From: Jason Xing <kernelxing@xxxxxxxxxxx>
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.
Reviewed-by: Yushan Zhou <katrinzhou@xxxxxxxxxxx>
Reviewed-by: Jens Axboe <axboe@xxxxxxxxx>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
Signed-off-by: Jason Xing <kernelxing@xxxxxxxxxxx>
---
include/linux/relay.h | 9 +++++++++
kernel/relay.c | 8 +++++++-
2 files changed, 16 insertions(+), 1 deletion(-)
diff --git a/include/linux/relay.h b/include/linux/relay.h
index e10a0fdf4325..cd77eb285a48 100644
--- a/include/linux/relay.h
+++ b/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 */
diff --git a/kernel/relay.c b/kernel/relay.c
index 94f79f52d826..eb3f630f3896 100644
--- a/kernel/relay.c
+++ b/kernel/relay.c
@@ -252,8 +252,13 @@ 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);
@@ -298,6 +303,7 @@ 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;
--
2.43.5
