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SubjectRe: [PATCH 1/2] rcu: Don't chase unnecessary quiescent states after extended grace periods
On Wed, Nov 24, 2010 at 04:45:11PM +0100, Frederic Weisbecker wrote:
> 2010/11/24 Paul E. McKenney <paulmck@linux.vnet.ibm.com>:
> > On Wed, Nov 24, 2010 at 02:48:46PM +0100, Frederic Weisbecker wrote:
> >> 2010/11/24 Paul E. McKenney <paulmck@linux.vnet.ibm.com>:
> >> > On Wed, Nov 24, 2010 at 03:33:21AM +0100, Frederic Weisbecker wrote:
> >> >> 2010/11/24 Frederic Weisbecker <fweisbec@gmail.com>:
> >> >> > On Tue, Nov 23, 2010 at 04:58:20PM -0800, Paul E. McKenney wrote:
> >> >> >> On Wed, Nov 24, 2010 at 01:31:12AM +0100, Frederic Weisbecker wrote:
> >> >> > The softirq fires, we call rcu_process_gp_end() which will
> >> >> > update rdp->completed into the global state:
> >> >> > (rsp->completed = rnp->pgnum = rnp->completed = rsp->pgnum = 4294967002).
> >> >> >
> >> >> > But rsp->pgnum is still 2 offsets backwards.
> >> >
> >> > This one is hard for me to believe -- rsp->gpnum drives the rest of
> >> > the ->gpnum fields, right?
> >>
> >> Oops, I meant rdp->pgnum is still 2 offsets backward, for CPU 1.
> >>
> >> Sorry.
> >
> > CPU 1?  Or CPU 0?
>
> CPU 1, the one that was idle :-D
>
> So CPU 1 rdp did catch up with node and state for its completed field.
> But not its pgnum yet.

OK, I will need to take a closer look at the rdp->gpnum setting.

> >> >> > Now we call rcu_check_quiescent_state() -> check_for_new_grace_period()
> >> >> > -> note_new_gpnum() and then we end up a requested quiescent state while
> >> >> > every grace periods are completed.
> >> >>
> >> >> Sorry I should have described that in the changelogs but my ideas
> >> >> weren't as clear as they
> >> >> are now (at least I feel they are, doesn't mean they actually are ;)
> >> >> Chasing these RCU bugs for too much hours has toasted my brain..
> >> >
> >> > Welcome to my world!!!  But keep in mind that an extra timer tick
> >> > or two is much preferable to a potential hang!  And you only get
> >> > the extra timer tick if there was some other reason that the
> >> > CPU came out of nohz mode, correct?
> >>
> >> Yeah, either because of a timer, hrtimer, or a reschedule.
> >> But you still generate a spurious softirq in this scheme.
> >
> > Eliminating spurious softirqs is a good thing, but let's review the
> > overall priorities (probably missing a few, but this should give you
> > an overall view):
> >
> > 1.      No too-short grace periods!!!
>
> Oh, I did not know that. Is it to avoid too much
> short series of callbacks execution or?

To avoid breaking RCU's fundamental guarantee. ;-)

It is easy to create bugs where a CPU thinks that it needs to respond
to the grace period based on a prior quiescent state, but unknown to
it that grace period has finished and a new one has started. This
CPU might then wrongly report a quiescent state against the new grace
period. Needless to say, this is very very very very very very bad.

> > 2.      No grace-period hangs!!
> >
> > 3.      Minimize read-side overhead!
> >
> > 4.      Maintain decent update-side performance and scalability!
> >
> > 5.      Avoid inflicting real-time latency problems!
> >
> > 6.      Avoid waking up sleeping CPUs!
> >
> > 7.      Let CPUs that can do so go to sleep immediately (as opposed to
> >        waiting a few milliseconds).
> >
> > 8.      Avoid spurious softirqs.
>
> Avoiding spurious softirqs bring us back to 7, even though it's not a matter
> of ms but rather us.

Microseconds should not be a problem as long as they are rare. Milliseconds
are a problem for the battery-powered folks as well as for people who care
about OS jitter.

> > 9.      Super-optimization of update and grace-period paths.  (I normally
> >        just say "no" -- it has to be an impressive optimization for me
> >        to be willing to risk messing up #1 through #7.)
> >
> > Nevertheless, simple changes that avoid spurious softirqs are good things.
>
> Ok, then the original patch of this discussion seems to fit then.
> Although I need to respin the changelog.

Sounds good -- I will then review it carefully.

> >> Two in fact: one because of the rnp->completed != rsp->completed condition
> >> in rcu_pending(), another one because when we update the pgnum, we
> >> always start chasing QS, regardless of the last GP beeing completed or not.
> >
> > OK -- is this an example of #8 above, or is it really #7?  I am absolutely
> > not worried about a pair of back-to-back softirqs, and I don't believe
> > that you should be, either.  ;-)
>
> This seems to be 8 and 7.
>
> A pair of back-to-back softirqs is no huge deal, but still time
> consuming, spurious, etc...
> if we can easily spott they are useless, why not get rid of them. At
> least we can easily
> and reliably avoid the second one from note_new_gpnum().

As long as I can prove to myself that the patch that gets rid of them
does not cause other problems. ;-)

> >> > Which is why checking the rnp fields makes more sense to me, actually.
> >> > Acquiring rnp->lock is much less painful than pinning down the rsp state.
> >>
> >> Right.
> >>
> >> Another thing, we  already have the (rnp->gpnum) != rdp->gpnu check in
> >> rcu_pending(),
> >> why also checking (rnp->completed) != rdp->completed) ?
> >
> > Because if (rnp->completed != rdp->completed), we might need to process
> > some callbacks, either advancing them or invoking them.
>
> The rdp->nxttail in rcu is still an obscur part for me, so I just believe you :)

It is just the place that the pending rcu_head callbacks are stored.

> > By the way, have you introduced a config variable for your HPC dynticks
> > changes?
>
> I will, because I'll have to touch some fast path and I would prefer to do
> that compile-conditional.
>
> But for now I don't care and experiment drafts :)

Well, if any of the softirq/tick streamlining in RCU is risky, it needs
to go under that same #ifdef! ;-)

> I'm quite close to something that seems to work well BTW, those series of
> softirq were my latest problem as it made rcu_pending() returning 1 for
> a good while and I coudn't stop the tick. After the 2nd patch it should be
> fine now, I hope.

By "couldn't stop the tick" you mean "the tick went on a few times more
than you like" or do you really mean "couldn't ever stop the tick"?
My guess is the former. If there were problems like the latter, Arjan
would probably have beat me up about it long ago!

> >  Longer term, __rcu_pending() for your HPC dynticks case
> > could check for the current CPU having any callbacks before the call to
> > check_cpu_stalls(), as in rcu_needs_cpu_quick_check().  That way, the
> > CPUs with callbacks would drive the RCU core state machine.
>
> Ah, because currently every CPUs calling rcu_pending() can be pulled into
> handling the state machine because of this check, right?
>
> /* Has an RCU GP gone long enough to send resched IPIs &c? */
> if (rcu_gp_in_progress(rsp) &&
> ULONG_CMP_LT(ACCESS_ONCE(rsp->jiffies_force_qs), jiffies)) {
> rdp->n_rp_need_fqs++;
> return 1;
>
> So the goal would be to let this job to CPUs that have callbacks, right?

In the dynticks-HPC case, yes.

> > We don't
> > necessarily want this in the common case because it can increase
> > grace-period latency
>
> Because it makes less CPUs to handle the grace period state machine?

Not quite. Here I am not worried about the dyntick-HPC case, but rather
the usual more-runnable-tasks-than-CPUs case. Here, CPUs are context
switching frequently, so if they report their own quiescent states (even
when they don't happen to have any callbacks queued) the grace period will
complete more quickly. After all, force_quiescent_state() doesn't even
get started until the third tick following the start of the grace period.

In contrast, if we let the context-switching callback-free CPUs do their
own work, the grace period will likely end in a tick or two.

> > but it could be very useful in the HPC-dyntick
> > case -- eliminate any number of sources of spurious ticks with low
> > risk to the high-priority RCU properties.  There are some corresponding
> > changes required on the force_quiescent_state() path, but these are
> > reasonably straightforward.
>
> Ok, I may have a try at it (if I completely understand what you suggest :-)
> But first I'll try to get that dyntick mode with the current state, which seems
> to be enough for the basic desired functionality.

If the two changes you have posted thus far are all you need, this does
sound like a good starting point.

Thanx, Paul
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