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 Wed, Aug 14, 2019 at 03:05:16PM -0700, Paul E. McKenney wrote:
[snip]
> > > > Arming a CPU timer could also be an alternative to tick_set_dep_cpu() for that.
> > > > 
> > > > What do you think?
> > > 
> > > Left to itself, RCU would take action only when a given nohz_full
> > > in-kernel CPU was delaying a grace period, which is what the (lightly
> > > tested) patch below is supposed to help with.  If that is all that is
> > > needed, well and good!
> > > 
> > > But should we need long-running in-kernel nohz_full CPUs to turn on
> > > their ticks when they are not blocking an RCU grace period, for example,
> > > when RCU is idle, more will be needed.  To that point, isn't there some
> > > sort of monitoring that checks up on nohz_full CPUs ever second or so?
> > 
> > Wouldn't such monitoring need to be more often than a second, given that
> > rcu_urgent_qs and rcu_need_heavy_qs are configured typically to be sooner
> > (200-300 jiffies on my system).
> 
> Either it would have to be more often than once per second, or RCU would
> need to retain its more frequent checks.  But note that RCU isn't going
> to check unless there is a grace period in progress.

Sure.

> > > 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

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
[   20.140077] rcu_perf-165     3d.h. 2591919us : rcu_sched_clock_irq: sched-tick
[   20.141041] rcu_perf-165     3d.h. 2591924us : rcu_sched_clock_irq: sched-tick
[   20.142001] rcu_perf-165     3d.h. 2591928us : rcu_sched_clock_irq: sched-tick
[   20.142961] rcu_perf-165     3d.h. 2591932us : rcu_sched_clock_irq: sched-tick
[   20.143925] rcu_perf-165     3d.h. 2591936us : rcu_sched_clock_irq: sched-tick
[   20.144885] rcu_perf-165     3d.h. 2591940us : rcu_sched_clock_irq: sched-tick
[   20.145876] rcu_perf-165     3d.h. 2591945us : rcu_sched_clock_irq: sched-tick
[   20.146835] rcu_perf-165     3d.h. 2591949us : rcu_sched_clock_irq: sched-tick
[   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
[   21.318104] rcu_pree-10      0d..1 3199975us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
[   21.899908] rcu_pree-10      0d..1 3503964us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
[   22.481316] rcu_pree-10      0d..1 3807990us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
[   23.065623] rcu_pree-10      0d..1 4111990us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
[   23.650875] rcu_pree-10      0d..1 4415989us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
[   24.233999] rcu_pree-10      0d..1 4719978us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
[   24.818397] rcu_pree-10      0d..1 5023982us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
[   25.402633] rcu_pree-10      0d..1 5327981us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
[   25.984104] rcu_pree-10      0d..1 5631976us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3
[   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

[snip]
> > > @@ -2906,7 +2927,7 @@ void rcu_barrier(void)
> > >  	/* Did someone else do our work for us? */
> > >  	if (rcu_seq_done(&rcu_state.barrier_sequence, s)) {
> > >  		rcu_barrier_trace(TPS("EarlyExit"), -1,
> > > -				   rcu_state.barrier_sequence);
> > > +				  rcu_state.barrier_sequence);
> > >  		smp_mb(); /* caller's subsequent code after above check. */
> > >  		mutex_unlock(&rcu_state.barrier_mutex);
> > >  		return;
> > > @@ -2938,11 +2959,11 @@ void rcu_barrier(void)
> > >  			continue;
> > >  		if (rcu_segcblist_n_cbs(&rdp->cblist)) {
> > >  			rcu_barrier_trace(TPS("OnlineQ"), cpu,
> > > -					   rcu_state.barrier_sequence);
> > > +					  rcu_state.barrier_sequence);
> > >  			smp_call_function_single(cpu, rcu_barrier_func, NULL, 1);
> > >  		} else {
> > >  			rcu_barrier_trace(TPS("OnlineNQ"), cpu,
> > > -					   rcu_state.barrier_sequence);
> > > +					  rcu_state.barrier_sequence);
> > >  		}
> > >  	}
> > >  	put_online_cpus();
> > > @@ -3168,6 +3189,7 @@ void rcu_cpu_starting(unsigned int cpu)
> > >  	rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);
> > >  	rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags);
> > >  	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?

thanks!

 - Joel