Re: [PATCH] rcu: Make jiffies_till_sched_qs writable

[Byungchul Park]

On Thu, Jul 18, 2019 at 02:34:19PM -0700, Paul E. McKenney wrote:
> 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.
> > - 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.
> 
> - CPU Y might have already passed through its quiescent state for
>   the current grace period, then disabled IRQs indefinitely.

Or for a longer time than the period that rcu considers as a stall. Or
preemption disabled for that long time. Or the stopper on Y even has yet
to be woken up inside scheduler because of any reasons but maybe locks.

>   Now, CPU Y would block a later grace period, but CPU X is
>   preventing the current grace period from ending, so no such
>   later grace period can start.
> 
> > Did I get that right? Would be exciting to run the rcutorture test
> > once Paul has it available to reproduce this problem.
> 
> Working on it!  Slow, I know!
> 
> 							Thanx, Paul