linux-kernel - Re: [PATCH v3] ring-buffer: Simplify reservation with try

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Message-ID: <20240220091922.45848d9a@gandalf.local.home>
Date: Tue, 20 Feb 2024 09:19:22 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mark Rutland
 <mark.rutland@....com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH v3] ring-buffer: Simplify reservation with try_cmpxchg()
 loop

On Mon, 19 Feb 2024 18:20:32 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> Instead of using local_add_return() to reserve the ring buffer data,
> Mathieu Desnoyers suggested using local_cmpxchg(). This would simplify the
> reservation with the time keeping code.
> 
> Although, it does not get rid of the double time stamps (before_stamp and
> write_stamp), using cmpxchg() does get rid of the more complex case when
> an interrupting event occurs between getting the timestamps and reserving
> the data, as when that happens, it just tries again instead of dealing
> with it.
> 
> Before we had:
> 
> 	w = local_read(&tail_page->write);
> 	/* get time stamps */
> 	write = local_add_return(length, &tail_page->write);
> 	if (write - length == w) {
> 		/* do simple case */
> 	} else {
> 		/* do complex case */
> 	}
> 
> By switching the local_add_return() to a local_try_cmpxchg() it can now be:
> 
> 	 w = local_read(&tail_page->write);
>  again:
> 	/* get time stamps */
> 	if (!local_try_cmpxchg(&tail_page->write, &w, w + length))
> 		goto again;
> 
> 	 /* do simple case */

Something about this logic is causing __rb_next_reserve() to sometimes
always return -EAGAIN and triggering the:

    RB_WARN_ON(cpu_buffer, ++nr_loops > 1000)

Which disables the ring buffer.

I'm not sure what it is, but until I do, I'm removing the patch from my
queue.

-- Steve