On Thu, Aug 15, 2019 at 11:39:37AM -0700, Paul E. McKenney wrote: > On Thu, Aug 15, 2019 at 02:15:00PM -0400, Joel Fernandes wrote: > > On Thu, Aug 15, 2019 at 10:23:51AM -0700, Paul E. McKenney wrote: > > > > [snip] > > > > > > > If so, perhaps that monitoring could periodically invoke an RCU function > > > > > > > that I provide for deciding when to turn the tick on. We would also need > > > > > > > to work out how to turn the tick off in a timely fashion once the CPU got > > > > > > > out of kernel mode, perhaps in rcu_user_enter() or rcu_nmi_exit_common(). > > > > > > > > > > > > > > If this would be called only every second or so, the separate grace-period > > > > > > > checking is still needed for its shorter timespan, though. > > > > > > > > > > > > > > Thoughts? > > > > > > > > > > > > Do you want me to test the below patch to see if it fixes the issue with my > > > > > > other test case (where I had a nohz full CPU holding up a grace period). > > > > > > > > > > Please! > > > > > > > > I tried the patch below, but it did not seem to make a difference to the > > > > issue I was seeing. My test tree is here in case you can spot anything I did > > > > not do right: https://github.com/joelagnel/linux-kernel/commits/rcu/nohz-test > > > > The main patch is here: > > > > https://github.com/joelagnel/linux-kernel/commit/4dc282b559d918a0be826936f997db0bdad7abb3 > > > > > > That is more aggressive that rcutorture's rcu_torture_fwd_prog_nr(), so > > > I am guessing that I need to up rcu_torture_fwd_prog_nr()'s game. I am > > > currently testing that. > > > > > > > On the trace output, I grep something like: egrep "(rcu_perf|cpu 3|3d)". I > > > > see a few ticks after 300ms, but then there are no more ticks and just a > > > > periodic resched_cpu() from rcu_implicit_dynticks_qs(): > > > > > > > > [ 19.534107] rcu_perf-165 12.... 2276436us : rcu_perf_writer: Start of rcuperf test > > > > [ 19.557968] rcu_pree-10 0d..1 2287973us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > > > [ 20.136222] rcu_perf-165 3d.h. 2591894us : rcu_sched_clock_irq: sched-tick > > > > [ 20.137185] rcu_perf-165 3d.h2 2591906us : rcu_sched_clock_irq: sched-tick > > > > [ 20.138149] rcu_perf-165 3d.h. 2591911us : rcu_sched_clock_irq: sched-tick > > > > [ 20.139106] rcu_perf-165 3d.h. 2591915us : rcu_sched_clock_irq: sched-tick > > [snip] > > > > [ 20.147797] rcu_perf-165 3d.h. 2591953us : rcu_sched_clock_irq: sched-tick > > > > [ 20.148759] rcu_perf-165 3d.h. 2591957us : rcu_sched_clock_irq: sched-tick > > > > [ 20.151655] rcu_pree-10 0d..1 2591979us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > > > [ 20.732938] rcu_pree-10 0d..1 2895960us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > [snip] > > > > [ 26.566100] rcu_pree-10 0d..1 5935982us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > > > [ 27.144497] rcu_pree-10 0d..1 6239973us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > > > [ 27.192661] rcu_perf-165 3d.h. 6276923us : rcu_sched_clock_irq: sched-tick > > > > [ 27.705789] rcu_pree-10 0d..1 6541901us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > > > [ 28.292155] rcu_pree-10 0d..1 6845974us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > > > [ 28.874049] rcu_pree-10 0d..1 7149972us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > > > [ 29.112646] rcu_perf-165 3.... 7275951us : rcu_perf_writer: End of rcuperf test > > > > > > That would be due to my own stupidity. I forgot to clear ->rcu_forced_tick > > > in rcu_disable_tick_upon_qs() inside the "if" statement. This of course > > > prevents rcu_nmi_exit_common() from ever re-enabling it. > > > > > > Excellent catch! Thank you for testing this!!! > > > > Ah I missed it too. Happy to help! I tried setting it as below but getting > > same results: > > > > +/* > > + * If the scheduler-clock interrupt was enabled on a nohz_full CPU > > + * in order to get to a quiescent state, disable it. > > + */ > > +void rcu_disable_tick_upon_qs(struct rcu_data *rdp) > > +{ > > + if (tick_nohz_full_cpu(rdp->cpu) && rdp->rcu_forced_tick) > > + tick_dep_clear_cpu(rdp->cpu, TICK_DEP_MASK_RCU); > > + rdp->rcu_forced_tick = false; > > I put this inside the "if" statement, though I would not expect that to > change behavior in this case. > > Does your test case still avoid turning on the tick more than once? Or > is it turning on the tick each time the grace period gets too long, but > without the tick managing to end the grace periods? I will put some more prints and let you know. But it looks like I see a print from rcu_sched_clock_irq() only once at around 700ms from the start of the test loop. After that I don't see prints at all for the rest of the 7 seconds of the test. Before the test starts, I see several rcu_sched_clock_irq() at the regular tick interval of 1 ms (HZ=1000). > > > > [snip] > > > > > Without that code, RCU can give false-positive splats at various points > > > > > in its processing. ("Wait! How can a task be blocked waiting on a > > > > > grace period that hasn't even started yet???") > > > > > > > > I did not fully understand the question in brackets though, a task can be on > > > > a different CPU though which has nothing to do with the CPU that's going > > > > offline/online so it could totally be waiting on a grace period right? > > > > > > > > Also waiting on a grace period that hasn't even started is totally possible: > > > > > > > > GP1 GP2 > > > > |<--------->|<-------->| > > > > ^ ^ > > > > | |____ task gets unblocked > > > > task blocks > > > > on synchronize_rcu > > > > but is waiting on > > > > GP2 which hasn't started > > > > > > > > Or did I misunderstand the question? > > > > > > There is a ->gp_tasks field in the leaf rcu_node structures that > > > references a list of tasks blocking the current grace period. When there > > > is no grace period in progress (as is the case from the end of GP1 to > > > the beginning of GP2, the RCU code expects ->gp_tasks to be NULL. > > > Without the curiosity code you pointed out above, ->gp_tasks could > > > in fact end up being non-NULL when no grace period was in progress. > > > > > > And did end up being non-NULL from time to time, initially every few > > > hundred hours of a particular rcutorture scenario. > > > > Oh ok! I will think more about it. I am not yet able to connect the gp_tasks > > being non-NULL to the CPU going offline/online scenario though. Maybe I > > should delete this code, run an experiment and trace for this condition > > (gp_tasks != NULL)? > > Or you could dig through the git logs for this code change. Ok will do. > > I love it how you found these issues by heavy testing and fixed them. > > Me, I would have rather foreseen them and avoided them in the first place, > but I agree that it is better for rcutorture to find them than for some > hapless user somewhere to be inconvenienced by them. ;-) True, forseeing is always better ;) thanks, - Joel
