linux-kernel - Re: [PATCH] ring-buffer: Do not update before stamp when switching sub-buffers

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Message-Id: <20231213093439.9c14b3dd0ac09d92aea70ec1@kernel.org>
Date:   Wed, 13 Dec 2023 09:34:39 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] ring-buffer: Do not update before stamp when switching
 sub-buffers

On Mon, 11 Dec 2023 11:44:20 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@...dmis.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) <mhiramat@...nel.org>

Thank you

> Cc: stable@...r.kernel.org
> Fixes: a389d86f7fd09 ("ring-buffer: Have nested events still record running time stamp")
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.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) <mhiramat@...nel.org>