Messages in this thread | | | Date | Thu, 5 Apr 2018 09:34:14 +1000 | From | Nicholas Piggin <> | Subject | rcu_process_callbacks irqsoff latency caused by taking spinlock with irqs disabled |
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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.
# 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.
Thanks, Nick
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