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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: Peter Zijlstra <peterz@infradead.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ Simplified WARN_ON_ONCE() ]
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Joel Fernandes (Google) 2018-07-30 15:24:22 -07:00 committed by Steven Rostedt (VMware)
parent 01f38497c6
commit e6753f23d9
2 changed files with 47 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

40
include/linux/tracepoint.h
View File

@ -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

16
kernel/tracepoint.c
View File

@ -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);
}
}