linux-kernel - Re: [RFC] The New and Improved Logdev (now with kprobes!)

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Date:	Thu, 5 Oct 2006 16:31:45 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Daniel Walker <dwalker@...sta.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Karim Yaghmour <karim@...rsys.com>,
	Andrew Morton <akpm@...l.org>,
	Chris Wright <chrisw@...s-sol.org>, fche@...hat.com,
	Tom Zanussi <zanussi@...ibm.com>
Subject: Re: [RFC] The New and Improved Logdev (now with kprobes!)

* Steven Rostedt (rostedt@...dmis.org) wrote:
> 
> On Thu, 5 Oct 2006, Mathieu Desnoyers wrote:
> 
> >
> > Just as a detail : LTTng traces NMI, which can happen on top of a
> > xtime_lock. So yes, I have to consider the impact of this kind of lock when I
> > choose my time source, which is currently a per architecture TSC read,
> > or a read of the jiffies counter when the architecture does not have a
> > synchronised TSC over the CPUs. This is abstracted in include/asm-*/ltt.h.
> >
> 
> I'm curious.  How do you show the interactions between two CPUs when the
> TSC isn't in sync?  Using jiffies is not fast enough to know the order of
> events that happen within usecs.
> 

I shift the jiffies and OR that with a logical clock which increments atomically
and is shared across the CPUs. It is slow and ugly, but it works. :)

Mathieu

> -- Steve
> 
> 
> > I know it doesn't support dynamic ticks, I'm working on using the HRtimers
> > instead, but I must make sure that the seqlock read will fail if it nests over
> > a write seqlock.
> >
> > MAthieu
> >
> > OpenPGP public key:              http://krystal.dyndns.org:8080/key/compudj.gpg
> > Key fingerprint:     8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
> >
OpenPGP public key:              http://krystal.dyndns.org:8080/key/compudj.gpg
Key fingerprint:     8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68 
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