Date:   Thu, 15 Aug 2019 15:42:57 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com
Subject: Re: [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu_stop()
 enable tick on all online CPUs

On Thu, Aug 15, 2019 at 11:39:37AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 15, 2019 at 02:15:00PM -0400, Joel Fernandes wrote:
> > On Thu, Aug 15, 2019 at 10:23:51AM -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;
> 
> I put this inside the "if" statement, though I would not expect that to
> change behavior in this case.
> 
> Does your test case still avoid turning on the tick more than once?  Or
> is it turning on the tick each time the grace period gets too long, but
> without the tick managing to end the grace periods?

I will put some more prints and let you know. But it looks like I see a print
from rcu_sched_clock_irq() only once at around 700ms from the start of the
test loop. After that I don't see prints at all for the rest of the 7 seconds
of the test.

Before the test starts, I see several rcu_sched_clock_irq() at the regular
tick interval of 1 ms (HZ=1000).

> > > > [snip]
> > > > > 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)?
> 
> Or you could dig through the git logs for this code change.

Ok will do.

> > I love it how you found these issues by heavy testing and fixed them.
> 
> Me, I would have rather foreseen them and avoided them in the first place,
> but I agree that it is better for rcutorture to find them than for some
> hapless user somewhere to be inconvenienced by them.  ;-)

True, forseeing is always better ;)

thanks,

 - Joel

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