linux-kernel - Re: Fwd: [GIT PULL] timer changes for v3.10

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Message-ID: <20130507215151.GB10061@amd.pavel.ucw.cz>
Date:	Tue, 7 May 2013 23:51:51 +0200
From:	Pavel Machek <pavel@....cz>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Ingo Molnar <mingo@...nel.org>, Feng Tang <feng.tang@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Fwd: [GIT PULL] timer changes for v3.10

Hi!

> >>>>>is even worse than that. Machine can stay is s2ram for weeks (for a
> >>>>>lot more if it is desktop and you do s2ram for powersaving). Also
> >>>>>temperature of CPU varies a lot between active and s2ram states. Is
> >>>>>TSC good enough?
> >>>>Yes, I think it is relatively precise. Per our test, system time backed
> >>>>by the S3 non stop TSC only has 1 second drift after 4 days running
> >>>>(with mixed running and S3 states). And before using this feature, we've
> >>>>seen many time drift problems due to the RTC HW or system FW with our
> >>>>platforms.
> >>>Nice result ...
> >>>
> >>>Is that with NTP running?
> >>>
> >>>Without NTP, the TSC fast-calibration on bootup is not (expected to be)
> >>>nearly as precise as the 1:345600 precision you've measured.
> >>We also do refined calibration now on the TSC asynchronously over a
> >>period of seconds at boot up that gives us much better accuracy then
> >>the fast calibration. This helps provide much more consistent
> >>boot-to-boot TSC frequencies.
> >On android (and this is targetted at android, right?) system is going
> >to suspend basically as soon as it boots. Will refined calibration
> >have enough time to do its job?
> I don't *think* this is a concern. The refined calibration only
> takes a few seconds while the system is booting and has always
> completed before userspace starts on the systems I have. Even so, I
> don't believe on boot Android will trigger the autosleep code until
> its userland is up and running, which takes more then a few seconds
> on the devices I've seen.

Userspace has to be running, right. It should boot in 10 seconds or
so.

> >And... reason for all this is that RTC has one second granularity when
> >accessed naively. But surely we could poll RTC X times a second,
> >getting error down by factor of X?
> Well, we can poll the RTC trying to get closer to the second edge,
> but that's somewhat expensive, and for suspend/resume would delay
> things more then whats acceptable.

Well, if 

> Sorry. You seem to not like the merged change, but I guess I'm not
> quite sure what exactly your objection is here.

I'm not exactly sure what my objections are.

TSC was not designed for long-term precise timekeeping. I guess it
may work ok for short naps, but some people suspend their machines for
longer than that. Plus I wonder how it will interfere with
/etc/adjtime.

									Pavel 
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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