Date:   Thu, 31 May 2018 10:51:53 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Joel Fernandes <joelaf@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Erick Reyes <erickreyes@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Julia Cartwright <julia@...com>,
        linux-kselftest <linux-kselftest@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        shuah <shuah@...nel.org>, rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Todd Kjos <tkjos@...gle.com>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>
Subject: Re: [PATCH v8 5/8] tracepoint: Make rcuidle tracepoint callers use
 SRCU

On Thu, May 31, 2018 at 02:50:41AM -0400, Mathieu Desnoyers wrote:
> ----- On May 30, 2018, at 2:04 AM, Joel Fernandes joelaf@...gle.com wrote:
> 
> > From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> > 
> > 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/
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > ---
> > include/linux/tracepoint.h | 48 +++++++++++++++++++++++++++++++-------
> > kernel/tracepoint.c        | 15 +++++++++++-
> > 2 files changed, 54 insertions(+), 9 deletions(-)
> > 
> > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > index c94f466d57ef..880794207921 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 +78,15 @@ 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) { }
> 
> In order to add some consistency to the style in this header file,
> this empty function should look like:
> 
> static inline void tracepoint_synchronize_unregister(void)
> { }
> 
> (with newline before "{")

Ok, I changed it to the style you're proposing.

> > +#endif
> > 
> > #ifdef CONFIG_HAVE_SYSCALL_TRACEPOINTS
> > extern int syscall_regfunc(void);
> > @@ -129,18 +137,38 @@ 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 */		\
> 
> I find it odd to have a "return" in a macro that consists of a
> do { } while (0). I'm tempted to replace "return" by "break" here,
> to break the macro do/while (0) loop.

"return;" is also used from "if (!(cond))" above so I prefer be consistent
and just use return than break as done above, but please let me know if you
still object.

> > +			}						\
> > +									\
> > +			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 +176,13 @@ 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) {						\
> > +			preempt_enable_notrace();			\
> > +			srcu_read_unlock_notrace(&tracepoint_srcu, idx);\
> > +		} else {						\
> > +			rcu_read_unlock_sched_notrace();		\
> > +		}							\
> > 	} while (0)
> > 
> > #ifndef MODULE
> > diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
> > index 1e37da2e0c25..54157792f5ab 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 +70,26 @@ 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 lets chain the SRCU and RCU callbacks.
> 
> lets -> let's
> 
> But I would use a different wording for that sentence that does not include
> "let's" which is short for "let us", e.g.
> 
> "Chain the SRCU and sched RCU callbacks to wait for both grace periods."

Fixed, thanks.

> With the comments above taken care of, please add my:
> 
> Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

Will do, and thanks for the review!

Below is the updated patch.

 - Joel

---8<-----------------------

From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
Date: Thu, 26 Apr 2018 20:44:07 -0700
Subject: [PATCH v8.1] tracepoint: Make rcuidle tracepoint callers use SRCU

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@...icios.com>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 include/linux/tracepoint.h | 49 +++++++++++++++++++++++++++++++-------
 kernel/tracepoint.c        | 16 ++++++++++++-
 2 files changed, 56 insertions(+), 9 deletions(-)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index c94f466d57ef..9cf2882d2ef4 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 +138,38 @@ 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 +177,13 @@ 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) {						\
+			preempt_enable_notrace();			\
+			srcu_read_unlock_notrace(&tracepoint_srcu, idx);\
+		} else {						\
+			rcu_read_unlock_sched_notrace();		\
+		}							\
 	} while (0)
 
 #ifndef MODULE
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 1e37da2e0c25..b2e3be589aff 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);
 	}
 }
-- 
2.17.0.921.gf22659ad46-goog

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