Re: [PATCH] rcu: Make jiffies_till_sched_qs writable

[Byungchul Park <byungchul.park@lge.com>]

On Tue, Jul 23, 2019 at 06:47:17AM -0700, Paul E. McKenney wrote:
> On Tue, Jul 23, 2019 at 08:05:21PM +0900, Byungchul Park wrote:
> > On Fri, Jul 19, 2019 at 04:33:56PM -0400, Joel Fernandes wrote:
> > > On Fri, Jul 19, 2019 at 3:57 PM Paul E. McKenney <paulmck@linux.ibm.com> wrote:
> > > >
> > > > On Fri, Jul 19, 2019 at 06:57:58PM +0900, Byungchul Park wrote:
> > > > > On Fri, Jul 19, 2019 at 4:43 PM Paul E. McKenney <paulmck@linux.ibm.com> wrote:
> > > > > >
> > > > > > On Thu, Jul 18, 2019 at 08:52:52PM -0400, Joel Fernandes wrote:
> > > > > > > On Thu, Jul 18, 2019 at 8:40 PM Byungchul Park <byungchul.park@lge.com> wrote:
> > > > > > > [snip]
> > > > > > > > > - There is a bug in the CPU stopper machinery itself preventing it
> > > > > > > > > from scheduling the stopper on Y. Even though Y is not holding up the
> > > > > > > > > grace period.
> > > > > > > >
> > > > > > > > Or any thread on Y is busy with preemption/irq disabled preventing the
> > > > > > > > stopper from being scheduled on Y.
> > > > > > > >
> > > > > > > > Or something is stuck in ttwu() to wake up the stopper on Y due to any
> > > > > > > > scheduler locks such as pi_lock or rq->lock or something.
> > > > > > > >
> > > > > > > > I think what you mentioned can happen easily.
> > > > > > > >
> > > > > > > > Basically we would need information about preemption/irq disabled
> > > > > > > > sections on Y and scheduler's current activity on every cpu at that time.
> > > > > > >
> > > > > > > I think all that's needed is an NMI backtrace on all CPUs. An ARM we
> > > > > > > don't have NMI solutions and only IPI or interrupt based backtrace
> > > > > > > works which should at least catch and the preempt disable and softirq
> > > > > > > disable cases.
> > > > > >
> > > > > > True, though people with systems having hundreds of CPUs might not
> > > > > > thank you for forcing an NMI backtrace on each of them.  Is it possible
> > > > > > to NMI only the ones that are holding up the CPU stopper?
> > > > >
> > > > > What a good idea! I think it's possible!
> > > > >
> > > > > But we need to think about the case NMI doesn't work when the
> > > > > holding-up was caused by IRQ disabled.
> > > > >
> > > > > Though it's just around the corner of weekend, I will keep thinking
> > > > > on it during weekend!
> > > >
> > > > Very good!
> > > 
> > > Me too will think more about it ;-) Agreed with point about 100s of
> > > CPUs usecase,
> > > 
> > > Thanks, have a great weekend,
> > 
> > BTW, if there's any long code section with irq/preemption disabled, then
> > the problem would be not only about RCU stall. And we can also use
> > latency tracer or something to detect the bad situation.
> > 
> > So in this case, sending ipi/nmi to the CPUs where the stoppers cannot
> > to be scheduled does not give us additional meaningful information.
> > 
> > I think Paul started to think about this to solve some real problem. I
> > seriously love to help RCU and it's my pleasure to dig deep into kind of
> > RCU stuff, but I've yet to define exactly what problem is. Sorry.
> > 
> > Could you share the real issue? I think you don't have to reproduce it.
> > Just sharing the issue that you got inspired from is enough. Then I
> > might be able to develop 'how' with Joel! :-) It's our pleasure!
> 
> It is unfortunately quite intermittent.  I was hoping to find a way
> to make it happen more often.  Part of the underlying problem appears
> to be lock contention, in that reducing contention made it even more
> intermittent.  Which is good in general, but not for exercising the
> CPU-stopper issue.
> 
> But perhaps your hardware will make this happen more readily than does
> mine.  The repeat-by is simple, namely run TREE04 on branch "dev" on an
> eight-CPU system.  It appear that the number of CPUs used by the test
> should match the number available on the system that you are running on,
> though perhaps affinity could allow mismatches.
> 
> So why not try it and see what happens?

Thank you. I'll try it too.