ring-buffer: Add a event_stamp to cpu_buffer for each level of nesting

Add a place to save the current event time stamp for each level of nesting.
This will be used to retrieve the time stamp of the current event before it
is committed.

Link: https://lkml.kernel.org/r/20210316164113.399089673@goodmis.orgReviewed-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/ring_buffer.c

@@ -492,6 +492,8 @@ struct rb_time_struct {
 #endif
 typedef struct rb_time_struct rb_time_t;
 
+#define MAX_NEST	5
+
 /*
  * head_page == tail_page && head == tail then buffer is empty.
  */
@@ -529,6 +531,7 @@ struct ring_buffer_per_cpu {
 	unsigned long			read_bytes;
 	rb_time_t			write_stamp;
 	rb_time_t			before_stamp;
+	u64				event_stamp[MAX_NEST];
 	u64				read_stamp;
 	/* ring buffer pages to update, > 0 to add, < 0 to remove */
 	long				nr_pages_to_update;
@@ -2715,6 +2718,10 @@ rb_update_event(struct ring_buffer_per_cpu *cpu_buffer,
 {
 	unsigned length = info->length;
 	u64 delta = info->delta;
+	unsigned int nest = local_read(&cpu_buffer->committing) - 1;
+
+	if (nest < MAX_NEST)
+		cpu_buffer->event_stamp[nest] = info->ts;
 
 	/*
 	 * If we need to add a timestamp, then we
@@ -3456,7 +3463,6 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
 				    info->after, ts)) {
 			/* Nothing came after this event between C and E */
 			info->delta = ts - info->after;
-			info->ts = ts;
 		} else {
 			/*
 			 * Interrupted between C and E:
@@ -3468,6 +3474,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
 			 */
 			info->delta = 0;
 		}
+		info->ts = ts;
 		info->add_timestamp &= ~RB_ADD_STAMP_FORCE;
 	}
 
@@ -5026,6 +5033,8 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
 	rb_time_set(&cpu_buffer->write_stamp, 0);
 	rb_time_set(&cpu_buffer->before_stamp, 0);
 
+	memset(cpu_buffer->event_stamp, 0, sizeof(cpu_buffer->event_stamp));
+
 	cpu_buffer->lost_events = 0;
 	cpu_buffer->last_overrun = 0;