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

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Message-ID: <66899.1316207693@turing-police.cc.vt.edu>
Date:	Fri, 16 Sep 2011 17:14:53 -0400
From:	Valdis.Kletnieks@...edu
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>, mingo <mingo@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ftrace: use a global counter for the global clock

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?

> 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)...

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

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