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

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