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Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] vl: Drain before (block) job c
From: |
Max Reitz |
Subject: |
Re: [Qemu-block] [Qemu-devel] [PATCH 1/2] vl: Drain before (block) job cancel when quitting |
Date: |
Wed, 19 Jun 2019 21:40:07 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 |
On 14.06.19 11:22, Vladimir Sementsov-Ogievskiy wrote:
> 13.06.2019 19:03, Max Reitz wrote:
>> [re-adding the original CCs, why not]
>>
>> On 13.06.19 16:30, Vladimir Sementsov-Ogievskiy wrote:
>>> 13.06.2019 17:21, Max Reitz wrote:
>>>> On 13.06.19 16:19, Vladimir Sementsov-Ogievskiy wrote:
>>>>> 13.06.2019 1:08, Max Reitz wrote:
>>>>>> If the main loop cancels all block jobs while the block layer is not
>>>>>> drained, this cancelling may not happen instantaneously. We can start a
>>>>>> drained section before vm_shutdown(), which entails another
>>>>>> bdrv_drain_all(); this nested bdrv_drain_all() will thus be a no-op,
>>>>>> basically.
>>>>>>
>>>>>> We do not have to end the drained section, because we actually do not
>>>>>> want any requests to happen from this point on.
>>>>>>
>>>>>> Signed-off-by: Max Reitz <address@hidden>
>>>>>> ---
>>>>>> I don't know whether it actually makes sense to never end this drained
>>>>>> section. It makes sense to me. Please correct me if I'm wrong.
>>>>>> ---
>>>>>> vl.c | 11 +++++++++++
>>>>>> 1 file changed, 11 insertions(+)
>>>>>>
>>>>>> diff --git a/vl.c b/vl.c
>>>>>> index cd1fbc4cdc..3f8b3f74f5 100644
>>>>>> --- a/vl.c
>>>>>> +++ b/vl.c
>>>>>> @@ -4538,6 +4538,17 @@ int main(int argc, char **argv, char **envp)
>>>>>> */
>>>>>> migration_shutdown();
>>>>>>
>>>>>> + /*
>>>>>> + * We must cancel all block jobs while the block layer is drained,
>>>>>> + * or cancelling will be affected by throttling and thus may block
>>>>>> + * for an extended period of time.
>>>>>> + * vm_shutdown() will bdrv_drain_all(), so we may as well include
>>>>>> + * it in the drained section.
>>>>>> + * We do not need to end this section, because we do not want any
>>>>>> + * requests happening from here on anyway.
>>>>>> + */
>>>>>> + bdrv_drain_all_begin();
>>>>>> +
>>>>>> /* No more vcpu or device emulation activity beyond this point */
>>>>>> vm_shutdown();
>>>>>>
>>>>>>
>>>>>
>>>>> So, actually, the problem is that we may wait for job requests twice:
>>>>> on drain and then on cancel.
>>>>
>>>> We don’t wait on drain. When the throttle node is drained, it will
>>>> ignore throttling (as noted in the cover letter).
>>>>
>>>> We do wait when cancelling a job while the throttle node isn’t drained,
>>>> though. That’s the problem.
>>>
>>> Ah, understand now.
>>>
>>> Is it safe to drain_begin before stopping cpus? We may finish up then with
>>> some queued
>>> somewhere IO requests..
>>
>> Hm... Aren’t guest devices prohibited from issuing requests to the
>> block layer while their respective block device is drained?
>
> It's at least a buggy place, I remember Denis Plotnikov sent patch to fix it
> and had a huge
> discussion with Kevin.
> And here it is:
> https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg00732.html
Ah, I even have that in my inbox... The latest reply I see came in April:
https://lists.nongnu.org/archive/html/qemu-block/2019-04/msg00243.html
Where Kevin asked for an RFC patch in the current state.
I’m not sure whether I should work around a potential bug here, if we
can agree that it is a bug, and if it isn’t clear whether this place
would actually be affected.
Max
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[Qemu-block] [PATCH 2/2] iotests: Test quitting with job on throttled node, Max Reitz, 2019/06/12
Re: [Qemu-block] [PATCH 0/2] vl: Drain before (block) job cancel when quitting, Max Reitz, 2019/06/12