linux-kernel - Re: [RFC PATCH v2 4/4] timekeeping: utilize the suspend-nonstop clocksource to count suspended time

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Message-ID: <20130305065935.GC5340@feng-snb>
Date:	Tue, 5 Mar 2013 14:59:35 +0800
From:	Feng Tang <feng.tang@...el.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	"H. Peter Anvin" <hpa@...ux.intel.com>, x86@...nel.org,
	Len Brown <lenb@...nel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	linux-kernel@...r.kernel.org, gong.chen@...ux.intel.com
Subject: Re: [RFC PATCH v2 4/4] timekeeping: utilize the suspend-nonstop
 clocksource to count suspended time

On Tue, Mar 05, 2013 at 02:47:27PM +0800, John Stultz wrote:
> On 03/05/2013 02:38 PM, Feng Tang wrote:
> >On Tue, Mar 05, 2013 at 02:27:34PM +0800, John Stultz wrote:
> >
> >>
> >>So this might be ok for an initial implementation, as on the
> >>non-stop-tsc hardware, the TSC is the best clocksource available.
> >>One concern long term is that there may be cases where the non-stop
> >>clocksource is not the most performant clocksource on a system. In
> >>that case, we may want to use a non-stop clocksource that is not the
> >>current timekeeping clocksource. So that may require some extra
> >>clocksource core interfaces to access the non-stop clocksource
> >>instead of using the timekeeper's clocksource, also we'll have to be
> >>sure to use something other then cycle_last in that case, since
> >>we'll need to read the nonstop clocksource at suspend, rather then
> >>trusting that forward_now updates cycle_last as is done here.
> >Yeah, I just realized this when Jason mentioned the counter_32k on
> >OMAP.
> >
> >So for next step, we may add something in timekeeping.c like
> >	static struct clocksource *suspend_time_cs;
> >read and save its counter righer before entering and after getting
> >out of suspended state. And create a new struct which includes
> >all time suspend related flags, counters, pointers, make it as a
> >member of struct timekeeper. Comments?
> I'd maybe add it to the clocksource code rather then the timekeeper.
> Have a clocksource_nonstop_clock() accessor which returns a pointer
> to the highest rated clocksource in the clocksource list that has
> the nonstop flag (updating the pointer at registration time, rather
> then scanning the list every time).

Yes, it's more natural to put it in clocksource code if we need to
pick and compare a best non-stop-tsc clocksource.

Thanks,
Feng


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