On Thu, Aug 15, 2019 at 10:23:51AM -0700, Paul E. McKenney wrote: > On Thu, Aug 15, 2019 at 11:07:35AM -0400, Joel Fernandes wrote: > > On Wed, Aug 14, 2019 at 03:05:16PM -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; +} + > > [snip] > > > > > if (rnp->qsmask & mask) { /* RCU waiting on incoming CPU? */ > > > > > + rcu_disable_tick_upon_qs(rdp); > > > > > /* Report QS -after- changing ->qsmaskinitnext! */ > > > > > rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags); > > > > > > > > Just curious about the existing code. If a CPU is just starting up (after > > > > bringing it online), how can RCU be waiting on it? I thought RCU would not be > > > > watching offline CPUs. > > > > > > Well, neither grace periods nor CPU-hotplug operations are atomic, > > > and each can take significant time to complete. > > > > > > So suppose we have a large system with multiple leaf rcu_node structures > > > (not that 17 CPUs is all that many these days, but please bear with me). > > > Suppose just after a new grace period initializes a given leaf rcu_node > > > structure, one of its CPUs goes offline (yes, that CPU would have to > > > have waited on a grace period, but that might have been the previous > > > grace period). But before the FQS scan notices that RCU is waiting on > > > an offline CPU, the CPU comes back online. > > > > > > That situation is exactly what the above code is intended to handle. > > > > That makes sense! > > > > > 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)? I love it how you found these issues by heavy testing and fixed them. thanks, - Joel
