linux-kernel - Re: [patch 14/55] timekeeping: Provide internal ktime

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Date:	Wed, 16 Jul 2014 09:12:52 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	John Stultz <john.stultz@...aro.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnd Bergmann <arnd.bergmann@...aro.org>
Subject: Re: [patch 14/55] timekeeping: Provide internal ktime_t based data

On Wed, 16 Jul 2014, Thomas Gleixner wrote:

> On Tue, 15 Jul 2014, John Stultz wrote:
> > Hrmm.. So I do understand why this is useful performance wise.
> > However, I'm really starting to feel that keeping all this duplicate
> > data is a real maintenance burden, as remembering to keep the values
> > in sync always is prone to error.
> > 
> > So I may have to just put up with it, but I'd like to start thinking
> > about how to reduce the duplicated data in the future. Arnd had an
> > interesting idea for something like storing fixed point seconds, which
> > could be cheaply converted to either ktime_t or timespec values.
> > However, I suspect that would be even more complex for folks to
> > understand, which I'd rather not do.
> > 
> > Overall, it might be best if we just kill the timespec
> > wall_to_monotonic/total_sleep_time/tai_offset values and keep the
> > timekeeper values almost all in timespecs. Then we can leave the
> 
> So we kill the time specs and store everything in timespecs :)
> 
> > conversion process to basically cache the timespec values to the
> > vsyscall_update logic?

Looking into it I think for now it's the least risky approach to keep
the core logic based on the timespec stuff unmodified and update the
ktime_t members in timekeeping_update(). Converting the whole thing to
a pure nsec based mechanism and update the timespec stuff in
timekeeping_update() needs a lot more thought and we should do that
later on. It wont change any of the interfaces.

Thanks,

	tglx
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