Re: [PATCH] rcu: Make jiffies_till_sched_qs writable

[Joel Fernandes]

On Fri, Jul 19, 2019 at 3:57 PM Paul E. McKenney <paulmck@xxxxxxxxxxxxx> 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@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!
>
> Very good!

Me too will think more about it ;-) Agreed with point about 100s of
CPUs usecase,

Thanks, have a great weekend,

 - Joel