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

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Date: Tue, 20 Feb 2024 10:40:23 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
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>
Subject: Re: [PATCH v3] ring-buffer: Simplify reservation with try_cmpxchg()
 loop

On Tue, 20 Feb 2024 09:50:13 -0500
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> On 2024-02-20 09:19, Steven Rostedt wrote:
> > 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.  
> 
> Try resetting the info->add_timestamp flags to add_ts_default on goto again
> within __rb_reserve_next().
>

I was looking at that too, but I don't know how it will make a difference.

Note, the test that fails is in my test suite, and takes about a half hour
to get there. Running that suite takes up resources (it's my main test
suite for all changes). I'm currently testing other patches so I either
need to figure it out through inspection, or this will need to wait a while.

-- Steve