or thereabouts. The system appears to be mostly
> idle during this time period.
>
> One oddity is that the _bh call to __rcu_process_callbacks() is invoking
> force_quiescent_state() each time, and force_quiescent_state() isn't
> doing anything. This is a possible mismatch between the conditions in
> rcu_pending() and force_quiescent_state(), and I will look into this.
>
> However, this sequence is consuming less than 10 microseconds per
> millisecond, so cannot be the main cause of the softirq issues you
> are seeing, though if there really is a mismatch, it needs to be fixed,
> and I will attend to this.
>
> The interesting portion of the trace is later on:
>
> 137.896992 | 1) ksoftir-2302 | | do_softirq() {
> 137.896993 | 1) ksoftir-2302 | | __do_softirq() {
> 137.896993 | 1) ksoftir-2302 | | /* #1 softirq pending: 00000100 */
> 137.896994 | 1) ksoftir-2302 | | /* #2 softirq pending: 00000000 */
> 137.896995 | 1) ksoftir-2302 | | rcu_process_callbacks() {
> 137.896995 | 1) ksoftir-2302 | | __rcu_process_callbacks() {
> 137.896996 | 1) ksoftir-2302 | 0.498 us | force_quiescent_state();
> 137.896997 | 1) ksoftir-2302 | 1.588 us | }
> 137.896997 | 1) ksoftir-2302 | | __rcu_process_callbacks() {
> 137.896998 | 1) ksoftir-2302 | 0.475 us | force_quiescent_state();
> 137.896999 | 1) ksoftir-2302 | | cpu_quiet() {
> 137.896999 | 1) ksoftir-2302 | 0.526 us | _spin_lock_irqsave();
> 137.897000 | 1) ksoftir-2302 | 0.511 us | _spin_unlock_irqrestore();
> 137.897001 | 1) ksoftir-2302 | 2.528 us | }
> 137.897002 | 1) ksoftir-2302 | 4.607 us | }
> 137.897002 | 1) ksoftir-2302 | 7.825 us | }
> 137.897003 | 1) ksoftir-2302 | 0.498 us | _local_bh_enable();
> 137.897004 | 1) ksoftir-2302 | + 11.430 us | }
> 137.897005 | 1) ksoftir-2302 | + 12.572 us | }
> 137.897005 | 1) ksoftir-2302 | 0.549 us | _cond_resched();
> 137.897006 | 1) ksoftir-2302 | 0.541 us | kthread_should_stop();
> 137.897007 | 1) ksoftir-2302 | | schedule() {
> 137.897008 | 1) ksoftir-2302 | | __schedule() {
> 137.897008 | 1) ksoftir-2302 | 0.514 us | _spin_lock_irq();
> 137.897009 | 1) ksoftir-2302 | 0.594 us | update_rq_clock();
> 137.897011 | 1) ksoftir-2302 | | deactivate_task() {
> 137.897011 | 1) ksoftir-2302 | | dequeue_task() {
> 137.897011 | 1) ksoftir-2302 | | dequeue_task_fair() {
> 137.897012 | 1) ksoftir-2302 | | update_curr() {
> 137.897012 | 1) ksoftir-2302 | | calc_delta_fair() {
> 137.897013 | 1) ksoftir-2302 | 0.506 us | calc_delta_mine();
> 137.897014 | 1) ksoftir-2302 | 1.528 us | }
> 137.897015 | 1) ksoftir-2302 | 2.563 us | }
> 137.897015 | 1) ksoftir-2302 | 0.513 us | hrtick_start_fair();
> 137.897019 | 1) ksoftir-2302 | 4.662 us | }
> 137.897019 | 1) ksoftir-2302 | 8.213 us | }
> 137.897020 | 1) ksoftir-2302 | 9.195 us | }
> 137.897020 | 1) ksoftir-2302 | 0.960 us | find_busiest_group();
> 137.897022 | 1) ksoftir-2302 | 0.493 us | msecs_to_jiffies();
> 137.897023 | 1) ksoftir-2302 | 0.511 us | put_prev_task_fair();
> 137.897024 | 1) ksoftir-2302 | | pick_next_task() {
> 137.897024 | 1) ksoftir-2302 | 0.481 us | pick_next_task_fair();
> 137.897025 | 1) ksoftir-2302 | 0.491 us | pick_next_task_rt();
> 137.897026 | 1) ksoftir-2302 | 0.474 us | pick_next_task_fair();
> 137.897027 | 1) ksoftir-2302 | 0.480 us | pick_next_task_idle();
> 137.897028 | 1) ksoftir-2302 | 4.516 us | }
> 137.897029 | 1) ksoftir-2302 | | perf_counter_task_sched_out() {
> 137.897029 | 1) ksoftir-2302 | | __perf_counter_sched_out() {
> 137.897030 | 1) ksoftir-2302 | 0.516 us | _spin_lock();
> 137.897031 | 1) ksoftir-2302 | 1.486 us | }
> 137.897031 | 1) ksoftir-2302 | 2.462 us | }
> 137.897032 | 1) ksoftir-2302 | 0.516 us | __lock_text_start();
> 137.897045 | ------------------------------------------
> 1) ksoftir-2302 => <idle>-0
> ------------------------------------------
>
> 1) <idle>-0 | | /* nr: 8 */
> ------------------------------------------
> 1) <idle>-0 => ksoftir-2302
> ------------------------------------------
>
> 137.897064 | 1) ksoftir-2302 | | finish_task_switch() {
> 137.897064 | 1) ksoftir-2302 | | perf_counter_task_sched_in() {
> 137.897065 | 1) ksoftir-2302 | 0.508 us | _spin_lock();
> 137.897066 | 1) ksoftir-2302 | 1.525 us | }
> 137.897066 | 1) ksoftir-2302 | 2.617 us | }
> 137.897067 | 1) ksoftir-2302 | + 58.928 us | }
> 137.897067 | 1) ksoftir-2302 | + 59.926 us | }
> 137.897068 | 1) ksoftir-2302 | | do_softirq() {
> 137.897068 | 1) ksoftir-2302 | | __do_softirq() {
> 137.897069 | 1) ksoftir-2302 | | /* #1 softirq pending: 00000100 */
> 137.897070 | 1) ksoftir-2302 | | /* #2 softirq pending: 00000000 */
> 137.897070 | 1) ksoftir-2302 | | rcu_process_callbacks() {
> 137.897071 | 1) ksoftir-2302 | | __rcu_process_callbacks() {
> 137.897071 | 1) ksoftir-2302 | | force_quiescent_state() {
> 137.897073 | 1) ksoftir-2302 | 1.575 us | }
> 137.897073 | 1) ksoftir-2302 | | __rcu_process_callbacks() {
> 137.897074 | 1) ksoftir-2302 | 0.474 us | force_quiescent_state();
> 137.897075 | 1) ksoftir-2302 | | cpu_quiet() {
> 137.897075 | 1) ksoftir-2302 | 0.526 us | _spin_lock_irqsave();
> 137.897076 | 1) ksoftir-2302 | 0.511 us | _spin_unlock_irqrestore();
> 137.897077 | 1) ksoftir-2302 | 2.532 us | }
> 137.897078 | 1) ksoftir-2302 | 4.632 us | }
> 137.897078 | 1) ksoftir-2302 | 7.815 us | }
> 137.897079 | 1) ksoftir-2302 | 0.501 us | _local_bh_enable();
> 137.897080 | 1) ksoftir-2302 | + 11.405 us | }
> 137.897080 | 1) ksoftir-2302 | + 12.542 us | }
>
> 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.

More likely it's a bug in the function graph tracer.
I will try to fix it.

> Certainly rcu_process_callbacks() can re-invoke itself, for example,
> when a large number of RCU callbacks has piled up. However, there are
> only 29 calls to __call_rcu() over the entire time period, so that does
> not appear to be the cause. Strangely enough, there appear to be no
> calls to rcu_do_batch() over the full trace, but this is invoked
> unconditionally from __rcu_process_callbacks(). So perhaps the trace
> wasn't covering that function?

Hmm. I have to check that too. I'm building CONFIG_RCU_TREE.
Will try to find what happens.

>
> Whatever, this pattern continues for more than 300 milliseconds(!).
>
> Would you be willing to enable CONFIG_RCU_TRACE and CONFIG_TREE_RCU,
> reproduce this and send the output of the debugfs files rcu/rcudata
> and rcu/rcuhier? The commands for this would be:
>
> mkdir /debug || :
> mount -t debugfs debugfs /debug
> cat /debug/rcu/rcuhier
> cat /debug/rcu/rcudata
>
> I will try to reproduce locally as well.
>
> Thanx, Paul