Message-ID: <20161002000049.GP19318@brightrain.aerifal.cx>
Date:   Sat, 1 Oct 2016 20:00:49 -0400
From:   Rich Felker <dalias@...c.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-sh@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v7 2/2] clocksource: add J-Core timer/clocksource driver

On Sat, Oct 01, 2016 at 10:58:37AM -0700, Paul E. McKenney wrote:
> On Sat, Oct 01, 2016 at 01:05:08PM -0400, Rich Felker wrote:
> > 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?
> 
> In recent kernels, it looks to me that the default is that none of the
> CPUs will be nohz by default.  You have to build with both NO_HZ_FULL
> and NO_HZ_FULL_ALL to get the everything-on-CPU-0 behavior.

Actually NO_HZ_FULL is not supported because
HAVE_VIRT_CPU_ACCOUNTING_GEN and HAVE_CONTEXT_TRACKING are missing, so
it's just normal NO_HZ. I listed the relevant config items somewhere
earlier in this thread I think.

> But are there rcu0 kthreads running on your system?

Apparently not:

# ps aux | grep rcu
    7 root       0:00 [rcu_sched]
    8 root       0:00 [rcu_bh]
  395 root       0:00 grep rcu
          
> > > > >        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.
> 
> That is indeed another alternative.  But either way, if your timers aren't
> waking rcu_sched up when it asks to be awakened, you will very likely get
> RCU CPU stall warning messages.  And on small systems, rcu_sched asks to
> be awakened every few jiffies by default.

Can you clarify which timer you think should be waking it up and
isn't? Maybe I'm mistaken but it looks to me like rcu_gp_kthread() is
waking up as expected, only to go back to sleep due to failure of
rcu_gp_fqs_check_wake to end the loop.

To demonstrate this to myself, I hacked rcu_gp_fqs_check_wake to show
2 low bits of rnp->qsmask on LEDs. I also put a hook to update them in
cpu_idle_loop just now, just in case. At least one, and usually two,
LEDs are on when the stalls happen.

> > > > > - 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.
> 
> No.  If the CPUs are entering and leaving idle, and if your timers
> were waking up rcu_sched every few jiffies like it asks, then the
> repeated idle entry/exit events would be noticed, courtesy of the atomic
> increments of ->dynticks and the rcu_sched kthread's snapshotting and
> checking of this value.

I don't see how rcu_sched could notice the changes if it's stuck in
the wait loop I think it's stuck in. There is no check of ->dynticks
in rcu_gp_fqs_check_wake. Just in case checking *gfp & RCU_GP_FLAG_FQS
accomplishes this, I updated my LED hacks to clear the LEDs in that
exit path (and killed the other place that could turn them back on
from cpu_idle_loop) but I still get 2 LEDs on for 21s followed by a
stall message.

> Even if the CPUs were always non-idle on every
> time force_quiescent_state() is invoked, give or take the possibility
> of counter wrap -- but even on a 32-bit system, that takes awhile.

Perhaps force_quiescent_state is not getting invoked? Does that sound
plausible, and if so, how should I go about debugging it? I see it is
called from the stall reporting code, so that's presumably what's
breaking the stalls.

> But if your timers don't wake up rcu_sched, then force_quiescent_state()
> will never be called, and the ->dynticks counter will be neither
> snapshotted nor checked, which will mean that the idle CPUs will never
> be noticed, which might mean that the grace period never completes.
> Which will definitely result in an RCU CPU stall warning like the one
> that you are seeing.

I don't follow. I thought force_quiescent_state was called by other
code to break rcu_sched out of the loop I'm finding it stuck in, not
by rcu_sched itself.

Rich

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