This is a note to let you know that I've just added the patch titled
ring-buffer: Fix wake ups when buffer_percent is set to 100
to the 5.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
ring-buffer-fix-wake-ups-when-buffer_percent-is-set-to-100.patch
and it can be found in the queue-5.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 623b1f896fa8a669a277ee5a258307a16c7377a3 Mon Sep 17 00:00:00 2001
From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
Date: Tue, 26 Dec 2023 12:59:02 -0500
Subject: ring-buffer: Fix wake ups when buffer_percent is set to 100
From: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
commit 623b1f896fa8a669a277ee5a258307a16c7377a3 upstream.
The tracefs file "buffer_percent" is to allow user space to set a
water-mark on how much of the tracing ring buffer needs to be filled in
order to wake up a blocked reader.
0 - is to wait until any data is in the buffer
1 - is to wait for 1% of the sub buffers to be filled
50 - would be half of the sub buffers are filled with data
100 - is not to wake the waiter until the ring buffer is completely full
Unfortunately the test for being full was:
dirty = ring_buffer_nr_dirty_pages(buffer, cpu);
return (dirty * 100) > (full * nr_pages);
Where "full" is the value for "buffer_percent".
There is two issues with the above when full == 100.
1. dirty * 100 > 100 * nr_pages will never be true
That is, the above is basically saying that if the user sets
buffer_percent to 100, more pages need to be dirty than exist in the
ring buffer!
2. The page that the writer is on is never considered dirty, as dirty
pages are only those that are full. When the writer goes to a new
sub-buffer, it clears the contents of that sub-buffer.
That is, even if the check was ">=" it would still not be equal as the
most pages that can be considered "dirty" is nr_pages - 1.
To fix this, add one to dirty and use ">=" in the compare.
Link: https://lore.kernel.org/linux-trace-kernel/20231226125902.4a057f1d@xxxxxxxxxxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
Fixes: 03329f9939781 ("tracing: Add tracefs file buffer_percentage")
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/trace/ring_buffer.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -579,9 +579,14 @@ static __always_inline bool full_hit(str
if (!nr_pages || !full)
return true;
- dirty = ring_buffer_nr_dirty_pages(buffer, cpu);
+ /*
+ * Add one as dirty will never equal nr_pages, as the sub-buffer
+ * that the writer is on is not counted as dirty.
+ * This is needed if "buffer_percent" is set to 100.
+ */
+ dirty = ring_buffer_nr_dirty_pages(buffer, cpu) + 1;
- return (dirty * 100) > (full * nr_pages);
+ return (dirty * 100) >= (full * nr_pages);
}
/*
Patches currently in stable-queue which might be from rostedt@xxxxxxxxxxx are
queue-5.4/ring-buffer-fix-wake-ups-when-buffer_percent-is-set-to-100.patch
