Re: Normal RCU grace period can be stalled for long because need-resched flags not set?

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Jul 03, 2019 at 02:57:14PM -0700, Paul E. McKenney wrote:
> On Wed, Jul 03, 2019 at 05:24:26PM -0400, Joel Fernandes wrote:
> > On Wed, Jul 03, 2019 at 10:39:35AM -0700, Paul E. McKenney wrote:
> > > On Wed, Jul 03, 2019 at 12:41:34PM -0400, Joel Fernandes wrote:
> > > > On Wed, Jul 03, 2019 at 11:30:36AM -0400, Steven Rostedt wrote:
> > > > > On Wed, 3 Jul 2019 11:25:20 -0400
> > > > > Joel Fernandes <joel@joelfernandes.org> wrote:
> > > > > 
> > > > > 
> > > > > > I am sorry if this is not a realistic real-life problem, but more a
> > > > > > "doctor it hurts if I do this" problem as Steven once said ;-)
> > > > > > 
> > > > > > I'll keep poking ;-)
> > > > > 
> > > > > Hi Joel,
> > > > > 
> > > > > Can you also share the tests you are performing as well as any
> > > > > module/code changes you made so that we can duplicate the results?
> > > > 
> > > > Sure thing. Below is the diff that I applied to Paul's /dev branch. But I
> > > > believe Linus's tree should have same results.
> > > > 
> > > > After applying the diff below, I run it like this:
> > > > tools/testing/selftests/rcutorture/bin/kvm.sh --bootargs rcuperf.pd_test=1 rcuperf.pd_busy_wait=5000 rcuperf.holdout=5 rcuperf.pd_resched=0 --duration 1 --torture rcuperf
> > > > 
> > > > Some new options I added:
> > > > pd_test=1 runs the preempt disable loop test
> > > > pd_busy_wait is the busy wait time each pass through the loop in microseconds
> > > > pd_resched is whether the loop should set the need-resched flag periodically.
> > > > 
> > > > If your qemu is a bit old or from debian, then you may also need to pass: --qemu-args "-net nic,model=e1000"
> > > > 
> > > > With pd_resched = 0, I get quite high average grace-period latencies. The
> > > > preempt-disable loop thread is running on its own CPU. Enabling the rcu:*
> > > > tracepoints, I see that for long periods of time, the FQS rcu loop can be
> > > > running while the scheduler tick learns from rcu_preempt_deferred_qs() that
> > > > there's nothing to worry about (at least this is what I remember tracing).
> > > > 
> > > > With pd_resched = 0, the output of the command above:
> > > > Average grace-period duration: 195629 microseconds
> > > > Minimum grace-period duration: 30111.7
> > > > 50th percentile grace-period duration: 211000
> > > > 90th percentile grace-period duration: 218000
> > > > 99th percentile grace-period duration: 222999
> > > > Maximum grace-period duration: 236351
> > > > 
> > > > With pd_resched = 1, you get more like twice (10ms) the busy-wait time (5ms).
> > > > I wonder why its twice, but that's still Ok. It is as follows:
> > > > Average grace-period duration: 12302.2 microseconds
> > > > Minimum grace-period duration: 5998.35
> > > > 50th percentile grace-period duration: 12000.4
> > > > 90th percentile grace-period duration: 15996.4
> > > > 99th percentile grace-period duration: 18000.6
> > > > Maximum grace-period duration: 20998.6
> > > 
> > > Both of these results are within the design range for normal
> > > RCU grace-period durations on busy systems.  See the code in
> > > adjust_jiffies_till_sched_qs(), which is setting one of the "panic
> > > durations" at which RCU starts taking more aggressive actions to end
> > > the current grace period.  See especially:
> > > 
> > > 	if (j < HZ / 10 + nr_cpu_ids / RCU_JIFFIES_FQS_DIV)
> > > 		j = HZ / 10 + nr_cpu_ids / RCU_JIFFIES_FQS_DIV;
> > > 	pr_info("RCU calculated value of scheduler-enlistment delay is %ld jiffies.\n", j);
> > > 	WRITE_ONCE(jiffies_to_sched_qs, j);
> > > 
> > > This usually gets you about 100 milliseconds, and if you are starting
> > > grace periods in quick succession from a single thread while other threads
> > > are doing likewise, each grace-period wait gets to wait about two grace
> > > periods worth due to the end of the previous grace period having started
> > > a new grace period before the thread is awakened.
> > > 
> > > Of course, if this is causing trouble for some use case, it would not
> > > be hard to create a tunable to override this panic duration.  But that
> > > would of course require a real use case in real use, given that RCU isn't
> > > exactly short on tunables at the moment.  Significantly shortening this
> > > panic duration caused 0day to complain about slowness last I tried it,
> > > just so you know.
> > 
> > Thanks a lot for the explanation.
> > Indeed this code in the tick is doing a good job and I just had to drop
> > jiffies_till_first_fqs to bring down the latencies. With a
> > jiffies_till_first_fqs of 50 instead of the default of 100, the latencies
> > drop by 4 fold.
> 
> You lost me on this one.  The normal value of jiffies_till_first_fqs
> is but three, for systems with 255 or fewer CPUs and HZ=1000.  So I
> have to ask...  What did you do to get jiffies_till_first_fqs=100?
> The normal default automatic settings would need something like 8,000
> CPUs to get it up to that level.
> 
> Or did you instead mean replacing the "HZ / 10" in the code snippet
> above with "HZ / 20" or similar?
> 

I meant jiffies_to_sched_qs. Without any changes, it is 100 jiffies on my
system. Setting rcutree.jiffies_till_sched_qs sets the jiffies_to_sched_qs. I
had set it to 50 and observed dramatic improvements.

        /* If jiffies_till_sched_qs was specified, respect the request. */
        if (jiffies_till_sched_qs != ULONG_MAX) {
                WRITE_ONCE(jiffies_to_sched_qs, jiffies_till_sched_qs);
                return;
        }

> Or did you mean jiffies_to_sched_qs instead of jiffies_till_first_fqs?
> -That- does default to 100, and you could set it using the
> rcutree.jiffies_to_sched_qs kernel boot parameter.  But even then, I
> must admit that I would naively expect halving jiffies_till_first_fqs to
> halve the latencies.  But I have not looked at it closely, and there are
> lots of moving parts in RCU's grace-period encouragement code, so maybe
> that is the effect.

It could also be my sloppy testing. Now I tried again with 50 and it cuts the
latencies by around half as you said. However my histogram does have several
really nasty outliers..

(rcu-kvm is my wrapper where I pass the -net qemu args I need)

rcu-kvm --rcuperf --boot-args "rcuperf.pd_test=1 rcuperf.pd_busy_wait=5000
rcuperf.holdout=5 rcuperf.pd_resched=0 rcutree.jiffies_till_sched_qs=50"
--kvm-args "--duration 1

Log says:
0.087440] rcu:     Boot-time adjustment of scheduler-enlistment delay to 50 jiffies.

Output:
Histogram bucket size: 1000
57000 4                     
58000 1                           
59000 1                   
60000 2
103000 1
104000 2
105000 8                       
106000 44                
107000 60                                          
108000 131           
109000 164
110000 143     <---------- most of the time its ~100ms.
111000 136            
112000 51                 
113000 45                             
114000 11             
115000 4                  
12464000 1                            
12466000 2   <--- But what are these :(
12467000 2                
12468000 1                                                        
12470000 1                                  

Average grace-period duration: 215642 microseconds <-- avg ended up skewed
Minimum grace-period duration: 57979.7
50th percentile grace-period duration: 110000
90th percentile grace-period duration: 112999  <-- but this reduced.
99th percentile grace-period duration: 115000


(And just did another test and the outliers are pretty consistent).

Also, I do occassionally see that preempt disable test not getting its Need
resched bit set yet. Even though we have multiple ticks. I am trying to trace
this out and understand it as well.

Thanks a lot!

 - Joel