linux-kernel - Re: [RFC][PATCH 5/7] x86: Use latch data structure for cyc2ns

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Date:	Fri, 29 Nov 2013 15:22:45 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tony Luck <tony.luck@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC][PATCH 5/7] x86: Use latch data structure for cyc2ns

On Fri, Nov 29, 2013 at 9:37 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> Use the 'latch' data structure for cyc2ns.
>
> This is a data structure first proposed by me and later named by
> Mathieu. If anybody's got a better name; do holler.

That structure must exist in the literature, but I have no idea what
it's called.  It's a multi-word lock-free atomic (I think -- maybe
it's just regular) register.  I even published a considerably fancier
version of much the same thing a few years ago.  :)

>
> Its a multi-version thing which allows always having a coherent
> object; we use this to avoid having to disable IRQs while reading
> sched_clock() and avoids a problem when getting an NMI while changing
> the cyc2ns data.
>
> The patch should have plenty comments actually explaining the thing.
>
> The hope is that the extra logic is offset by no longer requiring the
> sti;cli around reading the clock.

I've occasionally wondered whether it would be possible to make a
monotonicity-preserving version of this and use it for clock_gettime.
One approach: have the writer set the time for the update to be a bit
in the future and have the reader compare the current raw time to the
cutoff to see which set of frequency/offset to use.  (This requires
having some kind of bound on how long it takes to update the data
structures.)

The advantage: clock_gettime would never block.
The disadvantage: complicated, potentially nasty to implement, and it
would get complicated if anyone tried to allow multiple updates in
rapid succession.

Anyway, this is mostly irrelevant to your patches.

--Andy
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