Messages in this thread | | | Date | Wed, 4 Apr 2018 17:13:58 -0700 | From | "Paul E. McKenney" <> | Subject | Re: rcu_process_callbacks irqsoff latency caused by taking spinlock with irqs disabled |
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On Thu, Apr 05, 2018 at 09:34:14AM +1000, Nicholas Piggin wrote: > Hi Paul, > > Just looking at latencies, and RCU showed up as one of the maximums. > This is a 2 socket system with (176 CPU threads). Just doing a > `make -j 352` kernel build. Got a max latency of 3ms. I don't think > that's anything to worry about really, but I wanted to check the > cause.
Well, that 3 milliseconds would cause serious problems for some workloads...
> # tracer: irqsoff > # > # irqsoff latency trace v1.1.5 on 4.16.0-01530-g43d1859f0994 > # -------------------------------------------------------------------- > # latency: 3055 us, #19/19, CPU#135 | (M:server VP:0, KP:0, SP:0 HP:0 #P:176) > # ----------------- > # | task: cc1-58689 (uid:1003 nice:0 policy:0 rt_prio:0) > # ----------------- > # => started at: rcu_process_callbacks > # => ended at: _raw_spin_unlock_irqrestore > # > # > # _------=> CPU# > # / _-----=> irqs-off > # | / _----=> need-resched > # || / _---=> hardirq/softirq > # ||| / _--=> preempt-depth > # |||| / delay > # cmd pid ||||| time | caller > # \ / ||||| \ | / > <...>-58689 135d.s. 0us : rcu_process_callbacks > <...>-58689 135d.s. 1us : cpu_needs_another_gp <-rcu_process_callbacks > <...>-58689 135d.s. 2us : rcu_segcblist_future_gp_needed <-cpu_needs_another_gp > <...>-58689 135d.s. 3us#: _raw_spin_lock <-rcu_process_callbacks > <...>-58689 135d.s. 3047us : rcu_start_gp <-rcu_process_callbacks > <...>-58689 135d.s. 3048us : rcu_advance_cbs <-rcu_start_gp > <...>-58689 135d.s. 3049us : rcu_segcblist_pend_cbs <-rcu_advance_cbs > <...>-58689 135d.s. 3049us : rcu_segcblist_advance <-rcu_advance_cbs > <...>-58689 135d.s. 3050us : rcu_accelerate_cbs <-rcu_start_gp > <...>-58689 135d.s. 3050us : rcu_segcblist_pend_cbs <-rcu_accelerate_cbs > <...>-58689 135d.s. 3051us : rcu_segcblist_accelerate <-rcu_accelerate_cbs > <...>-58689 135d.s. 3052us : trace_rcu_future_gp.isra.0 <-rcu_accelerate_cbs > <...>-58689 135d.s. 3052us : trace_rcu_future_gp.isra.0 <-rcu_accelerate_cbs > <...>-58689 135d.s. 3053us : rcu_start_gp_advanced.isra.35 <-rcu_start_gp > <...>-58689 135d.s. 3053us : cpu_needs_another_gp <-rcu_start_gp_advanced.isra.35 > <...>-58689 135d.s. 3054us : _raw_spin_unlock_irqrestore <-rcu_process_callbacks > <...>-58689 135d.s. 3055us : _raw_spin_unlock_irqrestore > <...>-58689 135d.s. 3056us : trace_hardirqs_on <-_raw_spin_unlock_irqrestore > <...>-58689 135d.s. 3061us : <stack trace> > > So it's taking a contende lock with interrupts disabled: > > static void > __rcu_process_callbacks(struct rcu_state *rsp) > { > unsigned long flags; > bool needwake; > struct rcu_data *rdp = raw_cpu_ptr(rsp->rda); > > WARN_ON_ONCE(!rdp->beenonline); > > /* Update RCU state based on any recent quiescent states. */ > rcu_check_quiescent_state(rsp, rdp); > > /* Does this CPU require a not-yet-started grace period? */ > local_irq_save(flags); > if (cpu_needs_another_gp(rsp, rdp)) { > raw_spin_lock_rcu_node(rcu_get_root(rsp)); /* irqs disabled. */ > needwake = rcu_start_gp(rsp); > raw_spin_unlock_irqrestore_rcu_node(rcu_get_root(rsp), flags); > if (needwake) > rcu_gp_kthread_wake(rsp); > } else { > local_irq_restore(flags); > } > > Because irqs are disabled before taking the lock, we can't spin with > interrupts enabled. > > cpu_needs_another_gp needs interrupts off to prevent races with normal > callback registry, but that doesn't seem to be preventing any wider > races in this code, because we immediately re-enable interrupts anyway > if no gp is needed. So an interrupt can come in right after that and > queue something up. > > So then the question is whether it's safe-albeit-racy to call with > interrupts ensabled? Would be nice to move it to a spin_lock_irqsave.
If I recall correctly, the issue is that an unsynchronized (due to interrupts being enabled) check in the "if" statement can cause extra unneeded grace periods.
I am guessing that the long latency is caused by lots of CPUs suddenly needing a new grace period at about the same time. If so, this is a bottleneck that I have been expecting for some time, and one that I would resolve by introducing funnel locking, sort of like SRCU and expedited grace periods already use.
Or am I confused about the source of the contention?
Thanx, Paul
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