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

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Date:	Thu, 5 Oct 2006 16:18:20 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	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!)

* Daniel Walker (dwalker@...sta.com) wrote:
> On Thu, 2006-10-05 at 14:38 -0400, Steven Rostedt wrote:
> > On Thu, 5 Oct 2006, Daniel Walker wrote:
> > 
> > > On Thu, 2006-10-05 at 14:09 -0400, Steven Rostedt wrote:
> > >
> > > >
> > > > My problem with using a timestamp, is that I ran logdev on too many archs.
> > > > So I need to have a timestamp that I can get to that is always reliable.
> > > > How does LTTng get the time for different archs?  Does it have separate
> > > > code for each arch?
> > > >
> > >
> > > I just got done updating a patchset that exposes the clocksources from
> > > generic time to take low level time stamps.. But even without that you
> > > can just call gettimeofday() directly to get a timestamp .
> > >
> > 
> > unless you're tracing something that his holding the xtime_lock ;-)
> 
> That's part of the reason for the changes that I made to the clocksource
> API . It makes it so instrumentation, with other things, can generically
> read a low level cycle clock. Like on PPC you would read the
> decrementer, and on x86 you would read the TSC . However, the
> application has no idea what it's reading.
> 
> I submitted one version to LKML already, but I'm planning to submit
> another version shortly.
> 

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 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

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