On Mon, 11 Dec 2023 11:44:20 -0500
Steven Rostedt <rost...@goodmis.org> wrote:
> From: "Steven Rostedt (Google)" <rost...@goodmis.org>
>
> The ring buffer timestamps are synchronized by two timestamp placeholders.
> One is the "before_stamp" and the other is the "write_stamp" (sometimes
> referred to as the "after stamp" but only in the comments. These two
> stamps are key to knowing how to handle nested events coming in with a
> lockless system.
>
> When moving across sub-buffers, the before stamp is updated but the write
> stamp is not. There's an effort to put back the before stamp to something
> that seems logical in case there's nested events. But as the current event
> is about to cross sub-buffers, and so will any new nested event that happens,
> updating the before stamp is useless, and could even introduce new race
> conditions.
>
> The first event on a sub-buffer simply uses the sub-buffer's timestamp
> and keeps a "delta" of zero. The "before_stamp" and "write_stamp" are not
> used in the algorithm in this case. There's no reason to try to fix the
> before_stamp when this happens.
>
> As a bonus, it removes a cmpxchg() when crossing sub-buffers!
>
Looks good to me.
Reviewed-by: Masami Hiramatsu (Google) <mhira...@kernel.org>
Thank you
> Cc: sta...@vger.kernel.org
> Fixes: a389d86f7fd09 ("ring-buffer: Have nested events still record running
> time stamp")
> Signed-off-by: Steven Rostedt (Google) <rost...@goodmis.org>
> ---
> kernel/trace/ring_buffer.c | 9 +--------
> 1 file changed, 1 insertion(+), 8 deletions(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 2596fa7b748a..02bc9986fe0d 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -3607,14 +3607,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu
> *cpu_buffer,
>
> /* See if we shot pass the end of this buffer page */
> if (unlikely(write > BUF_PAGE_SIZE)) {
> - /* before and after may now different, fix it up*/
> - b_ok = rb_time_read(&cpu_buffer->before_stamp, &info->before);
> - a_ok = rb_time_read(&cpu_buffer->write_stamp, &info->after);
> - if (a_ok && b_ok && info->before != info->after)
> - (void)rb_time_cmpxchg(&cpu_buffer->before_stamp,
> - info->before, info->after);
> - if (a_ok && b_ok)
> - check_buffer(cpu_buffer, info, CHECK_FULL_PAGE);
> + check_buffer(cpu_buffer, info, CHECK_FULL_PAGE);
> return rb_move_tail(cpu_buffer, tail, info);
> }
>
> --
> 2.42.0
>
--
Masami Hiramatsu (Google) <mhira...@kernel.org>
