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

Jan Beulich wrote:

quoted

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.

quoted

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