linux-kernel - Re: [patch 54/55] timekeeping: Provide fast and NMI safe access to CLOCK_MONOTONIC[

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Date:	Sat, 12 Jul 2014 21:31:45 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [patch 54/55] timekeeping: Provide fast and NMI safe access to
 CLOCK_MONOTONIC[_RAW]

On Sat, 12 Jul 2014, Mathieu Desnoyers wrote:
> ----- Original Message -----
> > From: "Thomas Gleixner" <tglx@...utronix.de>
> > On Fri, 11 Jul 2014, Mathieu Desnoyers wrote:
> > > Any reason why the updater wouldn't do:
> > > 
> > > tkf->seq++;
> > > smp_wmb();
> > > update(tkf->base[1 - (tkf->seq & 0x01)], tk);
> > > 
> > > instead of updating both array entries each time ?
> > 
> >   base[0];   	      <- Current active
> >   seq++;	      -> Makes base[1] the active one for readers
> >   update(base[0]);
> > 
> > So readers are always one update cycle behind. Probably not an issue
> > most of the time, but think about fast wrapping clocksources and
> > NOHZ....
> 
> Ah, yep, got it.
> 
> So instead of calling update() twice per update, could we do
> the following instead ?
> 
> tkf->seq++;
> smp_wmb();
> update(tkf->base[0], tk);
> tkf->seq++;
> smp_wmb();
> memcpy(tkf->base[1], tkf->base[0], sizeof(tkf->base[1]);
> 
> just in case "update" would happen to be expensive for
> some clock implementation.

Sure, same end result ...

Thanks,

	tglx
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