Re: [PATCH 1/1] Reduce synchronize_rcu() waiting time

[Paul E. McKenney]

On Mon, Mar 27, 2023 at 10:29:31PM -0400, Joel Fernandes wrote:
> Hello,
> 
> > On Mar 27, 2023, at 9:06 PM, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
> > 
> > On Mon, Mar 27, 2023 at 11:21:23AM +0000, Zhang, Qiang1 wrote:
> >>>> From: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> >>>> Sent: Tuesday, March 21, 2023 6:28 PM
> >>>> [...]
> >>>> Subject: [PATCH 1/1] Reduce synchronize_rcu() waiting time
> >>>> 
> >>>> A call to a synchronize_rcu() can be expensive from time point of view.
> >>>> Different workloads can be affected by this especially the ones which use this
> >>>> API in its time critical sections.
> >>>> 
> >>> 
> >>> This is interesting and meaningful research. ;-)
> >>> 
> >>>> For example in case of NOCB scenario the wakeme_after_rcu() callback
> >>>> invocation depends on where in a nocb-list it is located. Below is an example
> >>>> when it was the last out of ~3600 callbacks:
> >>> 
> >> 
> >> 
> >> 
> >> Can it be implemented separately as follows?  it seems that the code is simpler
> >> (only personal opinion)  😊.
> >> 
> >> But I didn't test whether this reduce synchronize_rcu() waiting time
> >> 
> >> +static void rcu_poll_wait_gp(struct rcu_tasks *rtp)
> >> +{
> >> +       unsigned long gp_snap;
> >> +
> >> +       gp_snap = start_poll_synchronize_rcu();
> >> +       while (!poll_state_synchronize_rcu(gp_snap))
> >> +               schedule_timeout_idle(1);
> > 
> > I could be wrong, but my guess is that the guys working with
> > battery-powered devices are not going to be very happy with this loop.
> > 
> > All those wakeups by all tasks waiting for a grace period end up
> > consuming a surprisingly large amount of energy.
> 
> Is that really the common case? On the general topic of wake-ups:
> Most of the time there should be only one
> task waiting synchronously on a GP to end. If that is
> true, then it feels like waking
> up nocb Kthreads which indirectly wake other threads is doing more work than usual?

A good question, and the number of outstanding synchronize_rcu()
calls will of course be limited by the number of tasks in the system.
But I myself have raised the ire of battery-powered embedded folks with
a rather small number of wakeups, so...

And on larger systems there can be a tradeoff between contention on
the one hand and number of wakeups on the other.

The original nocb implementation in fact had the grace-period kthead
waking up all of what are now called rcuoc kthreads.  The indirect scheme
reduced the total number of wakeups by up to 50% and also reduced the
CPU consumption of the grace-period kthread, which otherwise would have
become a bottleneck on large systems.

And also, a scheme that directly wakes tasks waiting in synchronize_rcu()
might well use the same ->nocb_gp_wq[] waitqueues that are used by the
rcuog kthreads, if that is what you were getting at.

> I am curious to measure how much does Vlad patch reduce wakeups in the common case.

Sounds like a good thing to measure!

> I was also wondering how Vlad patch effects RCU-barrier ordering. I guess
> we want the wake up to happen in the order of
> other callbacks also waiting.

OK, I will bite.  Why would rcu_barrier() need to care about the
synchronize_rcu() invocations if they no longer used call_rcu()?

> One last note, most battery powered systems are perhaps already using expedited RCU ;-)

Good point.  And that does raise the question of exactly what workloads
and systems want faster wakeups from synchronize_rcu() and cannot get
this effect from expedited grace periods.

							Thanx, Paul

> Thoughts?
> 
>  - Joel 
> 
> > 
> >                            Thanx, Paul
> > 
> >> +}
> >> +
> >> +void call_rcu_poll(struct rcu_head *rhp, rcu_callback_t func);
> >> +DEFINE_RCU_TASKS(rcu_poll, rcu_poll_wait_gp, call_rcu_poll,
> >> +                 "RCU Poll");
> >> +void call_rcu_poll(struct rcu_head *rhp, rcu_callback_t func)
> >> +{
> >> +       call_rcu_tasks_generic(rhp, func, &rcu_poll);
> >> +}
> >> +EXPORT_SYMBOL_GPL(call_rcu_poll);
> >> +
> >> +void synchronize_rcu_poll(void)
> >> +{
> >> +       synchronize_rcu_tasks_generic(&rcu_poll);
> >> +}
> >> +EXPORT_SYMBOL_GPL(synchronize_rcu_poll);
> >> +
> >> +static int __init rcu_spawn_poll_kthread(void)
> >> +{
> >> +       cblist_init_generic(&rcu_poll);
> >> +       rcu_poll.gp_sleep = HZ / 10;
> >> +       rcu_spawn_tasks_kthread_generic(&rcu_poll);
> >> +       return 0;
> >> +}
> >> 
> >> Thanks
> >> Zqiang
> >> 
> >> 
> >>>> 
> >>>> <snip>
> >>>>  <...>-29      [001] d..1. 21950.145313: rcu_batch_start: rcu_preempt
> >>>> CBs=3613 bl=28
> >>>> ...
> >>>>  <...>-29      [001] ..... 21950.152578: rcu_invoke_callback: rcu_preempt
> >>>> rhp=00000000b2d6dee8 func=__free_vm_area_struct.cfi_jt
> >>>>  <...>-29      [001] ..... 21950.152579: rcu_invoke_callback: rcu_preempt
> >>>> rhp=00000000a446f607 func=__free_vm_area_struct.cfi_jt
> >>>>  <...>-29      [001] ..... 21950.152580: rcu_invoke_callback: rcu_preempt
> >>>> rhp=00000000a5cab03b func=__free_vm_area_struct.cfi_jt
> >>>>  <...>-29      [001] ..... 21950.152581: rcu_invoke_callback: rcu_preempt
> >>>> rhp=0000000013b7e5ee func=__free_vm_area_struct.cfi_jt
> >>>>  <...>-29      [001] ..... 21950.152582: rcu_invoke_callback: rcu_preempt
> >>>> rhp=000000000a8ca6f9 func=__free_vm_area_struct.cfi_jt
> >>>>  <...>-29      [001] ..... 21950.152583: rcu_invoke_callback: rcu_preempt
> >>>> rhp=000000008f162ca8 func=wakeme_after_rcu.cfi_jt
> >>>>  <...>-29      [001] d..1. 21950.152625: rcu_batch_end: rcu_preempt CBs-
> >>>> invoked=3612 idle=....
> >>>> <snip>
> >>>> 
> >>> 
> >>> Did the results above tell us that CBs-invoked=3612 during the time 21950.145313 ~ 21950.152625?
> >>> 
> >>> Yes.
> >>> 
> >>> 
> >>> If possible, may I know the steps, commands, and related parameters to produce the results above?
> >>> Thank you!
> >>> 
> >>> Build the kernel with CONFIG_RCU_TRACE configuration. Update your "set_event"
> >>> file with appropriate traces:
> >>> 
> >>> <snip>
> >>> XQ-DQ54:/sys/kernel/tracing # echo rcu:rcu_batch_start rcu:rcu_batch_end rcu:rcu_invoke_callback > set_event
> >>> 
> >>> XQ-DQ54:/sys/kernel/tracing # cat set_event
> >>> rcu:rcu_batch_start
> >>> rcu:rcu_invoke_callback
> >>> rcu:rcu_batch_end
> >>> XQ-DQ54:/sys/kernel/tracing #
> >>> <snip>
> >>> 
> >>> Collect traces as much as you want: XQ-DQ54:/sys/kernel/tracing # echo 1 > tracing_on; sleep 10; echo 0 > tracing_on
> >>> Next problem is how to parse it. Of course you will not be able to parse
> >>> megabytes of traces. For that purpose i use a special C trace parser.
> >>> If you need an example please let me know i can show here.
> >>> 
> >>> --
> >>> Uladzislau Rezki