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

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Message-ID: <20200211153452.GW14914@hirez.programming.kicks-ass.net>
Date:   Tue, 11 Feb 2020 16:34:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <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>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to
 perf trace point hook

On Tue, Feb 11, 2020 at 09:50:47AM -0500, Steven Rostedt wrote:
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 455451d24b4a..0abbf5e2ee62 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -8941,6 +8941,7 @@ void perf_tp_event(u16 event_type, u64 count, void *record, int entry_size,
>  {
>  	struct perf_sample_data data;
>  	struct perf_event *event;
> +	bool rcu_watching = rcu_is_watching();
>  
>  	struct perf_raw_record raw = {
>  		.frag = {
> @@ -8949,6 +8950,17 @@ void perf_tp_event(u16 event_type, u64 count, void *record, int entry_size,
>  		},
>  	};
>  
> +	if (!rcu_watching) {
> +		/*
> +		 * If nmi_enter() is traced, it is possible that
> +		 * RCU may not be watching "yet", and this is called.
> +		 * We can not call rcu_irq_enter_irqson() in this case.
> +		 */
> +		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();
> +	}
> +
>  	perf_sample_data_init(&data, 0, 0);
>  	data.raw = &raw;
>  
> @@ -8985,8 +8997,11 @@ void perf_tp_event(u16 event_type, u64 count, void *record, int entry_size,
>  unlock:
>  		rcu_read_unlock();
>  	}
> -
> +	if (!rcu_watching)
> +		rcu_irq_exit_irqson();
> +out:
>  	perf_swevent_put_recursion_context(rctx);
> +
>  }