Re: [PATCH RFC] tracepoint: Introduce tracepoint callbacks executing with preempt on

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Apr 26, 2018 at 09:26:56PM -0700, Joel Fernandes wrote:
> In recent tests with IRQ on/off tracepoints, a large performance
> overhead ~10% is noticed when running hackbench. This is root caused to
> calls to rcu_irq_enter_irqson and rcu_irq_exit_irqson from the
> tracepoint code. Following a long discussion on the list [1] about this,
> we concluded that srcu is a better alternative for use during rcu idle.
> Although it does involve extra barriers, its lighter than the sched-rcu
> version which has to do additional RCU calls to notify RCU idle about
> entry into RCU sections.
> 
> In this patch, we change the underlying implementation of the
> trace_*_rcuidle API to use SRCU. This has shown to improve performance
> alot for the high frequency irq enable/disable tracepoints.
> 
> In the future, we can add a new may_sleep API which can use this
> infrastructure for callbacks that actually can sleep which will support
> Mathieu's usecase of blocking probes.
> 
> Test: Tested idle and preempt/irq tracepoints.

Looks good overall!  One question and a few comments below.

							Thanx, Paul

> [1] https://patchwork.kernel.org/patch/10344297/
> 
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Peter Zilstra <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Tom Zanussi <tom.zanussi@linux.intel.com>
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Thomas Glexiner <tglx@linutronix.de>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Fenguang Wu <fengguang.wu@intel.com>
> Cc: Baohong Liu <baohong.liu@intel.com>
> Cc: Vedang Patel <vedang.patel@intel.com>
> Cc: kernel-team@android.com
> Signed-off-by: Joel Fernandes <joelaf@google.com>
> ---
>  include/linux/tracepoint.h | 37 +++++++++++++++++++++++++++++--------
>  kernel/tracepoint.c        | 10 +++++++++-
>  2 files changed, 38 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index c94f466d57ef..a1c1987de423 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -15,6 +15,7 @@
>   */
> 
>  #include <linux/smp.h>
> +#include <linux/srcu.h>
>  #include <linux/errno.h>
>  #include <linux/types.h>
>  #include <linux/cpumask.h>
> @@ -33,6 +34,8 @@ struct trace_eval_map {
> 
>  #define TRACEPOINT_DEFAULT_PRIO	10
> 
> +extern struct srcu_struct tracepoint_srcu;
> +
>  extern int
>  tracepoint_probe_register(struct tracepoint *tp, void *probe, void *data);
>  extern int
> @@ -77,6 +80,7 @@ int unregister_tracepoint_module_notifier(struct notifier_block *nb)
>   */
>  static inline void tracepoint_synchronize_unregister(void)
>  {
> +	synchronize_srcu(&tracepoint_srcu);
>  	synchronize_sched();
>  }
> 
> @@ -129,18 +133,26 @@ extern void syscall_unregfunc(void);
>   * as "(void *, void)". The DECLARE_TRACE_NOARGS() will pass in just
>   * "void *data", where as the DECLARE_TRACE() will pass in "void *data, proto".
>   */
> -#define __DO_TRACE(tp, proto, args, cond, rcucheck)			\
> +#define __DO_TRACE(tp, proto, args, cond, preempt_on)			\
>  	do {								\
>  		struct tracepoint_func *it_func_ptr;			\
>  		void *it_func;						\
>  		void *__data;						\
> +		int __maybe_unused idx = 0;				\
>  									\
>  		if (!(cond))						\
>  			return;						\
> -		if (rcucheck)						\
> -			rcu_irq_enter_irqson();				\
> -		rcu_read_lock_sched_notrace();				\
> -		it_func_ptr = rcu_dereference_sched((tp)->funcs);	\
> +		if (preempt_on) {					\
> +			WARN_ON_ONCE(in_nmi()); /* no srcu from nmi */	\

Very good on this check, thank you!

> +			idx = srcu_read_lock(&tracepoint_srcu);         \

Hmmm...  Do I need to create a _notrace variant of srcu_read_lock()
and srcu_read_unlock()?

> +			it_func_ptr = srcu_dereference((tp)->funcs,	\
> +						&tracepoint_srcu);	\
> +		} else {						\
> +			rcu_read_lock_sched_notrace();			\
> +			it_func_ptr =					\
> +				rcu_dereference_sched((tp)->funcs);	\
> +		}							\
> +									\
>  		if (it_func_ptr) {					\
>  			do {						\
>  				it_func = (it_func_ptr)->func;		\
> @@ -148,12 +160,21 @@ extern void syscall_unregfunc(void);
>  				((void(*)(proto))(it_func))(args);	\
>  			} while ((++it_func_ptr)->func);		\
>  		}							\
> -		rcu_read_unlock_sched_notrace();			\
> -		if (rcucheck)						\
> -			rcu_irq_exit_irqson();				\
> +									\
> +		if (preempt_on)					\
> +			srcu_read_unlock(&tracepoint_srcu, idx);	\
> +		else							\
> +			rcu_read_unlock_sched_notrace();		\
>  	} while (0)
> 
>  #ifndef MODULE
> +/*
> + * This is for tracepoints that may be called from an rcu idle path. To make
> + * sure we its safe in the idle path, use the srcu instead of sched-rcu by
> + * specifying the preempt_on flag below.  This has the obvious effect that any
> + * callback expecting preemption to be disabled should explicitly do so since
> + * with srcu it'd run with preempt on.
> + */
>  #define __DECLARE_TRACE_RCU(name, proto, args, cond, data_proto, data_args) \
>  	static inline void trace_##name##_rcuidle(proto)		\
>  	{								\
> diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> index 671b13457387..b3b1d65a2460 100644
> --- a/kernel/tracepoint.c
> +++ b/kernel/tracepoint.c
> @@ -31,6 +31,9 @@
>  extern struct tracepoint * const __start___tracepoints_ptrs[];
>  extern struct tracepoint * const __stop___tracepoints_ptrs[];
> 
> +DEFINE_SRCU(tracepoint_srcu);
> +EXPORT_SYMBOL_GPL(tracepoint_srcu);
> +
>  /* Set to 1 to enable tracepoint debug output */
>  static const int tracepoint_debug;
> 
> @@ -67,11 +70,16 @@ static inline void *allocate_probes(int count)
>  	return p == NULL ? NULL : p->probes;
>  }
> 
> -static void rcu_free_old_probes(struct rcu_head *head)
> +static void srcu_free_old_probes(struct rcu_head *head)
>  {
>  	kfree(container_of(head, struct tp_probes, rcu));
>  }
> 
> +static void rcu_free_old_probes(struct rcu_head *head)
> +{
> +	call_srcu(&tracepoint_srcu, head, srcu_free_old_probes);

Chaining them through the two callbacks works, good!

> +}
> +
>  static inline void release_probes(struct tracepoint_func *old)
>  {
>  	if (old) {
> -- 
> 2.17.0.441.gb46fe60e1d-goog
>