Re: [RFC][PATCH 4/4] time: Do leapsecond adjustment in gettime fastpaths

[Ingo Molnar]

* John Stultz <john.stultz@xxxxxxxxxx> wrote:

> > Instead of having these super rare special events, how about implementing leap 
> > second smearing instead? That's far less radical and a lot easier to test as 
> > well, as it's a continuous mechanism. It will also confuse user-space a lot 
> > less, because there are no sudden time jumps.
> 
> So yea. Leap smearing/slewing is an attractive solution. The first issue is that 
> there's no standard yet for the range of time that the slew occurs (or even if 
> the slew is linear or a curve). The second is I don't think we can actually get 
> away from supporting UTC w/ leap, as applications may depend on precision. Also 
> things like NTP sync w/ mixed systems would be problematic, as NTPd and others 
> need to become savvy of which mode they are working with.

Supporting it minimally is fine - supporting it with clearly unmaintainable 
complexity is not.

So as long as we offer good smearing of the leap second (with a configurable 
parameter for how long the period should be), people in need of better leap
second handling can take that.

> The leap smearing method of only doing it in private networks and controlling it 
> by the NTP server is becoming more widespread, but it has its own problems, 
> since it doesn't handle CLOCK_TAI properly, and since CLOCK_REALTIME isn't yet 
> frequency steerable separately from the other clockids, this method ends up 
> slowing down CLOCK_TAI and CLOCK_MONOTONIC as well.

All real time clock derived clocks should smear in sync as well.

> I'd like to try to get something working in the kernel so we could support 
> CLOCK_UTC and CLOCK_UTCSLS (smeared-leap-second) clockids, then allow 
> applications that care to migrate explicitly to the one they care about. 
> Possibly allowing CLOCK_REALTIME to be compile-time directed to CLOCK_UTCSLS so 
> that most applications that don't care can just ignore it.  But finding time to 
> do this has been hard (if anyone is interested in working on it, I'd be excited 
> to hear!).

There should definitely be a Kconfig option to just map all relevant clocks to 
smeared seconds. Hopefully this ends up being the standard in a few years and we 
can pin down the exact parameters as well.

Having separate clockids for mixed uses would be fine as well. Maybe.

> But if you think this patch is complicated, creating a new separately steered 
> clockid is not going to be trvial (as there will be lots of ugly edge cases, 
> like what if a leap second is cancelled mid-way through the slewing adjustment, 
> etc).

Well, I think the main advantage of leap second smearing is that it's not a 
binary, but a continuous interface, and so it's way easier to test than 'sudden' 
leap second insertions.

In fact we could essentially implement leap second smearing via the usual adjtimex 
mechanisms: as far as the time code is concerned it does not matter why a gradual 
adjustment occurs, only the rate of change and the method of convergence is an 
open parameter.

In fact I'd suggest we implement even original leap seconds by doing a high-rate 
'smearing' in the final X minutes leading up to the leap second, where 'X' could 
be 1 by default. This way we could eliminate leap seconds as a separate logical 
entity mostly.

This should be far more gentle to applications as well than sudden jumps, and 
timers will just work fine as well.

> > Secondly, why is there a directional flag? I thought leap seconds can only be 
> > inserted.
> 
> A leap delete isn't likely to occur, but its supported by the adjtimex 
> interface. And given the irregularity of the earths rotation, I'm not sure I'd 
> rule it out completely.

Well, the long term trend is clear and unambiguous: the rotation of Earth is 
slowing down (the main component of which is losing angular momentum to the Moon), 
hence the days are getting longer and we have to insert a leap second every second 
year or so.

The short term trends (discounting massive asteorid strikes, at which point leap 
seconds will be the least of our problems) are somewhat chaotic:

 - glaciation (which shifts water mass assymetrically)

 - global warming (one component of which is thermal expansion, which expands 
   oceans assymetrically and shifts water mass - the other component is changing
   climatology: different oceanic currents, etc. - which all shift mass around)

 - tectonics (slow rearrangement of mass plus earthquakes).

 - even slower scale rearrangement of mass (mantle plumes, etc.)

but the long term trend still dominates. Look at this graph of measurements of the 
Earth's rotation:

  http://en.wikipedia.org/wiki/File:Deviation_of_day_length_from_SI_day.svg

See how the mean (the green line) was always above zero in the measured past. The 
monotonically increasing nature comes from that.

and given how many problems we had with leap second insertion, on millions of 
installed systems, guess the likelihood of there being a leap second deleted? How 
many OSs that can do leap second insertion are unable to do leap second deletion?

Also note that leap second deletion means a jump in time backward. Daylight saving 
time is already causing problems with that.

> > So all in one, the leap second code is fragile and complex - lets re-think the 
> > whole topic instead of complicating it even more ...
> 
> So the core complexity with this patch is that we're basically having to do 
> state-machine transitions in a read-only path (since the reads may happen before 
> the update path runs). Since there's a number of read-paths, there's some 
> duplication, and in some cases variance if the read path exports more state (ie: 
> adjtimex).

My fundamental observation is: the cost/benefit ratio is insanely high.

Interrupts are fundamentally jittery, there's no guarantee of their accuracy - you 
yourself said that as a reply to PeterZ's suggestion to drive leap seconds via 
hrtimers - and the motivation was to make interrupts arrive more accurately around 
leap seconds.

So why make the code more fragile, more complex, just to solve a scenario that 
cannot really be done perfectly?

Especially as second smearing appears to be the way superior future method of 
handling leap seconds.

> I do agree that the complexity of the time subsystem is getting hard to manage. 

That's rather an understatement.

> I'm at the point where I think we need to avoid keeping duplicated timespec and 
> ktime_t data (we can leave the ktime->timespec caching to the VDSOs). That will 
> help cut down the read paths a bit, but will also simplify updates since we'll 
> have less data to keep in sync.  How we manage the ntp state also needs a 
> rework, since the locking rules are getting too complex (bit me in an earlier 
> version of this patch), and we're in effect duplicating some of that state in 
> the timekeeper with this patch to handle the reads safely.

Agreed.

> But even assuming all those changes were already made, I think we'd still need 
> something close to this patch.

I disagree rather strongly.

Thanks,

	Ingo
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