Re: [PATCH] rcu: Make jiffies_till_sched_qs writable

[Paul E. McKenney]

On Sun, Jul 14, 2019 at 10:39:58PM +0900, Byungchul Park wrote:
> On Sun, Jul 14, 2019 at 2:41 AM Paul E. McKenney <paulmck@xxxxxxxxxxxxx> wrote:
> >
> > On Sat, Jul 13, 2019 at 11:42:57AM -0400, Joel Fernandes wrote:
> > > On Sat, Jul 13, 2019 at 08:13:30AM -0700, Paul E. McKenney wrote:
> > > > On Sat, Jul 13, 2019 at 10:20:02AM -0400, Joel Fernandes wrote:
> > > > > On Sat, Jul 13, 2019 at 4:47 AM Byungchul Park
> > > > > <max.byungchul.park@xxxxxxxxx> wrote:
> > > > > >
> > > > > > On Fri, Jul 12, 2019 at 9:51 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> > > > > > >
> > > > > > > On Fri, Jul 12, 2019 at 03:32:40PM +0900, Byungchul Park wrote:
> > > > > > > > On Thu, Jul 11, 2019 at 03:58:39PM -0400, Joel Fernandes wrote:
> > > > > > > > > Hmm, speaking of grace period durations, it seems to me the maximum grace
> > > > > > > > > period ever is recorded in rcu_state.gp_max. However it is not read from
> > > > > > > > > anywhere.
> > > > > > > > >
> > > > > > > > > Any idea why it was added but not used?
> > > > > > > > >
> > > > > > > > > I am interested in dumping this value just for fun, and seeing what I get.
> > > > > > > > >
> > > > > > > > > I wonder also it is useful to dump it in rcutorture/rcuperf to find any
> > > > > > > > > issues, or even expose it in sys/proc fs to see what worst case grace periods
> > > > > > > > > look like.
> > > > > > > >
> > > > > > > > Hi,
> > > > > > > >
> > > > > > > >       commit ae91aa0adb14dc33114d566feca2f7cb7a96b8b7
> > > > > > > >       rcu: Remove debugfs tracing
> > > > > > > >
> > > > > > > > removed all debugfs tracing, gp_max also included.
> > > > > > > >
> > > > > > > > And you sounds great. And even looks not that hard to add it like,
> > > > > > > >
> > > > > > > > :)
> > > > > > > >
> > > > > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > > > > > index ad9dc86..86095ff 100644
> > > > > > > > --- a/kernel/rcu/tree.c
> > > > > > > > +++ b/kernel/rcu/tree.c
> > > > > > > > @@ -1658,8 +1658,10 @@ static void rcu_gp_cleanup(void)
> > > > > > > >       raw_spin_lock_irq_rcu_node(rnp);
> > > > > > > >       rcu_state.gp_end = jiffies;
> > > > > > > >       gp_duration = rcu_state.gp_end - rcu_state.gp_start;
> > > > > > > > -     if (gp_duration > rcu_state.gp_max)
> > > > > > > > +     if (gp_duration > rcu_state.gp_max) {
> > > > > > > >               rcu_state.gp_max = gp_duration;
> > > > > > > > +             trace_rcu_grace_period(something something);
> > > > > > > > +     }
> > > > > > >
> > > > > > > Yes, that makes sense. But I think it is much better off as a readable value
> > > > > > > from a virtual fs. The drawback of tracing for this sort of thing are:
> > > > > > >  - Tracing will only catch it if tracing is on
> > > > > > >  - Tracing data can be lost if too many events, then no one has a clue what
> > > > > > >    the max gp time is.
> > > > > > >  - The data is already available in rcu_state::gp_max so copying it into the
> > > > > > >    trace buffer seems a bit pointless IMHO
> > > > > > >  - It is a lot easier on ones eyes to process a single counter than process
> > > > > > >    heaps of traces.
> > > > > > >
> > > > > > > I think a minimal set of RCU counters exposed to /proc or /sys should not
> > > > > > > hurt and could do more good than not. The scheduler already does this for
> > > > > > > scheduler statistics. I have seen Peter complain a lot about new tracepoints
> > > > > > > but not much (or never) about new statistics.
> > > > > > >
> > > > > > > Tracing has its strengths but may not apply well here IMO. I think a counter
> > > > > > > like this could be useful for tuning of things like the jiffies_*_sched_qs,
> > > > > > > the stall timeouts and also any other RCU knobs. What do you think?
> > > > > >
> > > > > > I prefer proc/sys knob for it to tracepoint. Why I've considered it is just it
> > > > > > looks like undoing what Paul did at ae91aa0ad.
> > > > > >
> > > > > > I think you're rational enough. I just wondered how Paul think of it.
> > > > >
> > > > > I believe at least initially, a set of statistics can be made
> > > > > available only when rcutorture or rcuperf module is loaded. That way
> > > > > they are purely only for debugging and nothing needs to be exposed to
> > > > > normal kernels distributed thus reducing testability concerns.
> > > > >
> > > > > rcu_state::gp_max would be trivial to expose through this, but for
> > > > > other statistics that are more complicated - perhaps
> > > > > tracepoint_probe_register can be used to add hooks on to the
> > > > > tracepoints and generate statistics from them. Again the registration
> > > > > of the probe and the probe handler itself would all be in
> > > > > rcutorture/rcuperf test code and not a part of the kernel proper.
> > > > > Thoughts?
> > > >
> > > > 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 ;-)
> >
> > Evading the questions, are we?
> >
> > OK, I can be flexible.  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.
> >
> > What is the best way to work out what is really holding things up?
> 
> Either the stopper preempted another being in a critical section and
> has something wrong itself in case of PREEMPT or mechanisms for
> urgent control doesn't work correctly.
> 
> I don't know what exactly you intended but I would check things like
> (1) irq disable / eqs / tick / scheduler events and (2) whether special
> handling for each level of qs urgency has started correctly. For that
> purpose all the history of those events would be more useful.
> 
> And with thinking it more, we could come up with a good way to
> make use of those data to identify what the problem is. Do I catch
> the point correctly? If so, me and Joel can start to work on it.
> Otherwise, please correct me.

I believe you are on the right track.  In short, it would be great if
the kernel would automatically dump out the needed information when
cpu_stopper gets stalled, sort of like RCU does (much of the time,
anyway) in its CPU stall warnings.  Given a patch that did this, I would
be quite happy to help advocate for it!

							Thanx, Paul