[Xen-devel] Re: Very large value from get_nsec_offset() in timer_interrupt

[Ryan Harper <ryanh@xxxxxxxxxx>]

* Ryan Harper <ryanh@xxxxxxxxxx> [2006-04-20 17:47]:
> 
> ns_offset is calculated from get_nsec_offset().  I'm going to dig
> a little further, but I wanted to get what I'm seeing out there.

static u64 get_nsec_offset(struct shadow_time_info *shadow)
{
   u64 now, delta;
   rdtscll(now);
   delta = now - shadow->tsc_timestamp;
   return scale_delta(delta, shadow->tsc_to_nsec_mul, shadow->tsc_shift);
}

For some currently unknown reason, now is < shadow->tsc_timestamp;  This
results in a very large delta value, which is scaled up even larger.

The output from the multi-cpu clock info dump doesn't show anything
bogus:

(XEN) Min = 68672793737292 ; Max = 68672793747633 ; Diff = 10341 (10 
microseconds)
(XEN) Min = 68899728719792 ; Max = 68899728727283 ; Diff = 7491 (7 microseconds)

Any thoughts on why the shadow copy of the tsc_timestamp would be > than
the value returned from reading the tsc would be helpful.

Very likely this is related to the hardware, this is a 4-node x460
32-way.  I've not reproduced this on anything but multi-node setups.  

-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
(512) 838-9253   T/L: 678-9253
ryanh@xxxxxxxxxx

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