[PATCH] tracing: Fix to avoid wakeup loop in splice read of per-cpu buffer

[Masami Hiramatsu (Google)]

From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

ftrace user can set 0 or small number to the 'buffer_percent' for quick
response for the ring buffer. In that case wait_on_pipe() will return
before filling a page of the ring buffer. That is too soon for splice()
because ring_buffer_read_page() will fail again.
This leads unnecessary loop in tracing_buffers_splice_read().

Set a minimum percentage of the buffer which is enough to fill a page to
wait_on_pipe() to avoid this situation.

Fixes: 03329f993978 ("tracing: Add tracefs file buffer_percentage")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
---
 kernel/trace/trace.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index b8870078ef58..88448e8d8214 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -8462,6 +8462,8 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
 	/* did we read anything? */
 	if (!spd.nr_pages) {
 		long wait_index;
+		size_t nr_pages;
+		size_t full;
 
 		if (ret)
 			goto out;
@@ -8472,7 +8474,15 @@ tracing_buffers_splice_read(struct file *file, loff_t *ppos,
 
 		wait_index = READ_ONCE(iter->wait_index);
 
-		ret = wait_on_pipe(iter, iter->tr->buffer_percent);
+		/* For splice, we have to ensure at least 1 page is filled */
+		nr_pages = ring_buffer_nr_pages(iter->array_buffer->buffer, iter->cpu_file);
+		if (nr_pages * iter->tr->buffer_percent < 100) {
+			full = nr_pages + 99;
+			do_div(full, nr_pages);
+		} else
+			full = iter->tr->buffer_percent;
+
+		ret = wait_on_pipe(iter, full);
 		if (ret)
 			goto out;