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Re: [PATCH v3 0/8] blockdev: avoid acquiring AioContext lock twice at do
From: |
Sergio Lopez |
Subject: |
Re: [PATCH v3 0/8] blockdev: avoid acquiring AioContext lock twice at do_drive_backup and do_blockdev_backup |
Date: |
Tue, 19 Nov 2019 11:54:43 +0100 |
User-agent: |
mu4e 1.2.0; emacs 26.2 |
Max Reitz <address@hidden> writes:
> On 13.11.19 14:24, Sergio Lopez wrote:
>>
>> Sergio Lopez <address@hidden> writes:
>>
>>> address@hidden writes:
>>>
>>>> Patchew URL: https://patchew.org/QEMU/address@hidden/
>>>>
>>>>
>>>>
>>>> Hi,
>>>>
>>>> This series failed the docker-quick@centos7 build test. Please find the
>>>> testing commands and
>>>> their output below. If you have Docker installed, you can probably
>>>> reproduce it
>>>> locally.
>>>>
>>>> === TEST SCRIPT BEGIN ===
>>>> #!/bin/bash
>>>> make docker-image-centos7 V=1 NETWORK=1
>>>> time make docker-test-quick@centos7 SHOW_ENV=1 J=14 NETWORK=1
>>>> === TEST SCRIPT END ===
>>>>
>>>> TEST iotest-qcow2: 268
>>>> Failures: 141
>>>
>>> Hm... 141 didn't fail in my test machine. I'm going to have a look.
>>
>> So here's the output:
>>
>> --- /root/qemu/tests/qemu-iotests/141.out 2019-11-12 04:43:27.651557587
>> -0500
>> +++ /root/qemu/build/tests/qemu-iotests/141.out.bad 2019-11-13
>> 08:12:06.575967337 -0500
>> @@ -10,6 +10,8 @@
>> Formatting 'TEST_DIR/o.IMGFMT', fmt=IMGFMT size=1048576
>> backing_file=TEST_DIR/t.IMGFMT backing_fmt=IMGFMT
>> {"timestamp": {"seconds": TIMESTAMP, "microseconds": TIMESTAMP}, "event":
>> "JOB_STATUS_CHANGE", "data": {"status": "created", "id": "job0"}}
>> {"timestamp": {"seconds": TIMESTAMP, "microseconds": TIMESTAMP}, "event":
>> "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "job0"}}
>> +{"timestamp": {"seconds": TIMESTAMP, "microseconds": TIMESTAMP}, "event":
>> "JOB_STATUS_CHANGE", "data": {"status": "paused", "id": "job0"}}
>> +{"timestamp": {"seconds": TIMESTAMP, "microseconds": TIMESTAMP}, "event":
>> "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "job0"}}
>> {"error": {"class": "GenericError", "desc": "Node 'drv0' is busy: node is
>> used as backing hd of 'NODE_NAME'"}}
>> {"return": {}}
>> {"timestamp": {"seconds": TIMESTAMP, "microseconds": TIMESTAMP}, "event":
>> "JOB_STATUS_CHANGE", "data": {"status": "aborting", "id": "job0"}}
>>
>> Those extra lines, the "paused" and "running", are a result of the job
>> being done in a transaction, within a drained section.
>>
>> We can update 141.out, but now I'm wondering, was it safe creating the
>> job at do_drive_backup() outside of a drained section, as
>> qmp_drive_backup was doing?
>
> I think it is. Someone needs to drain the source node before attaching
> the job filter (which intercepts writes), and bdrv_backup_top_append()
> does precisely this.
>
> If the source node is in an I/O thread, you could argue that the drain
> starts later than when the user has invoked the backup command, and so
> some writes might slip through. That’s correct. But at the same time,
> it’s impossible to drain it the instant the command is received. So
> some writes might always slip through (and the drain will not stop them
> either, it will just let them happen).
>
> Therefore, I think it’s fine the way it is.
>
>> Do you think there may be any potential drawbacks as a result of always
>> doing it now inside a drained section?
>
> Well, one drawback is clearly visible. The job goes to paused for no
> reason.
This is something that already happens when requesting the drive-backup
through a transaction:
{"execute":"transaction","arguments":{"actions":[{"type":"drive-backup","data":{"device":"drv0","target":"o.qcow2","sync":"full","format":"qcow2"}}]}}
I don't think it makes sense to have two different behaviors for the
same action. So we either accept the additional pause+resume iteration
for qmp_drive_backup, or we remove the drained section from the
transaction based one.
What do you think?
Cheers,
Sergio.
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- [PATCH v3 2/8] blockdev: fix coding style issues in drive_backup_prepare, (continued)
- [PATCH v3 2/8] blockdev: fix coding style issues in drive_backup_prepare, Sergio Lopez, 2019/11/12
- [PATCH v3 8/8] blockdev: honor bdrv_try_set_aio_context() context requirements, Sergio Lopez, 2019/11/12
- [PATCH v3 6/8] blockdev: place blockdev_backup_prepare with the other related transaction helpers, Sergio Lopez, 2019/11/12
- [PATCH v3 7/8] blockdev: change qmp_blockdev_backup to make use of transactions, Sergio Lopez, 2019/11/12
- [PATCH v3 5/8] blockdev: merge blockdev_backup_prepare with do_blockdev_backup, Sergio Lopez, 2019/11/12
- [PATCH v3 3/8] blockdev: place drive_backup_prepare with the other related transaction functions, Sergio Lopez, 2019/11/12
- Re: [PATCH v3 0/8] blockdev: avoid acquiring AioContext lock twice at do_drive_backup and do_blockdev_backup, no-reply, 2019/11/12
- Re: [PATCH v3 0/8] blockdev: avoid acquiring AioContext lock twice at do_drive_backup and do_blockdev_backup, Sergio Lopez, 2019/11/13
- Re: [PATCH v3 0/8] blockdev: avoid acquiring AioContext lock twice at do_drive_backup and do_blockdev_backup, Sergio Lopez, 2019/11/13
- Re: [PATCH v3 0/8] blockdev: avoid acquiring AioContext lock twice at do_drive_backup and do_blockdev_backup, Max Reitz, 2019/11/19
- Re: [PATCH v3 0/8] blockdev: avoid acquiring AioContext lock twice at do_drive_backup and do_blockdev_backup,
Sergio Lopez <=
- Re: [PATCH v3 0/8] blockdev: avoid acquiring AioContext lock twice at do_drive_backup and do_blockdev_backup, Kevin Wolf, 2019/11/19
- Re: [PATCH v3 0/8] blockdev: avoid acquiring AioContext lock twice at do_drive_backup and do_blockdev_backup, Sergio Lopez, 2019/11/19
- Re: [PATCH v3 0/8] blockdev: avoid acquiring AioContext lock twice at do_drive_backup and do_blockdev_backup, Kevin Wolf, 2019/11/19
- Re: [PATCH v3 0/8] blockdev: avoid acquiring AioContext lock twice at do_drive_backup and do_blockdev_backup, Sergio Lopez, 2019/11/19