Re: [PATCH] ftrace: use a global counter for the global clock

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 2011-09-16 at 17:14 -0400, Valdis.Kletnieks@vt.edu wrote:
> On Fri, 16 Sep 2011 08:18:26 EDT, Steven Rostedt said:
> 
> > +	return atomic64_add_return(1, &trace_counter);
> 
> Given that the usefulness of this is probably directly proportional to the
> number of cores on the box, is this subject to cache line ping-ponging on
> systems with many cores?

It will cause hickups, but shouldn't really change much of the ordering.
If anything, it may synchronize more.

> 
> > When debugging tight race conditions, it can be helpful to have a
> > synchronized tracing method. Although in most cases the global clock
> > provides this functionality, if timings is not the issue, it is more
> > comforting to know that the order of events really happened in a precise
> > order.
> 
> One wonders if the overhead can end up being enough to change the
> ordering, and possibly cause a heisenbug (most likely if the race condition
> involves one CPU doing something we're tracing, and another CPU doing
> something we are *not* tracing)...

My use of logdev synchronized traces like this. But it actually caught a
lot of race conditions. Really, it's about catching the ordering of what
happens in a race condition, and having the tracer actually introduce
synchronization helped in analysis. But true, as a heisenbug effect,
enabling this tracer may make the bug go away.

> 
> If that's considered not an issue, feel free to stick this on it:
> Reviewed-By: Valdis Kletnieks <valdis.kletnieks@vt.edu>

Thanks, and yes, it is not too much of a concern. It just adds another
option developers can use. Thus, if you are worried about cache line
bouncing, just use the global or local clock, otherwise use the counter.
Which I plan on documenting this side effect.

-- Steve