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Date:   Tue, 11 Feb 2020 13:21:20 +0100
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] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf
 trace point hook
Message-ID: <20200211122120.GM14914@hirez.programming.kicks-ass.net>
References: <20200210170643.3544795d@gandalf.local.home>
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On Mon, Feb 10, 2020 at 05:06:43PM -0500, Steven Rostedt wrote:
> diff --git a/include/trace/perf.h b/include/trace/perf.h
> index dbc6c74defc3..1c94ce0cd4e2 100644
> --- a/include/trace/perf.h
> +++ b/include/trace/perf.h
> @@ -39,17 +39,27 @@ perf_trace_##call(void *__data, proto)					\
>  	u64 __count = 1;						\
>  	struct task_struct *__task = NULL;				\
>  	struct hlist_head *head;					\
> +	bool rcu_watching;						\
>  	int __entry_size;						\
>  	int __data_size;						\
>  	int rctx;							\
>  									\
> +	rcu_watching = rcu_is_watching();				\
> +									\
>  	__data_size = trace_event_get_offsets_##call(&__data_offsets, args); \
>  									\
> +	if (!rcu_watching) {						\
> +		/* Can not use RCU if rcu is not watching and in NMI */	\
> +		if (in_nmi())						\
> +			return;						\
> +		rcu_irq_enter_irqson();					\
> +	}								\
> +									\
>  	head = this_cpu_ptr(event_call->perf_events);			\
>  	if (!bpf_prog_array_valid(event_call) &&			\
>  	    __builtin_constant_p(!__task) && !__task &&			\
>  	    hlist_empty(head))						\
> -		return;							\
> +		goto out;						\
>  									\
>  	__entry_size = ALIGN(__data_size + sizeof(*entry) + sizeof(u32),\
>  			     sizeof(u64));				\
> @@ -57,7 +67,7 @@ perf_trace_##call(void *__data, proto)					\
>  									\
>  	entry = perf_trace_buf_alloc(__entry_size, &__regs, &rctx);	\
>  	if (!entry)							\
> -		return;							\
> +		goto out;						\
>  									\
>  	perf_fetch_caller_regs(__regs);					\
>  									\
> @@ -68,6 +78,9 @@ perf_trace_##call(void *__data, proto)					\
>  	perf_trace_run_bpf_submit(entry, __entry_size, rctx,		\
>  				  event_call, __count, __regs,		\
>  				  head, __task);			\
> +out:									\
> +	if (!rcu_watching)						\
> +		rcu_irq_exit_irqson();					\
>  }

It is probably okay to move that into perf_tp_event(), then this:

>  /*
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 1694a6b57ad8..3e6f07b62515 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -719,6 +719,7 @@ void rcu_irq_exit_irqson(void)
>  	rcu_irq_exit();
>  	local_irq_restore(flags);
>  }
> +EXPORT_SYMBOL_GPL(rcu_irq_exit_irqson);
>  
>  /*
>   * Exit an RCU extended quiescent state, which can be either the
> @@ -890,6 +891,7 @@ void rcu_irq_enter_irqson(void)
>  	rcu_irq_enter();
>  	local_irq_restore(flags);
>  }
> +EXPORT_SYMBOL_GPL(rcu_irq_enter_irqson);
>  
>  /*
>   * If any sort of urgency was applied to the current CPU (for example,

can go too. Those things really should not be exported.