Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook

From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	"Joel Fernandes (Google)" <joel@joelfernandes.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook
Date: Tue, 11 Feb 2020 16:34:52 +0100	[thread overview]
Message-ID: <20200211153452.GW14914@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20200211095047.58ddf750@gandalf.local.home>

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);
> +
>  }