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Re: [libvirt] [PATCH] qemu: lock: unlock vm during qemuBuildCommandLine



On 23.04.2015 11:32, Peter Krempa wrote:
> On Thu, Apr 23, 2015 at 11:19:34 +0200, Michal Privoznik wrote:
>> On 23.04.2015 10:30, Peter Krempa wrote:
>>> On Thu, Apr 23, 2015 at 11:44:49 +0800, zhang bo wrote:
>>>> The function qemuBuildCommandLine() may take a long time, for example
>>>> if we configure tens of vifs for the guest, each may cost hundrands of
>>>> milliseconds to create tap dev, senconds in total. Thus, unlock vm
>>>> before calling it.
>>>>
>>>> Signed-off-by: Zhang Bo <oscar zhangbo huawei com>
>>>> Signed-off-by: Zhou Yimin <zhouyimin huawei com>
>>>> ---
>>>>  src/qemu/qemu_process.c | 6 +++++-
>>>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
>>>> index 753afe8..d1aaaec 100644
>>>> --- a/src/qemu/qemu_process.c
>>>> +++ b/src/qemu/qemu_process.c
>>>> @@ -4628,14 +4628,18 @@ int qemuProcessStart(virConnectPtr conn,
>>>>      }
>>>>
>>>>      VIR_DEBUG("Building emulator command line");
>>>> +    virObjectUnlock(vm);
>>>>      if (!(cmd = qemuBuildCommandLine(conn, driver, vm->def, priv->monConfig,
>>>>                                       priv->monJSON, priv->qemuCaps,
>>>>                                       migrateFrom, stdin_fd, snapshot, vmop,
>>>>                                       &buildCommandLineCallbacks, false,
>>>>                                       qemuCheckFips(),
>>>>                                       priv->autoNodeset,
>>>> -                                     &nnicindexes, &nicindexes)))
>>>> +                                     &nnicindexes, &nicindexes))) {
>>>> +        virObjectLock(vm);
>>>>          goto cleanup;
>>>> +    }
>>>> +    virObjectLock(vm);
>>>
>>> Why do you need to unlock the object? The VM is starting at this point
>>> so you won't be able to save any time since APIs will either be blocked
>>> by a job or by the fact that the VM was not started.
>>
>> Not true. We still allow QUERY jobs, or APIs that lock a domain object
>> but don't necessarily take a job. For instance, if in one thread I'm
>> doing virDomainGetOSType(), virDomainGetInfo(), ... (essentially 'virsh
>> dominfo') while the other just starts the domain. I don't see a reason
>> why virDomainGetOSType() should wait for the domain to be started up.
>> Domain state should have no affect on the guest OS type, should it?
> 
> OK that is technically right. I wanted to point out that most of the
> stats are available only for online machines or it shouldn't be much of
> a problem to dealy the delivery.
> 
> Your example certainly doesn't illustrate why the delay to format the
> command line should be a problem when using libvirt. Or are you polling
> virDomainGetOSType every millisecond?
> 
> I am curious to see why the delay would be a problem.

Yep, I'm too. So far we don't really care much about libvirt response
time (otherwise our critical sections would be much shorter), but maybe
it's an issue for somebody.

> 
>>
>> On the other hand, I don't think we can just lock and unlock the domain
>> object as we please. qemuBuildCommandLine is a very complex function and
>> as such it calls many others. Some of them may rely on the fact, that
>> the object is locked by caller.
> 
> Well, you definitely can't since almost everything in there is accessing
> vm->def. Locking semantics would be broken right in the line after @vm
> was unlocked by dereferencing it.

Well, anything that would change a domain definition should grap a
MODIFY job. But such jobs are serialized, so even if we unlock the
domain we should be okay to still access vm->def. What I am more worried
about are all the small functions that interact with system or
something. Currently they are serialized by @vm lock, but one we remove
it ...

Michal


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