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

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Date: Sat, 20 Jan 2024 09:34:01 -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>, Philippe Proulx
 <pproulx@...icios.com>
Subject: Re: [PATCH] ring-buffer: Simplify reservation with try_cmpxchg()
 loop

On Sat, 20 Jan 2024 08:47:13 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> > I would also consider reducing code complexity as a worthwhile metric
> > in addition to speed. It makes it easier to extend in the future,
> > easier to understand for reviewers, and subtle bugs are less likely
> > to creep in.  
> 
> Really, it wouldn't make it that much simpler. The addition of the
> cmpxchg() of this patch removed the nasty part of the code.

Now let's look at the difference of the two. You still need to save the
current timestamp in one variable. I have to do it in two, so your
algorithm does have that advantage. I currently have (eliminating the
"always add absolute timestamp" switch):


  w = write;
  before = before_stamp;
again:
  after = write_stamp; (equivalent to your last_tsc)
  ts = rdtsc();
  if (!w)
	delta = 0; // event has same ts as subbuf
  else if (before == after)
	delta = ts - after;
  else {
	delta = 0;
	inject_absolute = true;
  }

  before_stamp = ts;

  if (!try_cmpxchg(&write, w, w + length))
	goto again;

  write_stamp = ts;


Now if I were to updated it to use a delta from the last injected
timestamp, where injecting a timestamp only happens on overflow.

#define TS_BITS 27
#define MAX_DELTA ((1 << TS_BITS) - 1)
#define BITMASK (~MAX_DELTA)

  w = write;
again:
  ts = rdtsc();

  delta = ts & MAX_DELTA;

  if (ts - (write_stamp & BITMASK) > MAX_DELTA)
	inject_absolute = true;

  if (!try_cmpxchg(&write, w, w + length))
	goto again;

  write_stamp = ts;

I admit that it does simplify the code a little, but does it do it
enough to be worth the process of deprecating an ABI with a new one?

-- Steve