Re: [PATCH v7 2/2] clocksource: add J-Core timer/clocksource driver

[Rich Felker]

On Fri, Sep 30, 2016 at 06:48:35AM -0700, Paul E. McKenney wrote:
> On Fri, Sep 30, 2016 at 03:15:11PM +0200, Thomas Gleixner wrote:
> > On Tue, 27 Sep 2016, Rich Felker wrote:
> > > I've managed to get a trace with a stall. I'm not sure what the best
> > > way to share the full thing is, since it's large, but here are the
> > > potentially interesting parts.
> 
> [ . . . ]
> 
> Some RCU commentary, on the off-chance that it helps...
> 
> > So that should kick rcu_sched-7 in 10ms, latest 20ms from now and CPU1 goes
> > into a NOHZ idle sleep.
> > 
> > >           <idle>-0     [001] d...   109.953436: tick_stop: success=1 dependency=NONE
> > >           <idle>-0     [001] d...   109.953617: hrtimer_cancel: hrtimer=109f449c
> > >           <idle>-0     [001] d...   109.953818: hrtimer_start: hrtimer=109f449c function=tick_sched_timer expires=109880000000 softexpires=109880000000
> > 
> > which is (using the 0.087621us delta between the trace clock and clock
> > MONO) at: 109.880 + 0.087621 = 109.968
> > 
> > Which is about correct as we expect the RCU timer to fire at:
> >       
> >       109.952633 + 0.01 = 109.963633
> > 
> > or latest at
> > 
> >       109.952633 + 0.02 = 109.983633
> >    
> > There is another caveat. That nohz stuff can queue the rcu timer on CPU0, which
> > it did not because:
> 
> Just for annoying completeness, the location of the timer depends on how
> the rcuo callback-offload kthreads are constrained.  And yes, in the most
> constrained case where all CPUs except for CPU 0 are nohz CPUs, they will
> by default all run on CPU 0.

In default full nohz configuration, am I correct that all cpus except
cpu0 willd be nohz and that the rcu callbacks then have to run on
cpu0?

> > >        rcu_sched-7     [001] d...   109.952633: timer_start: timer=160a9eb0 function=process_timeout expires=4294948284 [timeout=1] flags=0x00000001
> > 
> > The CPU nr encoded in flags is: 1
> > 
> > Now we cancel and restart the timer w/o seing the interrupt expiring
> > it. And that expiry should have happened at 109.968000 !?!
> > 
> > >           <idle>-0     [001] d...   109.968225: hrtimer_cancel: hrtimer=109f449c
> > >           <idle>-0     [001] d...   109.968526: hrtimer_start: hrtimer=109f449c function=tick_sched_timer expires=109890000000 softexpires=109890000000
> > 
> > So this advances the next tick even further out. And CPU 0 sets the timer to
> > the exact smae value:
> > 
> > >           <idle>-0     [000] d.h.   109.969104: hrtimer_start: hrtimer=109e949c function=tick_sched_timer expires=109890000000 softexpires=109890000000
> > 
> > 
> > >           <idle>-0     [000] d.h.   109.977690: irq_handler_entry: irq=16 name=jcore_pit
> > >           <idle>-0     [000] d.h.   109.977911: hrtimer_cancel: hrtimer=109e949c
> > >           <idle>-0     [000] d.h.   109.978053: hrtimer_expire_entry: hrtimer=109e949c function=tick_sched_timer now=109890434160
> > 
> > Which expires here. And CPU1 instead of getting an interrupt and expiring
> > the timer does the cancel/restart to the next jiffie again:
> > 
> > >           <idle>-0     [001] d...   109.978206: hrtimer_cancel: hrtimer=109f449c
> > >           <idle>-0     [001] d...   109.978495: hrtimer_start: hrtimer=109f449c function=tick_sched_timer expires=109900000000 softexpires=109900000000
> > 
> > And this repeats;
> > 
> > >           <idle>-0     [000] d.h.   109.987726: irq_handler_entry: irq=16 name=jcore_pit
> > >           <idle>-0     [000] d.h.   109.987954: hrtimer_cancel: hrtimer=109e949c
> > >           <idle>-0     [000] d.h.   109.988095: hrtimer_expire_entry: hrtimer=109e949c function=tick_sched_timer now=109900474620
> > 
> > >           <idle>-0     [001] d...   109.988243: hrtimer_cancel: hrtimer=109f449c
> > >           <idle>-0     [001] d...   109.988537: hrtimer_start: hrtimer=109f449c fun9c
> > 
> > There is something badly wrong here.
> > 
> > >           <idle>-0     [000] ..s.   110.019443: softirq_entry: vec=1 [action=TIMER]
> > >           <idle>-0     [000] ..s.   110.019617: softirq_exit: vec=1 [action=TIMER]
> > >           <idle>-0     [000] ..s.   110.019730: softirq_entry: vec=7 [action=SCHED]
> > >           <idle>-0     [000] ..s.   110.020174: softirq_exit: vec=7 [action=SCHED]
> > >           <idle>-0     [000] d.h.   110.027674: irq_handler_entry: irq=16 name=jcore_pit
> > > 
> > > The rcu_sched process does not run again after the tick_stop until
> > > 132s, and only a few RCU softirqs happen (all shown above). During
> > > this time, cpu1 has no interrupt activity and nothing in the trace
> > > except the above hrtimer_cancel/hrtimer_start pairs (not sure how
> > > they're happening without any interrupts).
> > 
> > If you drop out of the arch idle into the core idle loop then you might end
> > up with this. You want to add a few trace points or trace_printks() to the
> > involved functions. tick_nohz_restart() tick_nohz_stop_sched_tick()
> > tick_nohz_restart_sched_tick() and the idle code should be a good starting
> > point.
> > 
> > > This pattern repeats until almost 131s, where cpu1 goes into a frenzy
> > > of hrtimer_cancel/start:
> > 
> > It's not a frenzy. It's the same pattern as above. It arms the timer to the
> > next tick, but that timer never ever fires. And it does that every tick ....
> > 
> > Please put a tracepoint into your set_next_event() callback as well. SO
> > this changes here:
> > 
> > >           <idle>-0     [001] d...   132.198170: hrtimer_cancel: hrtimer=109f449c
> > >           <idle>-0     [001] d...   132.198451: hrtimer_start: hrtimer=109f449c function=tick_sched_timer expires=132120000000 softexpires=132120000000
> > 
> > >           <idle>-0     [001] dnh.   132.205860: irq_handler_entry: irq=20 name=ipi
> > >           <idle>-0     [001] dnh.   132.206041: irq_handler_exit: irq=20 ret=handle
> > 
> > So CPU1 gets an IPI
> > 
> > >           <idle>-0     [001] dn..   132.206650: hrtimer_cancel: hrtimer=109f449c
> > 49c function=tick_sched_timer now=132119115200
> > >           <idle>-0     [001] dn..   132.206936: hrtimer_start: hrtimer=109f449c function=tick_sched_timer expires=132120000000 softexpires=132120000000
> > 
> > And rcu-sched-7 gets running magically, but we don't know what woke it
> > up. Definitely not the timer, because that did not fire.
> > 
> > >        rcu_sched-7     [001] d...   132.207710: timer_cancel: timer=160a9eb0
> 
> It could have been an explicit wakeup at the end of a grace period.  That
> would explain its cancelling the timer, anyway.

I think the rcu stall handler kicked it, no? Looking at the code
again, maybe that behavior needs to be explicitly turned on, so maybe
it's just the uart interrupt activity/load from the stall message that
breaks the stall condition.

> > > - During the whole sequence, hrtimer expiration times are being set to
> > >   exact jiffies (@ 100 Hz), whereas before it they're quite arbitrary.
> > 
> > When a CPU goes into NOHZ idle and the next (timer/hrtimer) is farther out
> > than the next tick, then tick_sched_timer is set to this next event which
> > can be far out. So that's expected.
> > 
> > > - The CLOCK_MONOTONIC hrtimer times do not match up with the
> > >   timestamps; they're off by about 0.087s. I assume this is just
> > >   sched_clock vs clocksource time and not a big deal.
> > 
> > Yes. You can tell the tracer to use clock monotonic so then they should match.
> > 
> > > - The rcu_sched process is sleeping with timeout=1. This seems
> > >   odd/excessive.
> > 
> > Why is that odd? That's one tick, i.e. 10ms in your case. And that's not
> > the problem at all. The problem is your timer not firing, but the cpu is
> > obviously either getting out of idle and then moves the tick ahead for some
> > unknown reason.
> 
> And a one-jiffy timeout is in fact expected behavior when HZ=100.
> You have to be running HZ=250 or better to have two-jiffy timeouts,
> and HZ=500 or better for three-jiffy timeouts.

One possible theory I'm looking at is that the two cpus are both
waking up (leaving cpu_idle_poll or cpuidle_idle_call) every jiffy
with sufficient consistency that every time the rcu_gp_fqs_check_wake
loop wakes up in rcu_gp_kthread, the other cpu is in cpu_idle_loop but
outside the rcu_idle_enter/rcu_idle_exit range. Would this block
forward process? I added an LED indicator in rcu_gp_fqs_check_wake
that shows the low 2 bits of rnp->qsmask every time it's called, and
under normal operation the LEDs just flash on momentarily or just one
stays on for a few seconds then goes off. During a stall both are
stuck on. I'm still trying to make sense of the code but my impression
so far is that, on a 2-cpu machine, this is a leaf node and the 2 bits
correspond directly to cpus; is that right? If so I'm a bit confused
because I don't see how forward progress could ever happen if the cpu
on which rcu_gp_kthread is blocking forward progress of
rcu_gp_kthread.

Rich