Re: [PATCH v10 2/3] tracepoint: Make rcuidle tracepoint callers use SRCU

[Joel Fernandes]

On Sat, Jul 14, 2018 at 10:31:57AM -0400, Mathieu Desnoyers wrote:
> ----- On Jul 13, 2018, at 5:55 PM, Joel Fernandes, Google joel@xxxxxxxxxxxxxxxxx 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.
> > 
> > Test: Tested idle and preempt/irq tracepoints.
> > 
> > Here are some performance numbers:
> > 
> > With a run of the following 30 times on a single core x86 Qemu instance
> > with 1GB memory:
> > hackbench -g 4 -f 2 -l 3000
> > 
> > Completion times in seconds. CONFIG_PROVE_LOCKING=y.
> > 
> > No patches (without this series)
> > Mean: 3.048
> > Median: 3.025
> > Std Dev: 0.064
> > 
> > With Lockdep using irq tracepoints with RCU implementation:
> > Mean: 3.451   (-11.66 %)
> > Median: 3.447 (-12.22%)
> > Std Dev: 0.049
> > 
> > With Lockdep using irq tracepoints with SRCU implementation (this series):
> > Mean: 3.020   (I would consider the improvement against the "without
> >	       this series" case as just noise).
> > Median: 3.013
> > Std Dev: 0.033
> > 
> > [1] https://patchwork.kernel.org/patch/10344297/
> > 
> > Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> 
> I'm fine with the changes done since last iteration.
> 
> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>

Thanks a lot Mathieu!

Steve, Peter, looks good to you too now?

thanks,

- Joel


[...] 
> > Cleaned-up-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> > include/linux/tracepoint.h | 45 +++++++++++++++++++++++++++++++-------
> > kernel/tracepoint.c        | 16 +++++++++++++-
> > 2 files changed, 52 insertions(+), 9 deletions(-)
> > 
> > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > index 19a690b559ca..97e1d365a817 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
> > @@ -75,10 +78,16 @@ int unregister_tracepoint_module_notifier(struct
> > notifier_block *nb)
> >  * probe unregistration and the end of module exit to make sure there is no
> >  * caller executing a probe when it is freed.
> >  */
> > +#ifdef CONFIG_TRACEPOINTS
> > static inline void tracepoint_synchronize_unregister(void)
> > {
> > +	synchronize_srcu(&tracepoint_srcu);
> > 	synchronize_sched();
> > }
> > +#else
> > +static inline void tracepoint_synchronize_unregister(void)
> > +{ }
> > +#endif
> > 
> > #ifdef CONFIG_HAVE_SYSCALL_TRACEPOINTS
> > extern int syscall_regfunc(void);
> > @@ -129,18 +138,34 @@ 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);	\
> > +									\
> > +		/* srcu can't be used from NMI */			\
> > +		if (rcuidle && in_nmi())				\
> > +			WARN_ON_ONCE(1);				\
> > +									\
> > +		/* 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.			\
> > +		 */							\
> > +		if (rcuidle)						\
> > +			idx = srcu_read_lock_notrace(&tracepoint_srcu);	\
> > +		else							\
> > +			rcu_read_lock_sched_notrace();			\
> > +									\
> > +		it_func_ptr = rcu_dereference_raw((tp)->funcs);		\
> > +									\
> > 		if (it_func_ptr) {					\
> > 			do {						\
> > 				it_func = (it_func_ptr)->func;		\
> > @@ -148,9 +173,13 @@ 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)						\
> > +			srcu_read_unlock_notrace(&tracepoint_srcu, idx);\
> > +		else							\
> > +			rcu_read_unlock_sched_notrace();		\
> > +									\
> > +		preempt_enable_notrace();				\
> > 	} while (0)
> > 
> > #ifndef MODULE
> > diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> > index 6dc6356c3327..955148d91b74 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;
> > 
> > @@ -67,16 +70,27 @@ 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);
> > +}
> > +
> > static inline void release_probes(struct tracepoint_func *old)
> > {
> > 	if (old) {
> > 		struct tp_probes *tp_probes = container_of(old,
> > 			struct tp_probes, probes[0]);
> > +		/*
> > +		 * Tracepoint probes are protected by both sched RCU and SRCU,
> > +		 * by calling the SRCU callback in the sched RCU callback we
> > +		 * cover both cases. So let us chain the SRCU and sched RCU
> > +		 * callbacks to wait for both grace periods.
> > +		 */
> > 		call_rcu_sched(&tp_probes->rcu, rcu_free_old_probes);
> > 	}
> > }
> > --
> > 2.18.0.203.gfac676dfb9-goog
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com