linux-kernel - Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit

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Date:   Tue, 11 Feb 2020 12:35:21 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rostedt <rostedt@...dmis.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to
 perf trace point hook

----- On Feb 11, 2020, at 12:29 PM, Peter Zijlstra peterz@...radead.org wrote:

> 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@...radead.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.
> 
> ---
> 
> 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

Minor nits:

Why not make these an enum ?

> +
> +#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)

And convert these into static inline functions ?

Thanks,

Mathieu

> +
> #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
>  	/*

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com