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

Andi Kleen
2007-06-06T10:26:41Z

On Wednesday 06 June 2007 12:05:22 Jeremy Fitzhardinge wrote:
> 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).

This is being worked on.

> > 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.

But any per CPU setup likely needs this to avoid non monotonicity

-Andi