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

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Date:	Tue, 27 Feb 2007 11:02:06 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	Ingo Molnar <mingo@...e.hu>, 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

* Daniel Walker (dwalker@...sta.com) wrote:
> On Tue, 2007-02-27 at 02:38 -0500, Mathieu Desnoyers wrote:
> 
> > 
> > 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'm not sure what you mean by using the RCU

The original proposal of this thread uses a RCU (read-copy-update) style
update of the previous 64 bits counter : it swaps a pointer (atomically)
upon update by incrementing a word-sized counter that is used, by the
reader, to get the offest in the array (with a modulo operation) for the
current readable data and as a way to detect incorrect reads of
overwritten information (we re-read the word-sized counter after having
read the data structure to make sure is has not been incremented. If we
detect an increment, we redo the whole operation).

> > 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.
> 
> , but the pit clocksource
> does disable interrupts with a spin_lock_irqsave().
> 

When I say "clocksource read issue", I am talking about
race between the function you proposed earlier, which you say is used in
-rt kernels for latency tracing (get_monotonic_cycles), and HPET and TSC
"last cycles" updates.

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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