Re: [Qemu-devel] Fedora FC21 - Bug: 100% CPU and hangs in gettimeofday(&tp, NULL); forever

[Laine Stump <laine@redhat.com>]

On 01/13/2015 05:01 PM, Gerhard Wiesinger wrote:
> On 13.01.2015 22:16, Paolo Bonzini wrote:
>>
>> On 13/01/2015 22:14, Gerhard Wiesinger wrote:
>>> I also had a look at the kernel code again:
>>> http://lxr.free-electrons.com/source/kernel/time/timekeeping.c?v=3.17#L493
>>>
>>> 499         do {
>>> 500                 seq = read_seqcount_begin(&tk_core.seq);
>>> 501
>>> 502                 ts->tv_sec = tk->xtime_sec;
>>> 503                 nsecs = timekeeping_get_ns(&tk->tkr);
>>> 504
>>> 505         } while (read_seqcount_retry(&tk_core.seq, seq));
>>>
>>> So it looks like that the seqcount always changes and therefore loops
>>> forever here (as far as I digged it down this is the only loop here).
>>>
>>> Might be something wrong with the memory barriers in recent qemu-kvm
>>> releases?
>> No, that's not possible.  Unless you pause/resume or migrate the VM, all
>> of the handling of kvmclock is entirely in the kernel.
>
> Any other possible explaination of the problem?
>
> Had a look at the diff (I guess the right file at least in qemu tree):
> # no critical changes IHMO here
> git diff -u v1.6.2..v2.1.2 ./hw/i386/kvm/clock.c
>
> Trying to reproduce with a loop:
> #include <sys/time.h>
> #include <stdio.h>
>
> int main(int argc, char* argv[])
> {
>   struct timeval tv;
>   int i = 0;
>   for (;;)
>   {
>     gettimeofday(&tv, 0);
>     ++i;
>     if (i >= 10000000)
>     {
>       i = 0;
>       printf("%i\n", (int)tv.tv_sec);
>     }
>   }
>   return 0;
> }
>
> As I wrote this: "First tests seem to run well, so no quick win ....",
> I could reproduce it with a stall in 318s :-)
> (gdb) bt
> #0  0x00007fff6d9fefff in gettimeofday ()
> #1  0x00000000004005ad in main (argc=1, argv=0x7fff6d9b28b8) at
> gettimeofdayloop.c:10
>
> So we have at least a testcase which is quickly to reproduce.
>
> So we are digging down my second findings about a major bug in
> qemu-kvm  :-)
>
> Can someone try, too?
>
> Ciao,
> Gerhard
>
>

Take a look at the following kernel bug. It specifically deals with a
hang in gettimeofday() in a KVM guest:

https://bugzilla.redhat.com/show_bug.cgi?id=1178975

There is a link to a patched kernel you can try; it fixed my problems (I
was repeatedly getting hangs in python-urlgrabber during yum updates on
F21).