linux-kernel - Re: [RFC PATCH 16/22 -v2] add get_monotonic

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Date:	Wed, 16 Jan 2008 10:01:34 -0800
From:	Tim Bird <tim.bird@...sony.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Sam Ravnborg <sam@...nborg.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Daniel Walker <dwalker@...sta.com>
Subject: Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles

Steven Rostedt wrote:
> grmble. Then how do you trace preempt_disable?  As my tracer does that
> (see the last patch in the series).

One way is to make a tracer_preempt_disable() and tracer_preempt_enable(),
both of which would be 'notrace'. You could probably optimize them
as well.  The standard preempt_disable and preempt_enable don't look
very efficient (e.g. what's up with converting an increment operation into
an addition? - gak!)

Any lock you do is going to have a pretty bad effect.

In order to be able to trace as much as possible, for KFT, I implemented
my own synchronization mechanism using cmpxchg, to avoid using any of the
existing kernel locks (which change more often than you'd think, and have
weird side effects).

=============================
Tim Bird
Architecture Group Chair, CE Linux Forum
Senior Staff Engineer, Sony Corporation of America
=============================

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