Re: [RFC PATCH 13/22 -v2] handle accurate time keeping over long delays

Mathieu Desnoyers
2008-01-10T22:05:31Z

* john stultz (johnstul@us.ibm.com) wrote:
>
> On Thu, 2008-01-10 at 15:42 -0500, Mathieu Desnoyers wrote:
> > I think it's about time I introduce the approach I have taken for LTTng
> > timestamping. Basically, one of the main issues with the clock sources
> > is the xtime lock : having a read seqlock nested over a write seqlock is
> > a really, really bad idea. This can happen with NMIs. Basically, it
> > would cause a deadlock.
> >
> > What I have done is an RCU algorithm that extends a 32 bits TSC (that's
> > the case on MIPS, for instance) to 64 bits. The update of the MSBs is
> > done by a periodical timer (fired often enough to make sure we always
> > detect the 32 LSBs wrap-around) and the read-side only has to disable
> > preemption.
> >
> > I use a 2 slots array, each of them keeping, alternatively, the last 64
> > bits counter value, to implement the RCU algorithm.
> >
> > Since we are discussing time source modification, this is one that I
> > would really like to see in the Linux kernel : it would provide the kind
> > of time source needed for function entry/exit tracing and for generic
> > kernel tracing as well.
>
> Hmm. I know powerpc has had a similar lock-free dual structure method
> and for just a raw cycles based method you've shown below (or for some
> of the bits Steven is working on), I think it should be fine.
>
> The concern I've had with this method for general timekeeping, is that
> I'm not sure it can handle the frequency corrections made by NTP. Since
> we have to make sure time does not jump backwards, consider this
> exaggerated situation:
>
> time = base + (now - last)*mult;
>
> So we have two structures:
> base: 60 base: 180
> last: 10 last: 30
> mult: 06 mult: 05
>
> Where the second structure has just been updated lock-free, however just
> before the atomic pointer switch we were preempted, or somehow delayed,
> and some time has past.
>
> Now imagine two cpus now race to get the time. Both read the same now
> value, but get different structure pointer values. (Note: You can create
> the same race if you reverse the order and grab the pointer first, then
> the cycle. However I think this example makes it easier to understand).
>
> now = 50
> cpu1:
> 60 + (50-10)*6 = 300
> cpu2:
> 180 + (50-30)*5 = 280
>
>
> Alternatively:
> now=50: 60 + (50-10)*6 = 300
> now=51: 180 + (51-30)*5 = 285
>
> Eek. That's not good.
>
> I'm not sure how this can be avoided, but I'd be very interested in
> hearing ideas! Bounding the issue is a possibility, but then starts to
> run amok with NO_HZ and -rt deferment.
>
> thanks
> -john
>

I suggest we try to see the problem differently (and see how far we can
get with this) :

Let's suppose we have a 32 bits cycles counter given by the
architecture. We use the lockless algorithm to extend it to 64 bits : we
therefore have a 64 bits cycle counter that can be read locklessly.

Then, from this 64 bits counter (let's call it "now") (which has the
advantage of never overflowing, or after enough years so nobody
cares...), we can calculate the current time with :

time = base + (now - last) * mul

NTP would adjust time by modifying mul, last and base; base would be
recalculated from the formula with : base + (now - last) * mul each time
we modify the clock rate (mul), we also read the current "now" value
(which is saved as "last"). This would be done system-wide and would be
kept in a data structure separate from the 2 64 bits slots array.
Ideally, this NTP correction update could also be done atomically with a
2 slots array.

Whenever we need to read the time, we then have to insure that the "now"
value we use is consistent with the current NTP time correction. We want
to eliminate races where we would use value from the wrong NTP "window"
with a "now" value not belonging to this window. (an NTP window would be
defined by a tuple of base, last and mul values)

If "now" is lower than "last", we are using an old timestamp with a
new copy of the structure and must therefore re-read the "now" value.

If, when we are about to return the "time" value calculated, we figure
out that the current NTP window pointer have changed, we must make sure
that the time value we are about to return is lower than the new base or
otherwise time could go backward. If we detect that time is higher than
the new base, we re-read the "now" value and re-do the calculation.

Am I only partially crazy ? ;)

Mathieu

--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68