Re: [PATCH RFC v6 4/5] tracepoint: Make rcuidle tracepoint callers use SRCU

[Joel Fernandes]

On May 7, 2018 2:55:45 PM PDT, "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>On Mon, May 07, 2018 at 02:45:14PM -0700, Joel Fernandes wrote:
>> On Mon, May 07, 2018 at 02:08:01PM -0700, Paul E. McKenney wrote:
>> > On Mon, May 07, 2018 at 01:41:42PM -0700, Joel Fernandes wrote:
>> > > From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
>> > > 
>> > > 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.
>> [...]
>> >> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
>> > > ---
>> > >  include/linux/tracepoint.h | 46
>+++++++++++++++++++++++++++++++-------
>> > >  kernel/tracepoint.c        | 15 ++++++++++++-
>> > >  2 files changed, 52 insertions(+), 9 deletions(-)
>> > > 
>> > > diff --git a/include/linux/tracepoint.h
>b/include/linux/tracepoint.h
>> > > index c94f466d57ef..f56f290cf8eb 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 +80,9 @@ int
>unregister_tracepoint_module_notifier(struct notifier_block *nb)
>> > >   */
>> > >  static inline void tracepoint_synchronize_unregister(void)
>> > >  {
>> > > +#ifdef CONFIG_TRACEPOINTS
>> > > +	synchronize_srcu(&tracepoint_srcu);
>> > > +#endif
>> > >  	synchronize_sched();
>> > >  }
>> > > 
>> > > @@ -129,18 +135,38 @@ 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, rcuidle)			\
>> > >  	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);	\
>> > > +									\
>> > > +		/*							\
>> > > +		 * For rcuidle callers, use srcu since sched-rcu	\
>> > > +		 * doesn't work from the idle path.			\
>> > > +		 */							\
>> > > +		if (rcuidle) {						\
>> > > +			if (in_nmi()) {					\
>> > > +				WARN_ON_ONCE(1);			\
>> > > +				return; /* no srcu from nmi */		\
>> > > +			}						\
>> > > +									\
>> > > +			idx = srcu_read_lock_notrace(&tracepoint_srcu);	\
>> > > +			it_func_ptr =					\
>> > > +				srcu_dereference_notrace((tp)->funcs,	\
>> > > +						&tracepoint_srcu);	\
>> > > +			/* To keep it consistent with !rcuidle path */	\
>> > > +			preempt_disable_notrace();			\
>> > > +		} 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;		\
>> > > @@ -148,9 +174,13 @@ 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 (rcuidle) {						\
>> > 
>> > Don't we also need an in_nmi() check here in order to avoid
>unbalanced
>> > srcu_read_unlock_notrace() calls?
>> 
>>  The in_nmi() in the lock path should take care of making sure its
>balanced.
>> 
>>  The diff the way its formatted appears confusing as Mathieu pointed.
>
>Ah, right, I was for some reason thinking that the two hunks of the
>diff were two separate macros.  Apologies for my confusion!
>	

No worries! Thanks for the great idea to use in_nmi in the first place.

- Joel

>							Thanx, Paul

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