linux-kernel - Re: [PATCH 1/2] time: allow changing the timekeeper clock frequency

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Date:	Thu, 29 Aug 2013 14:40:59 -0400
From:	Chris Metcalf <cmetcalf@...era.com>
To:	John Stultz <john.stultz@...aro.org>
CC:	lkml <linux-kernel@...r.kernel.org>, <cpufreq@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 1/2] time: allow changing the timekeeper clock frequency

Ping!  I have this work queued up to push as part of the linux-tile tree for the
merge window.  Is that acceptable to the timekeeping/clocksource folks?
Should I hold it back pending further review?  Or does it make sense to
push it as-is and think about further improvements, if any, for a later release?

https://lkml.org/lkml/2013/8/9/497
https://lkml.org/lkml/2013/8/9/499

Thanks in advance!

On 8/14/2013 5:30 PM, Chris Metcalf wrote:
> On 8/14/2013 2:17 PM, John Stultz wrote:
>> So a long while back we had tried to adapt for clock frequency changes
>> on things like the TSC, but it resulting in *terrible* timekeeping as
>> the latency between the frequency change and the handling of the
>> notifications caused lots of clock drift, making it impossible for NTP
>> or other synchronization methods to work properly.
> We've done quite a bit of testing to show that our current implementation
> doesn't have any clock drift over time.  Basically, we take a machine
> running some workload, sync its time via ntpdate, and then run a script
> that changes the CPU speed up or down continually, with a delay of a couple
> seconds in between so we run for some decent amount of time at each speed.
> Every 5 minutes or so, the script runs ntpdate -q to see what the offset
> from real time is.  The skew we see doing that for a couple of days is
> identical to that seen when we _aren't_ changing the CPU frequency.
>
> A key part of making this work, as noted in the comments at the head of
> timekeeping_chfreq_prep(), is the fact that we do the frequency change
> under stop_machine() to make sure that no CPU gets an opportunity to
> sample the clock while it's being changed.
>
> However, I'm wondering whether you're talking about some other sort of
> much more local clock skew or other frequency effect that perhaps we
> haven't tested for.  (For instance, we haven't actually run this code
> on an NTP server.)  Can you give a bit more detail on exactly what sorts
> of bad behavior you saw with the previous implementation, and things one
> might do to detect them?
>
>
>> So early on we made
>> a requirement that all clocksources have a constant frequency and
>> provided a way to disqualify any clocksources that change frequency.
>>
>> So I'd be very hesitant to try to add any such behavior into the
>> timekeeping core. You may want to try to add some logic in the
>> clocksource driver itself to allow for the variable freq clocksource
>> to output what seems to be a fixed freq,
> So, just to be clear, you're suggesting that we claim our clocksource
> runs at some lower virtual speed (say, 1 MHz), and that internally to
> our clocksource drver we divide down the real frequency to the virtual
> one?
>
>
>> and if we get some time on it
>> to prove that it can be made to work well, then we can see about
>> making it more generic.
>>
>> Does that sound ok?
> That seems possible, although it would seem to make the whole process
> a bit less efficient (e.g., our clocksource will have to maintain its
> own multiplier and offset to convert from real ticks to virtual ticks,
> and then the core code will do the same operation again to convert to
> wall-clock time).  Obviously, we're not really anxious to re-test/re-qualify
> a new implementation of this, but if our current version is or might be
> incompatible with other code in the kernel perhaps that's a safer approach.
>
> What sort of eventual more-generic support were you thinking of?
>

-- 
Chris Metcalf, Tilera Corp.
http://www.tilera.com

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