Re: [PATCH] rcu: Make jiffies_till_sched_qs writable

[Byungchul Park]

On Thu, Jul 18, 2019 at 12:14:22PM -0400, Joel Fernandes wrote:
> Trimming the list a bit to keep my noise level low,
> 
> On Sat, Jul 13, 2019 at 1:41 PM Paul E. McKenney <paulmck@xxxxxxxxxxxxx> wrote:
> [snip]
> > > It still feels like you guys are hyperfocusing on this one particular
> > > > knob.  I instead need you to look at the interrelating knobs as a group.
> > >
> > > Thanks for the hints, we'll do that.
> > >
> > > > On the debugging side, suppose someone gives you an RCU bug report.
> > > > What information will you need?  How can you best get that information
> > > > without excessive numbers of over-and-back interactions with the guy
> > > > reporting the bug?  As part of this last question, what information is
> > > > normally supplied with the bug?  Alternatively, what information are
> > > > bug reporters normally expected to provide when asked?
> > >
> > > I suppose I could dig out some of our Android bug reports of the past where
> > > there were RCU issues but if there's any fires you are currently fighting do
> > > send it our way as debugging homework ;-)
> >
> >   Suppose that you were getting RCU CPU stall
> > warnings featuring multi_cpu_stop() called from cpu_stopper_thread().
> > Of course, this really means that some other CPU/task is holding up
> > multi_cpu_stop() without also blocking the current grace period.
> >
> 
> So I took a shot at this trying to learn how CPU stoppers work in
> relation to this problem.
> 
> I am assuming here say CPU X has entered MULTI_STOP_DISABLE_IRQ state
> in multi_cpu_stop() but another CPU Y has not yet entered this state.
> So CPU X is stalling RCU but it is really because of CPU Y. Now in the
> problem statement, you mentioned CPU Y is not holding up the grace
> period, which means Y doesn't have any of IRQ, BH or preemption
> disabled ; but is still somehow stalling RCU indirectly by troubling
> X.
> 
> This can only happen if :
> - CPU Y has a thread executing on it that is higher priority than CPU
> X's stopper thread which prevents it from getting scheduled. - but the
> CPU stopper thread (migration/..) is highest priority RT so this would
> be some kind of an odd scheduler bug.

I think this bug hardly can happen.

> - 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.

Am I missing something?

> Did I get that right? Would be exciting to run the rcutorture test
> once Paul has it available to reproduce this problem.

Thanks,
Byungchul