Messages in this thread Patch in this message | | | Date | Tue, 11 Feb 2020 18:29:52 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook |
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On Tue, Feb 11, 2020 at 11:18:28AM -0500, Steven Rostedt wrote: > On Tue, 11 Feb 2020 16:34:52 +0100 > Peter Zijlstra <peterz@infradead.org> wrote: > > > > + if (unlikely(in_nmi())) > > > + goto out; > > > > unless I'm mistaken, we can simply do rcu_nmi_enter() in this case, and > > rcu_nmi_exit() on the other end. > > > > > + rcu_irq_enter_irqson(); > > The thing is, I don't think this can ever happen.
Per your own argument it should be true in the trace_preempt_off() tracepoint from nmi_enter():
<idle> // rcu_is_watching() := false <NMI> nmi_enter() preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET) __preempt_count_add() // in_nmi() := true preempt_latency_start() // .oO -- we'll never hit this tracepoint because idle has // preempt_count() == 1, so we'll have: // preempt_count() != val
// .oO-2 we should be able to this this when the NMI // hits userspace and we have NOHZ_FULL on. rcu_nmi_enter() // function tracer
But the point is, if we ever do hit it, rcu_nmi_enter() is the right thing to call when '!rcu_is_watching() && in_nmi()', because, as per the rcu_nmi_enter() comments, that function fully supports nested NMIs.
How about something like so? And then you get to use the same in __trace_stack() or something, and anywhere else you're doing this dance.
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diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h index da0af631ded5..e987236abf95 100644 --- a/include/linux/hardirq.h +++ b/include/linux/hardirq.h @@ -89,4 +89,52 @@ extern void irq_exit(void); arch_nmi_exit(); \ } while (0) +/* + * Tracing vs rcu_is_watching() + * ---------------------------- + * + * tracepoints and function-tracing can happen when RCU isn't watching (idle, + * or early IRQ/NMI entry). + * + * When it happens during idle or early during IRQ entry, tracing will have + * to inform RCU that it ought to pay attention, this is done by calling + * rcu_irq_enter_irqon(). + * + * On NMI entry, we must be very careful that tracing only happens after we've + * incremented preempt_count(), otherwise we cannot tell we're in NMI and take + * the special path. + */ + +#define __TR_IRQ 1 +#define __TR_NMI 2 + +#define trace_rcu_enter() \ +({ \ + unsigned long state = 0; \ + if (!rcu_is_watching()) { \ + if (in_nmi()) { \ + state = __TR_NMI; \ + rcu_nmi_enter(); \ + } else { \ + state = __TR_IRQ; \ + rcu_irq_enter_irqson(); \ + } \ + } \ + state; \ +}) + +#define trace_rcu_exit(state) \ +do { \ + switch (state) { \ + case __TR_IRQ: \ + rcu_irq_exit_irqson(); \ + break; \ + case __IRQ_NMI: \ + rcu_nmi_exit(); \ + break; \ + default: \ + break; \ + } \ +} while (0) + #endif /* LINUX_HARDIRQ_H */ diff --git a/kernel/events/core.c b/kernel/events/core.c index e453589da97c..8f34592a1355 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -8950,6 +8950,7 @@ void perf_tp_event(u16 event_type, u64 count, void *record, int entry_size, { struct perf_sample_data data; struct perf_event *event; + unsigned long rcu_flags; struct perf_raw_record raw = { .frag = { @@ -8961,6 +8962,8 @@ void perf_tp_event(u16 event_type, u64 count, void *record, int entry_size, perf_sample_data_init(&data, 0, 0); data.raw = &raw; + rcu_flags = trace_rcu_enter(); + perf_trace_buf_update(record, event_type); hlist_for_each_entry_rcu(event, head, hlist_entry) { @@ -8996,6 +8999,8 @@ void perf_tp_event(u16 event_type, u64 count, void *record, int entry_size, } perf_swevent_put_recursion_context(rctx); + + trace_rcu_exit(rcu_flags); } EXPORT_SYMBOL_GPL(perf_tp_event); diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 45f79bcc3146..62f87807bbe6 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3781,7 +3781,7 @@ static inline void preempt_latency_start(int val) } } -void preempt_count_add(int val) +void notrace preempt_count_add(int val) { #ifdef CONFIG_DEBUG_PREEMPT /* @@ -3813,7 +3813,7 @@ static inline void preempt_latency_stop(int val) trace_preempt_on(CALLER_ADDR0, get_lock_parent_ip()); } -void preempt_count_sub(int val) +void notrace preempt_count_sub(int val) { #ifdef CONFIG_DEBUG_PREEMPT /*
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