On Tue, 26 Dec 2023 12:59:02 -0500
Steven Rostedt <rost...@goodmis.org> wrote:
> From: "Steven Rostedt (Google)" <rost...@goodmis.org>
>
> 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.
>
> Cc: sta...@vger.kernel.org
> Fixes: 03329f9939781 ("tracing: Add tracefs file buffer_percentage")
> Signed-off-by: Steven Rostedt (Google) <rost...@goodmis.org>
> ---
> kernel/trace/ring_buffer.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 83eab547f1d1..32c0dd2fd1c3 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -881,9 +881,14 @@ static __always_inline bool full_hit(struct trace_buffer
> *buffer, int cpu, int f
> 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;
Is this "+ 1" required? If we have 200 pages and 1 buffer is dirty,
it is 0.5% dirty. Consider @full = 1%.
@dirty = 1 + 1 = 2 and @dirty * 100 == 200. but
@full * @nr_pages = 1 * 200 = 200.
Thus it hits (200 >= 200 is true) even if dirty pages are 0.5%.
>
> - return (dirty * 100) > (full * nr_pages);
> + return (dirty * 100) >= (full * nr_pages);
Thank you,
> }
>
> /*
> --
> 2.42.0
>
--
Masami Hiramatsu (Google) <mhira...@kernel.org>
