[PATCH v4 20/22] perf: Allocate ring buffers for inherited per-task kernel events

[Alexander Shishkin]

When a new event is inherited from a per-task kernel event that has a
ring buffer, allocate a new buffer for this event so that data from the
child task is collected and can later be retrieved for sample annotation
or whatnot.

Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
---
 kernel/events/core.c     |  9 +++++++++
 kernel/events/internal.h | 11 +++++++++++
 2 files changed, 20 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 3b1550fd0e..9d7f6086d5 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8230,6 +8230,15 @@ inherit_event(struct perf_event *parent_event,
 		(void)perf_event_set_output(child_event, parent_event);
 
 	/*
+	 * For per-task kernel events with ring buffers, set_output doesn't
+	 * make sense, but we can allocate a new buffer here.
+	 */
+	if (parent_event->cpu == -1 && kernel_rb_event(parent_event)) {
+		(void)rb_alloc_kernel(child_event, parent_event->rb->nr_pages,
+				      parent_event->rb->aux_nr_pages);
+	}
+
+	/*
 	 * Precalculate sample_data sizes
 	 */
 	perf_event__header_size(child_event);
diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index bb035dd645..b306bc9307 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -120,6 +120,17 @@ static inline unsigned long perf_aux_size(struct ring_buffer *rb)
 	return rb->aux_nr_pages << PAGE_SHIFT;
 }
 
+static inline bool kernel_rb_event(struct perf_event *event)
+{
+	/*
+	 * Having a ring buffer and not being on any ring buffers' wakeup
+	 * list means it was attached by rb_alloc_kernel() and not
+	 * ring_buffer_attach(). It's the only case when these two
+	 * conditions take place at the same time.
+	 */
+	return event->rb && list_empty(&event->rb_entry);
+}
+
 #define DEFINE_OUTPUT_COPY(func_name, memcpy_func)			\
 static inline unsigned long						\
 func_name(struct perf_output_handle *handle,				\
-- 
2.1.0

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