Re: [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu_stop() enable tick on all online CPUs

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Aug 15, 2019 at 10:23:51AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 15, 2019 at 11:07:35AM -0400, Joel Fernandes wrote:
> > On Wed, Aug 14, 2019 at 03:05:16PM -0700, Paul E. McKenney wrote:
> > [snip]
> > > > > If so, perhaps that monitoring could periodically invoke an RCU function
> > > > > that I provide for deciding when to turn the tick on.  We would also need
> > > > > to work out how to turn the tick off in a timely fashion once the CPU got
> > > > > out of kernel mode, perhaps in rcu_user_enter() or rcu_nmi_exit_common().
> > > > > 
> > > > > If this would be called only every second or so, the separate grace-period
> > > > > checking is still needed for its shorter timespan, though.
> > > > > 
> > > > > Thoughts?
> > > > 
> > > > Do you want me to test the below patch to see if it fixes the issue with my
> > > > other test case (where I had a nohz full CPU holding up a grace period).
> > > 
> > > Please!
> > 
> > I tried the patch below, but it did not seem to make a difference to the
> > issue I was seeing. My test tree is here in case you can spot anything I did
> > not do right: https://github.com/joelagnel/linux-kernel/commits/rcu/nohz-test
> > The main patch is here:
> > https://github.com/joelagnel/linux-kernel/commit/4dc282b559d918a0be826936f997db0bdad7abb3
> 
> That is more aggressive that rcutorture's rcu_torture_fwd_prog_nr(), so
> I am guessing that I need to up rcu_torture_fwd_prog_nr()'s game.  I am
> currently testing that.
> 
> > On the trace output, I grep something like: egrep "(rcu_perf|cpu 3|3d)". I
> > see a few ticks after 300ms, but then there are no more ticks and just a
> > periodic resched_cpu() from rcu_implicit_dynticks_qs():
> > 
> > [   19.534107] rcu_perf-165    12.... 2276436us : rcu_perf_writer: Start of rcuperf test
> > [   19.557968] rcu_pree-10      0d..1 2287973us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
> > [   20.136222] rcu_perf-165     3d.h. 2591894us : rcu_sched_clock_irq: sched-tick
> > [   20.137185] rcu_perf-165     3d.h2 2591906us : rcu_sched_clock_irq: sched-tick
> > [   20.138149] rcu_perf-165     3d.h. 2591911us : rcu_sched_clock_irq: sched-tick
> > [   20.139106] rcu_perf-165     3d.h. 2591915us : rcu_sched_clock_irq: sched-tick
[snip]
> > [   20.147797] rcu_perf-165     3d.h. 2591953us : rcu_sched_clock_irq: sched-tick
> > [   20.148759] rcu_perf-165     3d.h. 2591957us : rcu_sched_clock_irq: sched-tick
> > [   20.151655] rcu_pree-10      0d..1 2591979us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
> > [   20.732938] rcu_pree-10      0d..1 2895960us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
[snip]
> > [   26.566100] rcu_pree-10      0d..1 5935982us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
> > [   27.144497] rcu_pree-10      0d..1 6239973us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
> > [   27.192661] rcu_perf-165     3d.h. 6276923us : rcu_sched_clock_irq: sched-tick
> > [   27.705789] rcu_pree-10      0d..1 6541901us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
> > [   28.292155] rcu_pree-10      0d..1 6845974us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
> > [   28.874049] rcu_pree-10      0d..1 7149972us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
> > [   29.112646] rcu_perf-165     3.... 7275951us : rcu_perf_writer: End of rcuperf test
> 
> That would be due to my own stupidity.  I forgot to clear ->rcu_forced_tick
> in rcu_disable_tick_upon_qs() inside the "if" statement.  This of course
> prevents rcu_nmi_exit_common() from ever re-enabling it.
> 
> Excellent catch!  Thank you for testing this!!!

Ah I missed it too. Happy to help! I tried setting it as below but getting
same results:

+/*
+ * If the scheduler-clock interrupt was enabled on a nohz_full CPU
+ * in order to get to a quiescent state, disable it.
+ */
+void rcu_disable_tick_upon_qs(struct rcu_data *rdp)
+{
+       if (tick_nohz_full_cpu(rdp->cpu) && rdp->rcu_forced_tick)
+               tick_dep_clear_cpu(rdp->cpu, TICK_DEP_MASK_RCU);
+       rdp->rcu_forced_tick = false;
+}
+

> > [snip]
> > > > >  	if (rnp->qsmask & mask) { /* RCU waiting on incoming CPU? */
> > > > > +		rcu_disable_tick_upon_qs(rdp);
> > > > >  		/* Report QS -after- changing ->qsmaskinitnext! */
> > > > >  		rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
> > > > 
> > > > Just curious about the existing code. If a CPU is just starting up (after
> > > > bringing it online), how can RCU be waiting on it? I thought RCU would not be
> > > > watching offline CPUs.
> > > 
> > > Well, neither grace periods nor CPU-hotplug operations are atomic,
> > > and each can take significant time to complete.
> > > 
> > > So suppose we have a large system with multiple leaf rcu_node structures
> > > (not that 17 CPUs is all that many these days, but please bear with me).
> > > Suppose just after a new grace period initializes a given leaf rcu_node
> > > structure, one of its CPUs goes offline (yes, that CPU would have to
> > > have waited on a grace period, but that might have been the previous
> > > grace period).  But before the FQS scan notices that RCU is waiting on
> > > an offline CPU, the CPU comes back online.
> > > 
> > > That situation is exactly what the above code is intended to handle.
> > 
> > That makes sense!
> > 
> > > Without that code, RCU can give false-positive splats at various points
> > > in its processing.  ("Wait!  How can a task be blocked waiting on a
> > > grace period that hasn't even started yet???")
> > 
> > I did not fully understand the question in brackets though, a task can be on
> > a different CPU though which has nothing to do with the CPU that's going
> > offline/online so it could totally be waiting on a grace period right?
> > 
> > Also waiting on a grace period that hasn't even started is totally possible:
> > 
> >      GP1         GP2
> > |<--------->|<-------->|
> >      ^                 ^
> >      |                 |____  task gets unblocked
> > task blocks
> > on synchronize_rcu
> > but is waiting on
> > GP2 which hasn't started
> > 
> > Or did I misunderstand the question?
> 
> There is a ->gp_tasks field in the leaf rcu_node structures that
> references a list of tasks blocking the current grace period.  When there
> is no grace period in progress (as is the case from the end of GP1 to
> the beginning of GP2, the RCU code expects ->gp_tasks to be NULL.
> Without the curiosity code you pointed out above, ->gp_tasks could
> in fact end up being non-NULL when no grace period was in progress.
> 
> And did end up being non-NULL from time to time, initially every few
> hundred hours of a particular rcutorture scenario.

Oh ok! I will think more about it. I am not yet able to connect the gp_tasks
being non-NULL to the CPU going offline/online scenario though. Maybe I
should delete this code, run an experiment and trace for this condition
(gp_tasks != NULL)?

I love it how you found these issues by heavy testing and fixed them.

thanks,

 - Joel