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. BTW, the other question I have is "why do we need to call rcu_pending() and rcu_check_callbacks() from the idle loop of 32-bit x86, especially given that no other architecture does this?". Don't get me wrong, it would be good to get rcutree's rcu_pending() to avoid spuriously saying that rcu_check_callbacks() should be invoked, so I would still like the trace with my patch, but... Thanx, Paul > 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.) > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > --- > > rcupdate.c | 23 +++++++++++++++++++++++ > rcutree.c | 31 +++++++++++++++++++++++++------ > 2 files changed, 48 insertions(+), 6 deletions(-) > > diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c > index d92a76a..42bbf03 100644 > --- a/kernel/rcupdate.c > +++ b/kernel/rcupdate.c > @@ -175,3 +175,26 @@ void __init rcu_init(void) > __rcu_init(); > } > > +void __rcu_pending_qs_pending(void) > +{ > +} > + > +void __rcu_pending_callbacks_ready(void) > +{ > +} > + > +void __rcu_pending_needs_gp(void) > +{ > +} > + > +void __rcu_pending_new_completed(void) > +{ > +} > + > +void __rcu_pending_new_gp(void) > +{ > +} > + > +void __rcu_pending_fqs(void) > +{ > +} > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > index b2fd602..e2d72c3 100644 > --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c > @@ -1234,6 +1234,13 @@ void call_rcu_bh(struct rcu_head *head, void (*func)(struct rcu_head *rcu)) > } > EXPORT_SYMBOL_GPL(call_rcu_bh); > > +extern void __rcu_pending_qs_pending(void); > +extern void __rcu_pending_callbacks_ready(void); > +extern void __rcu_pending_needs_gp(void); > +extern void __rcu_pending_new_completed(void); > +extern void __rcu_pending_new_gp(void); > +extern void __rcu_pending_fqs(void); > + > /* > * Check to see if there is any immediate RCU-related work to be done > * by the current CPU, for the specified type of RCU, returning 1 if so. > @@ -1249,30 +1256,42 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp) > check_cpu_stall(rsp, rdp); > > /* Is the RCU core waiting for a quiescent state from this CPU? */ > - if (rdp->qs_pending) > + if (rdp->qs_pending) { > + __rcu_pending_qs_pending(); > return 1; > + } > > /* Does this CPU have callbacks ready to invoke? */ > - if (cpu_has_callbacks_ready_to_invoke(rdp)) > + if (cpu_has_callbacks_ready_to_invoke(rdp)) { > + __rcu_pending_callbacks_ready(); > return 1; > + } > > /* Has RCU gone idle with this CPU needing another grace period? */ > - if (cpu_needs_another_gp(rsp, rdp)) > + if (cpu_needs_another_gp(rsp, rdp)) { > + __rcu_pending_needs_gp(); > return 1; > + } > > /* Has another RCU grace period completed? */ > - if (ACCESS_ONCE(rsp->completed) != rdp->completed) /* outside of lock */ > + if (ACCESS_ONCE(rsp->completed) != rdp->completed) /* outside of lock */ { > + __rcu_pending_new_completed(); > return 1; > + } > > /* Has a new RCU grace period started? */ > - if (ACCESS_ONCE(rsp->gpnum) != rdp->gpnum) /* outside of lock */ > + if (ACCESS_ONCE(rsp->gpnum) != rdp->gpnum) /* outside of lock */ { > + __rcu_pending_new_gp(); > return 1; > + } > > /* Has an RCU GP gone long enough to send resched IPIs &c? */ > if (ACCESS_ONCE(rsp->completed) != ACCESS_ONCE(rsp->gpnum) && > ((long)(ACCESS_ONCE(rsp->jiffies_force_qs) - jiffies) < 0 || > - (rdp->n_rcu_pending_force_qs - rdp->n_rcu_pending) < 0)) > + (rdp->n_rcu_pending_force_qs - rdp->n_rcu_pending) < 0)) { > + __rcu_pending_fqs(); > return 1; > + } > > /* nothing to do */ > return 0; -- 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
