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Re: [libvirt] [PATCH 1/8] qemu: bulk stats: extend internal collection API



On 09/10/14 15:55, Francesco Romani wrote:
> ----- Original Message -----
>> From: "Peter Krempa" <pkrempa redhat com>
>> To: "Francesco Romani" <fromani redhat com>, libvir-list redhat com
>> Sent: Wednesday, September 10, 2014 10:07:33 AM
>> Subject: Re: [libvirt] [PATCH 1/8] qemu: bulk stats: extend internal collection API
> 
> [...]
>>>> Hmm this skips offline domains entirely if one of the stats groups needs
>>>> the monitor.
>>>>
>>>> I think we should rather skip individual stats groups, or better stats
>>>> fields that we can't provide.
>>>>
>>>> Any ideas?
>>>
>>> What about this (pseudo-C):
>>>
>>>     unsigned int privflags = 0;
>>>
>>>     if (needmon && qemuDomainObjBeginJob(driver, dom, QEMU_JOB_QUERY)
>>>         needmon = false;
>>>         /* auto disable monitoring, the remainder of the function should be
>>>         unchanged */
>>>     else
>>>         privflags |= MONITOR_AVAILABLE;
>>>  
>>>     if ((needmon && virDomainObjIsActive(dom)) || !needmon) {

Drop this condition ...

>>>         if (qemuDomainGetStats(conn, dom, stats, &tmp, privflags) < 0)

The individual stats groups will decide if they need the VM alive they
can skip it individually rather than omitting the vm completely.

>>>             /* pass monitor availability down the chain. Individual workers
>>>             will
>>>                bail out immediately and silently if they need monitor but
>>>                it is
>>>                not available
>>>              */
>>>             goto endjob;
>>>
>>>         if (tmp)
>>>             tmpstats[nstats++] = tmp;
>>>     }
>>>
>>>
>>> No other change should be needed to this patch, and with trivial changes
>>> all the others can be fixed.
>>
>> Also you can just grab the lock always and the workers will exit if the
>> VM is not alive. Having a domain job should be fine.
> 
> As you prefer. For oVirt, we do care of all the stats group implemented, and always all of them,
> (actually I may have missed some bits, e.g. the pininfo as you pointed out elsewhere - going to fix),
> so for our needs we'll always need to enter the monitor.
> 
> But other users of this API may beg to differ, and I believe is fair to assume that other consumers
> of this API could just use stats which doesn't require to enter the monitor: e.g. CPU/VCPU/interface.

In case you don't have any stat group that requires the monitor it is
imho fine not to get a job. If there is a group where we can enter it we
can just grab the monitor always ... see above.

> 
> Hence, I was trying to be a good citizen and do not require monitor access unless it is actually
> needed; but if turns out it is OK to do a domain job anyway, I'll happily simplify my code :)
> 
> Thanks and bests,
> 

Peter

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