On Wed, Aug 14, 2019 at 03:05:16PM -0700, Paul E. McKenney wrote: [snip] > > > > Arming a CPU timer could also be an alternative to tick_set_dep_cpu() for that. > > > > > > > > What do you think? > > > > > > Left to itself, RCU would take action only when a given nohz_full > > > in-kernel CPU was delaying a grace period, which is what the (lightly > > > tested) patch below is supposed to help with. If that is all that is > > > needed, well and good! > > > > > > But should we need long-running in-kernel nohz_full CPUs to turn on > > > their ticks when they are not blocking an RCU grace period, for example, > > > when RCU is idle, more will be needed. To that point, isn't there some > > > sort of monitoring that checks up on nohz_full CPUs ever second or so? > > > > Wouldn't such monitoring need to be more often than a second, given that > > rcu_urgent_qs and rcu_need_heavy_qs are configured typically to be sooner > > (200-300 jiffies on my system). > > Either it would have to be more often than once per second, or RCU would > need to retain its more frequent checks. But note that RCU isn't going > to check unless there is a grace period in progress. Sure. > > > 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 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 [ 20.140077] rcu_perf-165 3d.h. 2591919us : rcu_sched_clock_irq: sched-tick [ 20.141041] rcu_perf-165 3d.h. 2591924us : rcu_sched_clock_irq: sched-tick [ 20.142001] rcu_perf-165 3d.h. 2591928us : rcu_sched_clock_irq: sched-tick [ 20.142961] rcu_perf-165 3d.h. 2591932us : rcu_sched_clock_irq: sched-tick [ 20.143925] rcu_perf-165 3d.h. 2591936us : rcu_sched_clock_irq: sched-tick [ 20.144885] rcu_perf-165 3d.h. 2591940us : rcu_sched_clock_irq: sched-tick [ 20.145876] rcu_perf-165 3d.h. 2591945us : rcu_sched_clock_irq: sched-tick [ 20.146835] rcu_perf-165 3d.h. 2591949us : rcu_sched_clock_irq: sched-tick [ 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 [ 21.318104] rcu_pree-10 0d..1 3199975us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 [ 21.899908] rcu_pree-10 0d..1 3503964us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 [ 22.481316] rcu_pree-10 0d..1 3807990us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 [ 23.065623] rcu_pree-10 0d..1 4111990us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 [ 23.650875] rcu_pree-10 0d..1 4415989us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 [ 24.233999] rcu_pree-10 0d..1 4719978us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 [ 24.818397] rcu_pree-10 0d..1 5023982us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 [ 25.402633] rcu_pree-10 0d..1 5327981us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 [ 25.984104] rcu_pree-10 0d..1 5631976us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 [ 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 [snip] > > > @@ -2906,7 +2927,7 @@ void rcu_barrier(void) > > > /* Did someone else do our work for us? */ > > > if (rcu_seq_done(&rcu_state.barrier_sequence, s)) { > > > rcu_barrier_trace(TPS("EarlyExit"), -1, > > > - rcu_state.barrier_sequence); > > > + rcu_state.barrier_sequence); > > > smp_mb(); /* caller's subsequent code after above check. */ > > > mutex_unlock(&rcu_state.barrier_mutex); > > > return; > > > @@ -2938,11 +2959,11 @@ void rcu_barrier(void) > > > continue; > > > if (rcu_segcblist_n_cbs(&rdp->cblist)) { > > > rcu_barrier_trace(TPS("OnlineQ"), cpu, > > > - rcu_state.barrier_sequence); > > > + rcu_state.barrier_sequence); > > > smp_call_function_single(cpu, rcu_barrier_func, NULL, 1); > > > } else { > > > rcu_barrier_trace(TPS("OnlineNQ"), cpu, > > > - rcu_state.barrier_sequence); > > > + rcu_state.barrier_sequence); > > > } > > > } > > > put_online_cpus(); > > > @@ -3168,6 +3189,7 @@ void rcu_cpu_starting(unsigned int cpu) > > > rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq); > > > rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags); > > > 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? thanks! - Joel
