Re: [PATCH] rcu: Make jiffies_till_sched_qs writable

[Byungchul Park]

On Fri, Jul 19, 2019 at 4:43 PM Paul E. McKenney <paulmck@xxxxxxxxxxxxx> 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@xxxxxxx> 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!

Thanks,
Byungchul

>                                                         Thanx, Paul
>
> > But yeah I don't see why just the stacks of those CPUs that are
> > blocking the CPU X would not suffice for the trivial cases where a
> > piece of misbehaving code disable interrupts / preemption and
> > prevented the stopper thread from executing.
> >
> > May be once the test case is ready (no rush!) , then it will be more
> > clear what can help.
> >
> > J.
> >



-- 
Thanks,
Byungchul