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

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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/20250612061201.34272-3-kerneljasonxing@gmail.com
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Yushan Zhou <katrinzhou@tencent.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Jason Xing 2025-06-12 14:11:58 +08:00 • committed by Andrew Morton
parent 2489e95812
commit ca01a90ae7
2 changed files with 16 additions and 1 deletions
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9
include/linux/relay.h
View file

@ -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 */

8
kernel/relay.c
View file

@ -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;