Re: [Xen-users] xenstat_domain_cpu_ns() occasionally returns a huge value

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.10.2019 11:01, Jürgen Groß wrote:
> On 06.10.19 07:19, Andy Smith wrote:
>> Hi,
>>
>> I was writing a little utility to dump out domain CPU times and I
>> noticed that occasionally xenstat_domain_cpu_ns() returns an
>> erroneous huge value like 9223488034477457013.
>>
>> Attached is a small test program that just requests every domain's
>> CPU time in a tight loop; it received such a result after less than
>> 3 minutes running in dom0 of a host with only dom0 and two other PV
>> domains running:
>>
>> $ make cpu_time_test
>> cc -Wall  -lxenstat -lyajl -Wl,-rpath,/usr/lib/xen-4.12/lib 
>> -L/usr/lib/xen-4.12/lib  cpu_time_test.c   -o cpu_time_test
>> $ sudo time ./cpu_time_test
>> Got a weird CPU time 9223488108.867305 >100 years 
>> (cpu_ns=9223488108867304818)
>> Command exited with non-zero status 1
>> 84.07user 41.90system 2:40.20elapsed 78%CPU (0avgtext+0avgdata 
>> 39780maxresident)k
>> 0inputs+0outputs (0major+9541minor)pagefaults 0swaps
>>
>> The erroneous results are always somewhere above 922xxxxxxxxxxxxxxxx
>> nanoseconds (some 285 years of CPU time if it were genuine!). Then
>> the next reading will be normal. Very occasionally I've seen two in
>> a row. I see this on both 4.12 and 4.10.
>>
>> My C is very rusty so I've probably made a simple error and don't
>> want to bother xen-devel with it; can someone familiar with using
>> the xenstat interface please tell me what I've done wrong here?
> 
> I believe chances are rather high this is the bug which was corrected
> recently with Xen commit f28c4c4c10bdacb.
> 
> Andy, you can easily avoid that problem by removing the highest bit
> of the runtime value, e.g.
> 
>     correct_value = reported_runtime & ~(1ULL << 63);
> 
> Jan, I think that patch should be included in stable versions.

I have it queued already; I've merely been waiting for it to pass the
pus gate to master.

Jan

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