Re: [Xen-devel] [patch 14/33] xen: xen time implementation

Jeremy Fitzhardinge
2007-06-06T10:05:40Z

Jan Beulich wrote:
> Xen itself knows to deal with this (by using an error correction factor to
> slow down the local [TSC-based] clock), but for the kernel such a situation
> may be fatal: If clocksource->cycle_last was most recently set on a CPU
> with shadow->tsc_to_nsec_mul sufficiently different from that where
> getnstimeofday() is being used, timekeeping.c's __get_nsec_offset() will
> calculate a huge nanosecond value (due to cyc2ns() doing unsigned
> operations), worth abut 4000s. This value may then be used to set a
> timeout that was intended to be a few milliseconds, effectively yielding
> a hung app (and perhaps system).
>

Hm. I had a similar situation in the stolen time code, and I ended up
using signed values so I could clamp at zero. Though that might have
been another bug; either way, the clamp is still there.

I wonder if cyc2ns might not be better using signed operations? Or
perhaps better, the time code should endevour to do things on a
completely per-cpu basis (haven't really given this any thought).

> I'm sure the time keeping code can't deal with negative values returned
> from __get_nsec_offset() (timespec_add_ns() is an example, used in
> __get_realtime_clock_ts()), otherwise a potential solution might have
> been to set the clock source's multiplier and shift to one and zero
> respectively.

I don't quite follow you here, but wouldn't setting the multiplier/shift
to 1/0 preclude being able to warp the clocksource with ntp?

> But I think that a clock source can be expected to be
> monotonic anyway, which Xen's interpolation mechanism doesn't
> guarantee across multiple CPUs. (I'm actually beginning to think that
> this might also be the reason for certain test suites occasionally reporting
> timeouts to fire early.)
>

Does the kernel expect the tsc clocksource to be completely monotonic
across cpus? Any form of cpu-local clocksource is going to have this
problem; I wonder if clocksources can really only be useful if they're
always referring to a single system-wide time reference - seems like a
bit of a limitation.

> Unfortunately so far I haven't been able to think of a reasonable solution
> to this - a simplistic approach like making xen_clocksource_read() check
> the value it is about to return against the last value it returned doesn't
> seem to be a good idea (time might appear to have stopped over some
> period of time otherwise), nor does attempting to adjust the shadowed
> tsc_to_nsec_mul values (because the kernel can't know whether it should
> boost the lagging CPU or throttle the rushing one).

I once had some code in there to do that, implemented in very boneheaded
way with a spinlock to protect the "last time returned" variable. I
expect there's a better way to implement it.

J