relayfs: support a counter tracking if per-cpu buffers is full

When using relay mechanism, we often encounter the case where new data are
lost or old unconsumed data are overwritten because of slow reader.

Add 'full' field in per-cpu buffer structure to detect if the above case
is happening.  Relay has two modes: 1) non-overwrite mode, 2) overwrite
mode.  So buffer being full here respectively means: 1) relayfs doesn't
intend to accept new data and then simply drop them, or 2) relayfs is
going to start over again and overwrite old unread data with new data.

Note: this counter doesn't need any explicit lock to protect from being
modified by different threads for the better performance consideration. 
Writers calling __relay_write/relay_write should consider how to use the
lock and ensure it performs under the lock protection, thus it's not
necessary to add a new small lock here.

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

@@ -28,6 +28,14 @@
  */
 #define RELAYFS_CHANNEL_VERSION		7
 
+/*
+ * Relay buffer statistics
+ */
+struct rchan_buf_stats
+{
+	unsigned int full_count;	/* counter for buffer full */
+};
+
 /*
  * Per-cpu relay channel buffer
  */
@@ -43,6 +51,7 @@ struct rchan_buf
 	struct irq_work wakeup_work;	/* reader wakeup */
 	struct dentry *dentry;		/* channel file dentry */
 	struct kref kref;		/* channel buffer refcount */
+	struct rchan_buf_stats stats;	/* buffer stats */
 	struct page **page_array;	/* array of current buffer pages */
 	unsigned int page_count;	/* number of current buffer pages */
 	unsigned int finalized;		/* buffer has been finalized */

kernel/relay.c

@@ -251,8 +251,13 @@ EXPORT_SYMBOL_GPL(relay_buf_full);
 static int relay_subbuf_start(struct rchan_buf *buf, void *subbuf,
 			      void *prev_subbuf)
 {
+	int full = relay_buf_full(buf);
+
+	if (full)
+		buf->stats.full_count++;
+
 	if (!buf->chan->cb->subbuf_start)
-		return !relay_buf_full(buf);
+		return !full;
 
 	return buf->chan->cb->subbuf_start(buf, subbuf,
 					   prev_subbuf);
@@ -297,6 +302,7 @@ static void __relay_reset(struct rchan_buf *buf, unsigned int init)
 	buf->finalized = 0;
 	buf->data = buf->start;
 	buf->offset = 0;
+	buf->stats.full_count = 0;
 
 	for (i = 0; i < buf->chan->n_subbufs; i++)
 		buf->padding[i] = 0;