linux-kernel - Re: tracepoints for kernel/mutex.c

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Message-Id: <1224192879.16038.82.camel@lappy.programming.kicks-ass.net>
Date:	Thu, 16 Oct 2008 23:34:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Baron <jbaron@...hat.com>
Cc:	linux-kernel@...r.kernel.org, compudj@...stal.dyndns.org,
	fche@...hat.com, fweisbec@...il.com, edwintorok@...il.com,
	mingo@...e.hu, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: tracepoints for kernel/mutex.c

On Thu, 2008-10-16 at 17:04 -0400, Jason Baron wrote:

> Below are 3 tracepoints I've been playing with in kernel/mutex.c using
> a SystemTap script. The idea is to detect and determine the cause of
> lock contention. Currently I get the following output:
> 
> <contended mutex nam> <process name and pid of the contention> <time of
> contention> <pid that woke me up(caused the contention?)>

> I think this approach has a number of advantages. It has low
> overhead in the off case, since its based on tracepoints. It is
> minimally invasive in the code path (3 tracepoints). It also allows me
> to explore data structures and parts of the kernel by simply modifying
> the SystemTap script. I do not need to re-compile the kernel and reboot.

*sigh* this is why I hate markers and all related things...

_IFF_ you want to place tracepoints, get them in the same place as the
lock-dep/stat hooks, that way you get all the locks, not only mutexes.

This is the same reason I absolutely _hate_ Edwin's rwsem tracer.

Folks, lets please start by getting the tracing infrastructure in and
those few high-level trace-points google proposed.

Until we get the basics in, I think I'm going to NAK any and all
tracepoint/marker patches.

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