Re: [PATCH v2] rcu/tree: Add a trace event for RCU stall warnings

[Uladzislau Rezki <urezki@gmail.com>]

> The event allows us to trace the RCU stall when
> sysctl_panic_on_rcu_stall is disabled.
> 
> The first parameter is the name of RCU flavour like other trace
> events. The second one shows us which function detected stalls.
> 
> The RCU stall is mainly caused by external factors such as interrupt
> handling or task scheduling or something else. Therefore, this event
> uses TRACE_EVENT macro, not dedicated one, so that someone interested
> in the RCU stall can use it without CONFIG_RCU_TRACE.
> 
> Signed-off-by: Sangmoon Kim <sangmoon.kim@samsung.com>
> ---
> Changes in v2:
>  - Move the tracepoints before printing out stall messages.
> 
>  include/trace/events/rcu.h | 28 ++++++++++++++++++++++++++++
>  kernel/rcu/tree_exp.h      |  1 +
>  kernel/rcu/tree_stall.h    |  2 ++
>  3 files changed, 31 insertions(+)
> 
> diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
> index 155b5cb43cfd..8476f3161bd0 100644
> --- a/include/trace/events/rcu.h
> +++ b/include/trace/events/rcu.h
> @@ -432,6 +432,34 @@ TRACE_EVENT_RCU(rcu_fqs,
>  		  __entry->cpu, __entry->qsevent)
>  );
>  
> +/*
> + * Tracepoint for RCU stall events. Takes a string identifying the RCU flavor
> + * and a string identifying which function detected the RCU stall as follows:
> + *
> + *	"StallDetected": Scheduler-tick detects other CPU's stalls.
> + *	"SelfDetected": Scheduler-tick detects a current CPU's stall.
> + *	"ExpeditedStall": Expedited grace period detects stalls.
> + */
> +TRACE_EVENT(rcu_stall_warning,
> +
> +	TP_PROTO(const char *rcuname, const char *msg),
> +
> +	TP_ARGS(rcuname, msg),
> +
> +	TP_STRUCT__entry(
> +		__field(const char *, rcuname)
> +		__field(const char *, msg)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->rcuname = rcuname;
> +		__entry->msg = msg;
> +	),
> +
> +	TP_printk("%s %s",
> +		  __entry->rcuname, __entry->msg)
> +);
> +
>  #endif /* #if defined(CONFIG_TREE_RCU) */
>  
>  /*
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index 8760b6ead770..6e1ac5b41f69 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -521,6 +521,7 @@ static void synchronize_rcu_expedited_wait(void)
>  		if (rcu_stall_is_suppressed())
>  			continue;
>  		panic_on_rcu_stall();
> +		trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall"));
>  		pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
>  		       rcu_state.name);
>  		ndetected = 0;
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index 70d48c52fabc..ed50056a3c7b 100644
> --- a/kernel/rcu/tree_stall.h
> +++ b/kernel/rcu/tree_stall.h
> @@ -488,6 +488,7 @@ static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
>  	 * See Documentation/RCU/stallwarn.rst for info on how to debug
>  	 * RCU CPU stall warnings.
>  	 */
> +	trace_rcu_stall_warning(rcu_state.name, TPS("StallDetected"));
>  	pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
>  	rcu_for_each_leaf_node(rnp) {
>  		raw_spin_lock_irqsave_rcu_node(rnp, flags);
> @@ -554,6 +555,7 @@ static void print_cpu_stall(unsigned long gps)
>  	 * See Documentation/RCU/stallwarn.rst for info on how to debug
>  	 * RCU CPU stall warnings.
>  	 */
> +	trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected"));
>  	pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
>  	raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
>  	print_cpu_stall_info(smp_processor_id());
> -- 
> 2.17.1
> 
One small nit, the patch is not cleanelly applied on latest dev
so it should be rebased unless Paul fixes it on his own :)

Anyway to me it looks OK.

Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

--
Vlad Rezki