relayfs: introduce getting relayfs statistics function

In this version, only support getting the counter for buffer full and
implement the framework of how it works.

Users can pass certain flag to fetch what field/statistics they expect to
know.  Each time it only returns one result.  So do not pass multiple
flags.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Jason Xing <[email protected]>
Reviewed-by: Yushan Zhou <[email protected]>
Reviewed-by: Masami Hiramatsu (Google) <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Steven Rostedt <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: JasonXing

include/linux/relay.h

@@ -31,6 +31,12 @@
 /*
  * Relay buffer statistics
  */
+enum {
+	RELAY_STATS_BUF_FULL = (1 << 0),
+
+	RELAY_STATS_LAST = RELAY_STATS_BUF_FULL,
+};
+
 struct rchan_buf_stats
 {
 	unsigned int full_count;	/* counter for buffer full */
@@ -167,6 +173,7 @@ struct rchan *relay_open(const char *base_filename,
 			 void *private_data);
 extern void relay_close(struct rchan *chan);
 extern void relay_flush(struct rchan *chan);
+size_t relay_stats(struct rchan *chan, int flags);
 extern void relay_subbufs_consumed(struct rchan *chan,
 				   unsigned int cpu,
 				   size_t consumed);

kernel/relay.c

@@ -700,6 +700,36 @@ void relay_flush(struct rchan *chan)
 }
 EXPORT_SYMBOL_GPL(relay_flush);
 
+/**
+ *	relay_stats - get channel buffer statistics
+ *	@chan: the channel
+ *	@flags: select particular information to get
+ *
+ *	Returns the count of certain field that caller specifies.
+ */
+size_t relay_stats(struct rchan *chan, int flags)
+{
+	unsigned int i, count = 0;
+	struct rchan_buf *rbuf;
+
+	if (!chan || flags > RELAY_STATS_LAST)
+		return 0;
+
+	if (chan->is_global) {
+		rbuf = *per_cpu_ptr(chan->buf, 0);
+		if (flags & RELAY_STATS_BUF_FULL)
+			count = rbuf->stats.full_count;
+	} else {
+		for_each_online_cpu(i) {
+			rbuf = *per_cpu_ptr(chan->buf, i);
+			if (rbuf && flags & RELAY_STATS_BUF_FULL)
+				count += rbuf->stats.full_count;
+		}
+	}
+
+	return count;
+}
+
 /**
  *	relay_file_open - open file op for relay files
  *	@inode: the inode