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Subject[PATCH RFC] tracepoint: Introduce tracepoint callbacks executing with preempt on
Date
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.

In the future, we can add a new may_sleep API which can use this
infrastructure for callbacks that actually can sleep which will support
Mathieu's usecase of blocking probes.

Test: Tested idle and preempt/irq tracepoints.

[1] https://patchwork.kernel.org/patch/10344297/

Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zilstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Tom Zanussi <tom.zanussi@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Thomas Glexiner <tglx@linutronix.de>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Fenguang Wu <fengguang.wu@intel.com>
Cc: Baohong Liu <baohong.liu@intel.com>
Cc: Vedang Patel <vedang.patel@intel.com>
Cc: kernel-team@android.com
Signed-off-by: Joel Fernandes <joelaf@google.com>
---
include/linux/tracepoint.h | 37 +++++++++++++++++++++++++++++--------
kernel/tracepoint.c | 10 +++++++++-
2 files changed, 38 insertions(+), 9 deletions(-)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index c94f466d57ef..a1c1987de423 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 +80,7 @@ int unregister_tracepoint_module_notifier(struct notifier_block *nb)
*/
static inline void tracepoint_synchronize_unregister(void)
{
+ synchronize_srcu(&tracepoint_srcu);
synchronize_sched();
}

@@ -129,18 +133,26 @@ 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, preempt_on) \
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); \
+ if (preempt_on) { \
+ WARN_ON_ONCE(in_nmi()); /* no srcu from nmi */ \
+ idx = srcu_read_lock(&tracepoint_srcu); \
+ it_func_ptr = srcu_dereference((tp)->funcs, \
+ &tracepoint_srcu); \
+ } 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; \
@@ -148,12 +160,21 @@ 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 (preempt_on) \
+ srcu_read_unlock(&tracepoint_srcu, idx); \
+ else \
+ rcu_read_unlock_sched_notrace(); \
} while (0)

#ifndef MODULE
+/*
+ * This is for tracepoints that may be called from an rcu idle path. To make
+ * sure we its safe in the idle path, use the srcu instead of sched-rcu by
+ * specifying the preempt_on flag below. This has the obvious effect that any
+ * callback expecting preemption to be disabled should explicitly do so since
+ * with srcu it'd run with preempt on.
+ */
#define __DECLARE_TRACE_RCU(name, proto, args, cond, data_proto, data_args) \
static inline void trace_##name##_rcuidle(proto) \
{ \
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 671b13457387..b3b1d65a2460 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;

@@ -67,11 +70,16 @@ 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) {
--
2.17.0.441.gb46fe60e1d-goog
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