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

[Joel Fernandes]

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.

 thanks,

 - Joel