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

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Message-ID: <alpine.DEB.2.10.1407160859110.24854@nanos>
Date:	Wed, 16 Jul 2014 09:00:38 +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 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?

> Or if we do want to maintain performance for non-vdso implementations,
> maybe we can split the timespec functions out of the timekeeping core
> and do a similar single shot update each tick to set the update the
> tiemspec cached values. But that can be layered over the timekeeping
> core and we can avoid the crazy updating 2 values any time we change
> one logic we currently have.

Makes sense. Lemme have a stab on it.

Thanks,

	tglx
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