Commit e6753f23 authored by Joel Fernandes (Google)'s avatar Joel Fernandes (Google) Committed by Steven Rostedt (VMware)
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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/

[remove rcu_read_lock_sched_notrace as its the equivalent of
preempt_disable_notrace and is unnecessary to call in tracepoint code]
Link: http://lkml.kernel.org/r/20180730222423.196630-3-joel@joelfernandes.org



Cleaned-up-by: default avatarPeter Zijlstra <peterz@infradead.org>
Acked-by: default avatarPeter Zijlstra <peterz@infradead.org>
Reviewed-by: default avatarMathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
[ Simplified WARN_ON_ONCE() ]
Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
parent 01f38497

include/linux/tracepoint.h

+32 −8
Original line number Diff line number Diff line
@@ -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 +138,31 @@ 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 */			\

		WARN_ON_ONCE(rcuidle && in_nmi());			\

									\

		/* 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);	\

									\

		it_func_ptr = rcu_dereference_raw((tp)->funcs);		\

									\

		if (it_func_ptr) {					\

			do {						\

				it_func = (it_func_ptr)->func;		\
@@ -148,9 +170,11 @@ 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);\

									\

		preempt_enable_notrace();				\

	} while (0)



#ifndef MODULE

kernel/tracepoint.c

+15 −1
Original line number Diff line number Diff line
@@ -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);

	}

}

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