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Re: [libvirt] [PATCH 1/2] Timeout QEMU monitor replies after 30 seconds



On 24.06.2011 07:19, Daniel Veillard wrote:
> On Wed, Jun 22, 2011 at 11:26:27AM -0600, Eric Blake wrote:
>> On 06/22/2011 11:05 AM, Jiri Denemark wrote:
>>> On Wed, Jun 22, 2011 at 16:47:18 +0100, Daniel P. Berrange wrote:
>>>> If the QEMU process has been stopped (kill -STOP/gdb), or the
>>>> QEMU process has live-locked itself, then we will never get a
>>>> reply from the monitor. We should not wait forever in this
>>>> case, but instead timeout after a reasonable amount of time.
>>>>
>>>> NB if the host has high CPU load, or a single monitor command
>>>> intentionally takes a long time, then this will cause bogus
>>>> failures. In the case of high CPU load, arguably the guest
>>>> should have been migrated elsewhere, since you can't effectively
>>>> manage guests on a host if QEMU is taking > 30 seconds to reply
>>>> to simply commands. Since we use background migration, there
>>>> should not be any commands which take significant time to
>>>> execute any more
>>>
>>> The thing I'm most concerned about is that is far too easy to get into such
>>> situations especially since disk cache subsystem in Linux kernel is not the
>>> best thing in the world. While I agree that running guests on a loaded host is
>>> not very clever and guests should rather be migrated elsewhere, such situation
>>> doesn't have to be intentional. In other words, in case of a malfunction of
>>> some kind (some processes go crazy, network disruptions, ...) QEMU may require
>>> more than a timeout seconds to respond and we will penalize an innocent QEMU
>>> process because we won't be able to control it anymore even though the issues
>>> get fixed.
>>
>> Is there any way to measure time spent by the child process, rather than
>> just relying on wall-time elapsed?  That is, when libvirt hits 30
>> seconds of wall time in waiting for a monitor, can it then check whether
>> the child process has accumulated any execution time (likely hung) vs.
>> no execution time (likely a starved system situation), and only give up
>> in the former case?
> 
>   Well a STOP'ed child process won't accumulate any execution time,
> and you won't be able to discriminate just based on this, but I think
> we should be able to poke linux to see if the process is in D state for
> example and if we do mark the guest as non reponding then being able
> to provide an useful error information upon the associated API failure
> like
>    "Failed to contact domain: process stopped"
>    "Failed to contact domain: blocked on I/O"
>    "Failed to contact domain: process looping"
> 
> would be a really good thing. That probing and reporting can be done
> as a separate step though
> 
> Daniel
> 

To me this looks like solving the Halting problem. That means - for some
cases we might be able to tell qemu will not answer anymore, but for
others we will not.

I agree if qemu (and thus libvirt API call) does not return in ~30
seconds, users get anxious, but it would be nice if we could send
destroy to a unresponsive domains at least.

Michal


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