On Wed, Jul 03, 2019 at 08:32:13PM -0400, Joel Fernandes wrote: > On Wed, Jul 03, 2019 at 08:21:30PM -0400, Joel Fernandes wrote: > > On Wed, Jul 03, 2019 at 04:01:03PM -0700, Paul E. McKenney wrote: > > > On Wed, Jul 03, 2019 at 06:24:06PM -0400, Joel Fernandes wrote: > > > > 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@xxxxxxxxxxxxxxxxx> 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. > > > > > > Whew!!! ;-) > > > > > > > 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.. > > > > > > OK, that is more what I would expect. > > > > > > > (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 > > > > > > Well, those are a bit over one second, which is when .b.need_qs is set > > > in CONFIG_PREEMPT=y systems. Are you possibly seeing vCPU preeemption? > > > (Not that .b.need_qs does anything about vCPU preemption just yet.) > > > > Actually, I just realized, there is an extra 0 on those outliers. So it is > > really 12 seconds, for example: 12,466,000 microseconds is ~12.4 seconds. So > > these outliers are really odd since I don't see any RCU stalls, possible theories: > > > > Looking closer into the logs, it is always the last iteration of a writer: > > > > Third last iteration.. > > [ 62.157951] rcu-perf: 0 writer-duration: 99 109999933 > > [ 62.258131] rcu-perf: 1 writer-duration: 99 110999466 > > [ 62.353607] rcu-perf: 2 writer-duration: 99 112000830 > > [ 62.433249] rcu-perf: 3 writer-duration: 99 114000321 > > [ 62.510405] rcu-perf: 4 writer-duration: 99 109000240 > > [ 62.603643] rcu-perf: 5 writer-duration: 99 108999164 > > [ 62.702109] rcu-perf: 6 writer-duration: 99 111000721 > > [ 62.799970] rcu-perf: 7 writer-duration: 99 109000536 > > > > Second last iteration.. > > [ 62.158920] rcu-perf: 0 writer-duration: 100 109998415 > > [ 62.259061] rcu-perf: 1 writer-duration: 100 104024906 > > [ 62.354395] rcu-perf: 2 writer-duration: 100 105019175 > > [ 62.434072] rcu-perf: 3 writer-duration: 100 108019926 > > [ 62.511154] rcu-perf: 4 writer-duration: 100 109998839 > > [ 62.604572] rcu-perf: 5 writer-duration: 100 109000969 > > [ 62.703005] rcu-perf: 6 writer-duration: 100 107015416 > > [ 62.800894] rcu-perf: 7 writer-duration: 100 111018680 > > > > And last one which is over blown over.. > > [ 62.161123] rcu-perf: 0 writer-duration: 101 42665751175 > > [ 62.261115] rcu-perf: 1 writer-duration: 101 42693148470 > > [ 62.357093] rcu-perf: 2 writer-duration: 101 42692066685 > > [ 62.436059] rcu-perf: 3 writer-duration: 101 42692831737 > > [ 62.513063] rcu-perf: 4 writer-duration: 101 42693195036 > > [ 62.705105] rcu-perf: 6 writer-duration: 101 42692114079 > > [ 62.803104] rcu-perf: 7 writer-duration: 101 42693153435 > > > > I am wondering if this because the torture_stop() is having a hard time > > stopping my preempt disable thread, which is odd because I am checking for > > torture_must_stop() to break out of the loop as are other threads. > > > > Anyway this is clearly more of a test issue than an RCU one ;-) > > > > > > 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 > > > > > > Indeed, medians and percentiles are often more stable than are averages. > > > (But yes, you can cook up distributions that work the other way around.) > > If I add an rcu_perf_wait_shutdown() to the end of the loop, the outliers go away. > > Still can't explain that :) > > do { > ... > ... > + rcu_perf_wait_shutdown(); > } while (!torture_must_stop()); Might it be the cond_resched_tasks_rcu_qs() invoked from within rcu_perf_wait_shutdown()? So I have to ask... What happens if you use cond_resched_tasks_rcu_qs() at the end of that loop instead of rcu_perf_wait_shutdown()? Thanx, Paul