[PATCH v4 2/6] perf/core: Provide a kernel-internal interface to pause perf_event

[Like Xu <like.xu@linux.intel.com>]

Exporting perf_event_pause() as an external accessor for kernel users (such
as KVM) who may do both disable perf_event and read count with just one
time to hold perf_event_ctx_lock. Also the value could be reset optionally.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Like Xu <like.xu@linux.intel.com>
---
 include/linux/perf_event.h |  5 +++++
 kernel/events/core.c       | 18 ++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index d601df36e671..e9768bfc76f6 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1337,6 +1337,7 @@ extern void perf_event_disable_inatomic(struct perf_event *event);
 extern void perf_event_task_tick(void);
 extern int perf_event_account_interrupt(struct perf_event *event);
 extern int perf_event_period(struct perf_event *event, u64 value);
+extern u64 perf_event_pause(struct perf_event *event, bool reset);
 #else /* !CONFIG_PERF_EVENTS: */
 static inline void *
 perf_aux_output_begin(struct perf_output_handle *handle,
@@ -1420,6 +1421,10 @@ static inline int perf_event_period(struct perf_event *event, u64 value)
 {
 	return -EINVAL;
 }
+static inline u64 perf_event_pause(struct perf_event *event, bool reset)
+{
+	return 0;
+}
 #endif
 
 #if defined(CONFIG_PERF_EVENTS) && defined(CONFIG_CPU_SUP_INTEL)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2fbb485ae880..35cc901e0e4f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5029,6 +5029,24 @@ static void _perf_event_reset(struct perf_event *event)
 	perf_event_update_userpage(event);
 }
 
+/* Assume it's not an event with inherit set. */
+u64 perf_event_pause(struct perf_event *event, bool reset)
+{
+	struct perf_event_context *ctx;
+	u64 count;
+
+	ctx = perf_event_ctx_lock(event);
+	WARN_ON_ONCE(event->attr.inherit);
+	_perf_event_disable(event);
+	count = local64_read(&event->count);
+	if (reset)
+		local64_set(&event->count, 0);
+	perf_event_ctx_unlock(event, ctx);
+
+	return count;
+}
+EXPORT_SYMBOL_GPL(perf_event_pause);
+
 /*
  * Holding the top-level event's child_mutex means that any
  * descendant process that has inherited this event will block
-- 
2.21.0