linux-kernel - Re: [RFC] Fast assurate clock readable from user space and NMI handler

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Date:	Tue, 27 Feb 2007 02:38:15 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Daniel Walker <dwalker@...sta.com>, mbligh@...gle.com,
	linux-kernel@...r.kernel.org, johnstul@...ibm.com,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC] Fast assurate clock readable from user space and NMI handler

* Ingo Molnar (mingo@...e.hu) wrote:
> 
> * Daniel Walker <dwalker@...sta.com> wrote:
> 
> > The pit clocksource could be dropped pretty easy with my clocksource 
> > update patches, which I'm still working on but you could easily drop 
> > clock sources that aren't atomic like the pit .. Also the pit is 
> > generally undesirable, so it's not going to be missed.
> 
> that's totally unacceptable, and i'm amazed you are even suggesting it - 
> often the PIT ends up being the most reliable hardware clock in a PC. 
> Btw., what's wrong with the spinlock that is protecting PIT access? It 
> expresses the non-atomic property of the PIT just fine.
> 

I am concerned about the automatic fallback to the PIT when no other
clock source is available. A clocksource read would be atomic when TSC
or HPET are available, but would fall back on PIT otherwise. There
should be some way to specify that a caller is only interested in atomic
clock sources (if none are available, the call should simply return an
error, or 0).

I still think that an RCU style update mechanism would be a good way  to
fix the current clocksource read issue. Another, slower and non NMI
safe way to do this would be with a read seqlock and with IRQ disabling.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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