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

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Date:	Tue, 5 Mar 2013 14:38:30 +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 02:27:34PM +0800, John Stultz wrote:
> On 03/05/2013 10:27 AM, Feng Tang wrote:
> >There are some new processors whose TSC clocksource won't stop during
> >suspend. Currently, after system resumes, kernel will use persistent
> >clock or RTC to compensate the sleep time, but for those new types of
> >clocksources, we could skip the special compensation from external
> >sources, and just use current clocksource for time recounting.
> >
> >This can solve some time drift bugs caused by some not-so-accurate or
> >error-prone RTC devices.
> >
> >The current way to count suspened time is first try to use the persistent
> >clock, and then try the rtc if persistent clock can't be used. This
> >patch will change the trying order to:
> >	suspend-nonstop clocksource -> persistent clock -> rtc
> 
> Thanks for sending out another iteration of this code. Jason's
> feedback has been good, but I think this is starting to shape up
> nicely.

Thanks :)

> >Signed-off-by: Feng Tang <feng.tang@...el.com>
> >---
> >  kernel/time/timekeeping.c |   57 ++++++++++++++++++++++++++++++++++++++------
> >  1 files changed, 49 insertions(+), 8 deletions(-)
> >
> >diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> >index 9a0bc98..15cc086 100644
> >--- a/kernel/time/timekeeping.c
> >+++ b/kernel/time/timekeeping.c
> >@@ -788,22 +788,63 @@ void timekeeping_inject_sleeptime(struct timespec *delta)
> >  static void timekeeping_resume(void)
> >  {
> >  	struct timekeeper *tk = &timekeeper;
> >+	struct clocksource *clock = tk->clock;
> >  	unsigned long flags;
> >-	struct timespec ts;
> >+	struct timespec ts_new, ts_delta;
> >+	cycle_t cycle_now, cycle_delta;
> >+	s64 nsec;
> >-	read_persistent_clock(&ts);
> >+	ts_delta.tv_sec = 0;
> >+	read_persistent_clock(&ts_new);
> >  	clockevents_resume();
> >  	clocksource_resume();
> >  	write_seqlock_irqsave(&tk->lock, flags);
> >-	if (timespec_compare(&ts, &timekeeping_suspend_time) > 0) {
> >-		ts = timespec_sub(ts, timekeeping_suspend_time);
> >-		__timekeeping_inject_sleeptime(tk, &ts);
> >-	}
> >-	/* re-base the last cycle value */
> >-	tk->clock->cycle_last = tk->clock->read(tk->clock);
> >+	/*
> >+	 * After system resumes, we need to calculate the suspended time and
> >+	 * compensate it for the OS time. There are 3 sources that could be
> >+	 * used: Nonstop clocksource during suspend, persistent clock and rtc
> >+	 * device.
> >+	 *
> >+	 * One specific platform may have 1 or 2 or all of them, and the
> >+	 * preference will be:
> >+	 *	suspend-nonstop clocksource > persistent clock > rtc
> >+	 * The less preferred source will only be tried if there is no better
> >+	 * usable source. The rtc part is handled separately in rtc core code.
> >+	 */
> >+	cycle_now = clock->read(clock);
> 
> 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?

Thanks,
Feng


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