Re: [PATCH] tracepoint: rcuidle: use rcu_is_watching() and tree-rcu

["Paul E. McKenney" <paulmck@kernel.org>]

On Tue, Mar 10, 2020 at 04:20:54PM -0400, Mathieu Desnoyers wrote:
> commit e6753f23d961 ("tracepoint: Make rcuidle tracepoint callers use
> SRCU") aimed at improving performance of rcuidle tracepoints by using
> SRCU rather than temporarily enabling tree-rcu every time.
> 
> commit 865e63b04e9b ("tracing: Add back in rcu_irq_enter/exit_irqson()
> for rcuidle tracepoints") adds back the high-overhead enabling of
> tree-rcu because perf expects RCU to be watching when called from
> rcuidle tracepoints.
> 
> It turns out that by using "rcu_is_watching()" and conditionally
> calling the high-overhead rcu_irq_enter/exit_irqson(), the original
> motivation for using SRCU in the first place disappears.

Adding Alexei on CC for his thoughts, given that these were his
benchmarks.  I believe that he also has additional use cases.

But given the use cases you describe, this seems plausible.  This does
mean that tracepoints cannot be attached to the CPU-hotplug code that
runs on the incoming/outgoing CPU early/late in that process, though
that might be OK.

						Thanx, Paul

> I suspect that the original benchmarks justifying the introduction
> of SRCU to handle rcuidle tracepoints was caused by preempt/irq
> tracepoints, which are typically invoked from contexts that have
> RCU watching.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> CC: Thomas Gleixner <tglx@linutronix.de>
> CC: Steven Rostedt <rostedt@goodmis.org>
> CC: Joel Fernandes <joel@joelfernandes.org>
> CC: "Paul E. McKenney" <paulmck@kernel.org>
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: Frederic Weisbecker <fweisbec@gmail.com>
> CC: Ingo Molnar <mingo@kernel.org>
> ---
>  include/linux/tracepoint.h | 25 ++++++++++---------------
>  1 file changed, 10 insertions(+), 15 deletions(-)
> 
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index 1fb11daa5c53..8e0e94fee29a 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -165,25 +165,22 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  		void *it_func;						\
>  		void *__data;						\
>  		int __maybe_unused __idx = 0;				\
> +		bool __exit_rcu = false;				\
>  									\
>  		if (!(cond))						\
>  			return;						\
>  									\
> -		/* srcu can't be used from NMI */			\
> -		WARN_ON_ONCE(rcuidle && in_nmi());			\
> -									\
> -		/* keep srcu and sched-rcu usage consistent */		\
> -		preempt_disable_notrace();				\
> -									\
>  		/*							\
> -		 * For rcuidle callers, use srcu since sched-rcu	\
> -		 * doesn't work from the idle path.			\
> +		 * For rcuidle callers, temporarily enable RCU if	\
> +		 * it is not currently watching.			\
>  		 */							\
> -		if (rcuidle) {						\
> -			__idx = srcu_read_lock_notrace(&tracepoint_srcu);\
> +		if (rcuidle && !rcu_is_watching()) {			\
>  			rcu_irq_enter_irqson();				\
> +			__exit_rcu = true;				\
>  		}							\
>  									\
> +		preempt_disable_notrace();				\
> +									\
>  		it_func_ptr = rcu_dereference_raw((tp)->funcs);		\
>  									\
>  		if (it_func_ptr) {					\
> @@ -194,12 +191,10 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>  			} while ((++it_func_ptr)->func);		\
>  		}							\
>  									\
> -		if (rcuidle) {						\
> -			rcu_irq_exit_irqson();				\
> -			srcu_read_unlock_notrace(&tracepoint_srcu, __idx);\
> -		}							\
> -									\
>  		preempt_enable_notrace();				\
> +									\
> +		if (__exit_rcu)						\
> +			rcu_irq_exit_irqson();				\
>  	} while (0)
>  
>  #ifndef MODULE
> -- 
> 2.17.1
>