Re: RCU vs NOHZ

From: Paul E. McKenney
Date: Fri Sep 23 2022 - 12:01:28 EST

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On Fri, Sep 23, 2022 at 11:12:17AM -0400, Joel Fernandes wrote:
> On 9/21/2022 5:36 PM, Paul E. McKenney wrote:
> > On Sat, Sep 17, 2022 at 07:25:08AM -0700, Paul E. McKenney wrote:
> >> On Fri, Sep 16, 2022 at 11:20:14AM +0200, Peter Zijlstra wrote:
> >>> On Fri, Sep 16, 2022 at 12:58:17AM -0700, Paul E. McKenney wrote:
> >>>
> >>>> To the best of my knowledge at this point in time, agreed. Who knows
> >>>> what someone will come up with next week? But for people running certain
> >>>> types of real-time and HPC workloads, context tracking really does handle
> >>>> both idle and userspace transitions.
> >>>
> >>> Sure, but idle != nohz. Nohz is where we disable the tick, and currently
> >>> RCU can inhibit this -- rcu_needs_cpu().
> >>
> >> Exactly. For non-nohz userspace execution, the tick is still running
> >> anyway, so RCU of course won't be inhibiting its disabling. And in that
> >> case, RCU's hook is the tick interrupt itself. RCU's hook is passed a
> >> flag saying whether the interrupt came from userspace or from kernel.
> >>
> >>> AFAICT there really isn't an RCU hook for this, not through context
> >>> tracking not through anything else.
> >>
> >> There is a directly invoked RCU hook for any transition that enables or
> >> disables the tick, namely the ct_*_enter() and ct_*_exit() functions,
> >> that is, those functions formerly known as rcu_*_enter() and rcu_*_exit().
> >>
> >>>> It wasn't enabled for ChromeOS.
> >>>>
> >>>> When fully enabled, it gave them the energy-efficiency advantages Joel
> >>>> described. And then Joel described some additional call_rcu_lazy()
> >>>> changes that provided even better energy efficiency. Though I believe
> >>>> that the application should also be changed to avoid incessantly opening
> >>>> and closing that file while the device is idle, as this would remove
> >>>> -all- RCU work when nearly idle. But some of the other call_rcu_lazy()
> >>>> use cases would likely remain.
> >>>
> >>> So I'm thinking the scheme I outlined gets you most if not all of what
> >>> lazy would get you without having to add the lazy thing. A CPU is never
> >>> refused deep idle when it passes off the callbacks.
> >>>
> >>> The NOHZ thing is a nice hook for 'this-cpu-wants-to-go-idle-long-term'
> >>> and do our utmost bestest to move work away from it. You *want* to break
> >>> affinity at this point.
> >>>
> >>> If you hate on the global, push it to a per rcu_node offload list until
> >>> the whole node is idle and then push it up the next rcu_node level until
> >>> you reach the top.
> >>>
> >>> Then when the top rcu_node is full idle; you can insta progress the QS
> >>> state and run the callbacks and go idle.
> >>
> >> Unfortunately, the overhead of doing all that tracking along with
> >> resolving all the resulting race conditions will -increase- power
> >> consumption. With RCU, counting CPU wakeups is not as good a predictor
> >> of power consumption as one might like. Sure, it is a nice heuristic
> >> in some cases, but with RCU it is absolutely -not- a replacement for
> >> actually measuring power consumption on real hardware. And yes, I did
> >> learn this the hard way. Why do you ask? ;-)
> >>
> >> And that is why the recently removed CONFIG_RCU_FAST_NO_HZ left the
> >> callbacks in place and substituted a 4x slower timer for the tick.
> >> -That- actually resulted in significant real measured power savings on
> >> real hardware.
> >>
> >> Except that everything that was building with CONFIG_RCU_FAST_NO_HZ
> >> was also doing nohz_full on each and every CPU. Which meant that all
> >> that CONFIG_RCU_FAST_NO_HZ was doing for them was adding an additional
> >> useless check on each transition to and from idle. Which in turn is why
> >> CONFIG_RCU_FAST_NO_HZ was removed. No one was using it in any way that
> >> made any sense.
> >>
> >> And more recent testing with rcu_nocbs on both ChromeOS and Android has
> >> produced better savings than was produced by CONFIG_RCU_FAST_NO_HZ anyway.
> >>
> >> Much of the additional savings from Joel et al.'s work is not so much
> >> from reducing the number of ticks, but rather from reducing the number
> >> of grace periods, which are of course much heavier weight.
> >>
> >> And this of course means that any additional schemes to reduce RCU's
> >> power consumption must be compared (with real measurements on real
> >> hardware!) to Joel et al.'s work, whether in combination or as an
> >> alternative. And either way, the power savings must of course justify
> >> the added code and complexity.
> >
> > And here is an untested patch that in theory might allow much of the
> > reduction in power with minimal complexity/overhead for kernels without
> > rcu_nocbs CPUs. On the off-chance you know of someone who would be
> > willing to do a realistic evaluation of it.
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit 80fc02e80a2dfb6c7468217cff2d4494a1c4b58d
> > Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Date: Wed Sep 21 13:30:24 2022 -0700
> >
> > rcu: Let non-offloaded idle CPUs with callbacks defer tick
> >
> > When a CPU goes idle, rcu_needs_cpu() is invoked to determine whether or
> > not RCU needs the scheduler-clock tick to keep interrupting. Right now,
> > RCU keeps the tick on for a given idle CPU if there are any non-offloaded
> > callbacks queued on that CPU.
> >
> > But if all of these callbacks are waiting for a grace period to finish,
> > there is no point in scheduling a tick before that grace period has any
> > reasonable chance of completing. This commit therefore delays the tick
> > in the case where all the callbacks are waiting for a specific grace
> > period to elapse. In theory, this should result in a 50-70% reduction in
> > RCU-induced scheduling-clock ticks on mostly-idle CPUs. In practice, TBD.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >
> > diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> > index 9bc025aa79a3..84e930c11065 100644
> > --- a/include/linux/rcutiny.h
> > +++ b/include/linux/rcutiny.h
> > @@ -133,7 +133,7 @@ static inline void rcu_softirq_qs(void)
> > rcu_tasks_qs(current, (preempt)); \
> > } while (0)
> >
> > -static inline int rcu_needs_cpu(void)
> > +static inline int rcu_needs_cpu(u64 basemono, u64 *nextevt)
> > {
> > return 0;
> > }
> > diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
> > index 70795386b9ff..3066e0975022 100644
> > --- a/include/linux/rcutree.h
> > +++ b/include/linux/rcutree.h
> > @@ -19,7 +19,7 @@
> >
> > void rcu_softirq_qs(void);
> > void rcu_note_context_switch(bool preempt);
> > -int rcu_needs_cpu(void);
> > +int rcu_needs_cpu(u64 basemono, u64 *nextevt);
> > void rcu_cpu_stall_reset(void);
> >
> > /*
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 5ec97e3f7468..47cd3b0d2a07 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -676,12 +676,33 @@ void __rcu_irq_enter_check_tick(void)
> > * scheduler-clock interrupt.
> > *
> > * Just check whether or not this CPU has non-offloaded RCU callbacks
> > - * queued.
> > + * queued that need immediate attention.
> > */
> > -int rcu_needs_cpu(void)
> > +int rcu_needs_cpu(u64 basemono, u64 *nextevt)
> > {
> > - return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist) &&
> > - !rcu_rdp_is_offloaded(this_cpu_ptr(&rcu_data));
> > + struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> > + struct rcu_segcblist *rsclp = &rdp->cblist;
> > +
> > + // Disabled, empty, or offloaded means nothing to do.
> > + if (!rcu_segcblist_is_enabled(rsclp) ||
> > + rcu_segcblist_empty(rsclp) || rcu_rdp_is_offloaded(rdp)) {
> > + *nextevt = KTIME_MAX;
> > + return 0;
> > + }
> > +
> > + // Callbacks ready to invoke or that have not already been
> > + // assigned a grace period need immediate attention.
> > + if (!rcu_segcblist_segempty(rsclp, RCU_DONE_TAIL) ||
> > + !rcu_segcblist_segempty(rsclp, RCU_NEXT_TAIL))
> > + return 1;> +
> > + // There are callbacks waiting for some later grace period.
> > + // Wait for about a grace period or two for the next tick, at which
> > + // point there is high probability that this CPU will need to do some
> > + // work for RCU.
> > + *nextevt = basemono + TICK_NSEC * (READ_ONCE(jiffies_till_first_fqs) > + READ_ONCE(jiffies_till_next_fqs) + 1);
>
> Looks like nice idea. Could this race with the main GP thread on another CPU
> completing the grace period, then on this CPU there is actually some work to do
> but rcu_needs_cpu() returns 0.
>
> I think it is plausible but not common, in which case the extra delay is
> probably Ok.

Glad you like it!

Yes, that race can happen, but it can also happen today.
A scheduling-clock interrupt might arrive at a CPU just as a grace
period finishes. Yes, the delay is longer with this patch. If this
proves to be a problem, then the delay heuristic might expanded to
include the age of the current grace period.

But keeping it simple to start with.

> Also, if the RCU readers take a long time, then we'd still wake up the system
> periodically although with the above change, much fewer times, which is a good
> thing.

And the delay heuristic could also be expanded to include a digitally
filtered estimate of grace-period duration. But again, keeping it simple
to start with. ;-)

My guess is that offloading gets you more power savings, but I don't
have a good way of testing this guess.

Thanx, Paul

> Thanks,
>
> - Joel
>
>
>
> > + return 0;
> > }
> >
> > /*
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index b0e3c9205946..303ea15cdb96 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -784,7 +784,7 @@ static inline bool local_timer_softirq_pending(void)
> >
> > static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
> > {
> > - u64 basemono, next_tick, delta, expires;
> > + u64 basemono, next_tick, next_tmr, next_rcu, delta, expires;
> > unsigned long basejiff;
> > unsigned int seq;
> >
> > @@ -807,7 +807,7 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
> > * minimal delta which brings us back to this place
> > * immediately. Lather, rinse and repeat...
> > */
> > - if (rcu_needs_cpu() || arch_needs_cpu() ||
> > + if (rcu_needs_cpu(basemono, &next_rcu) || arch_needs_cpu() ||
> > irq_work_needs_cpu() || local_timer_softirq_pending()) {
> > next_tick = basemono + TICK_NSEC;
> > } else {
> > @@ -818,8 +818,10 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
> > * disabled this also looks at the next expiring
> > * hrtimer.
> > */
> > - next_tick = get_next_timer_interrupt(basejiff, basemono);
> > - ts->next_timer = next_tick;
> > + next_tmr = get_next_timer_interrupt(basejiff, basemono);
> > + ts->next_timer = next_tmr;
> > + /* Take the next rcu event into account */
> > + next_tick = next_rcu < next_tmr ? next_rcu : next_tmr;
> > }
> >
> > /*

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