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Re: [Qemu-devel] [PATCH COLO-Frame v5 00/29] COarse-grain LOck-stepping(

From: Wen Congyang
Subject: Re: [Qemu-devel] [PATCH COLO-Frame v5 00/29] COarse-grain LOck-stepping(COLO) Virtual Machines for Non-stop Service
Date: Fri, 29 May 2015 09:29:40 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 05/29/2015 12:24 AM, Dr. David Alan Gilbert wrote:
> * zhanghailiang (address@hidden) wrote:
>> This is the 5th version of COLO, here is only COLO frame part, include: VM 
>> checkpoint,
>> failover, proxy API, block replication API, not include block replication.
>> The block part has been sent by wencongyang:
>> "[Qemu-devel] [PATCH COLO-Block v5 00/15] Block replication for continuous 
>> checkpoints"
>> we have finished some new features and optimization on COLO (As a 
>> development branch in github),
>> but for easy of review, it is better to keep it simple now, so we will not 
>> add too much new 
>> codes into this frame patch set before it been totally reviewed. 
>> You can get the latest integrated qemu colo patches from github (Include 
>> Block part):
>> https://github.com/coloft/qemu/commits/colo-v1.2-basic
>> https://github.com/coloft/qemu/commits/colo-v1.2-developing (more features)
>> Please NOTE the difference between these two branch.
>> colo-v1.2-basic is exactly same with this patch series, which has basic 
>> features of COLO.
>> Compared with colo-v1.2-basic, colo-v1.2-developing has some optimization in 
>> the 
>> process of checkpoint, including: 
>>    1) separate ram and device save/load process to reduce size of extra 
>> memory
>>       used during checkpoint
>>    2) live migrate part of dirty pages to slave during sleep time.
>> Besides, we add some statistic info in colo-v1.2-developing, which you can 
>> get these stat
>> info by using command 'info migrate'.
> Hi,
>   I have that running now.
> Some notes:
>   1) The colo-proxy is working OK until qemu quits, and then it gets an RCU 
> problem; see below
>   2) I've attached some minor tweaks that were needed to build with the 4.1rc 
> kernel I'm using;
>      they're very minor changes and I don't think related to (1).
>   3) I've also included some minor fixups I needed to get the -developing 
> world
>      to build;  my compiler is fussy about unused variables etc - but I think 
> the code
>      in ram_save_complete in your -developing patch is wrong because there 
> are two
>      'pages' variables and the one in the inner loop is the only one changed.
>   4) I've started trying simple benchmarks and tests now:
>     a) With a simple web server most requests have very little overhead, the 
> comparison
>        matches most of the time;  I do get quite large spikes (0.04s->1.05s) 
> which I guess
>        corresponds to when a checkpoint happens, but I'm not sure why the 
> spike is so big,
>        since the downtime isn't that big.
>     b) I tried something with more dynamic pages - the front page of a simple 
> bugzilla
>        install;  it failed the comparison every time; it took me a while to 
> figure out
>        why, but it generates a unique token in it's javascript each time (for 
> a password reset
>        link), and I guess the randomness used by that doesn't match on the 
> two hosts.
>        It surprised me, because I didn't expect this page to have much 
> randomness
>        in.
>   4a is really nice - it shows the benefit of COLO over the simple 
> checkpointing;
> checkpoints happen very rarely.
> The colo-proxy rcu problem I hit shows as rcu-stalls in both primary and 
> secondary
> after the qemu quits; the backtrace of the qemu stack is:

How to reproduce it? Use monitor command quit to quit qemu? Or kill the qemu?

> [<ffffffff810d8c0c>] wait_rcu_gp+0x5c/0x80
> [<ffffffff810ddb05>] synchronize_rcu+0x45/0xd0
> [<ffffffffa0a251e5>] colo_node_release+0x35/0x50 [nfnetlink_colo]
> [<ffffffffa0a25795>] colonl_close_event+0xe5/0x160 [nfnetlink_colo]
> [<ffffffff81090c96>] notifier_call_chain+0x66/0x90
> [<ffffffff8109154c>] atomic_notifier_call_chain+0x6c/0x110
> [<ffffffff815eee07>] netlink_release+0x5b7/0x7f0
> [<ffffffff815878bf>] sock_release+0x1f/0x90
> [<ffffffff81587942>] sock_close+0x12/0x20
> [<ffffffff812193c3>] __fput+0xd3/0x210
> [<ffffffff8121954e>] ____fput+0xe/0x10
> [<ffffffff8108d9f7>] task_work_run+0xb7/0xf0
> [<ffffffff81002d4d>] do_notify_resume+0x8d/0xa0
> [<ffffffff81722b66>] int_signal+0x12/0x17
> [<ffffffffffffffff>] 0xffffffffffffffff

Thanks for your test. The backtrace is very useful, and we will fix it soon.

> that's with both the 423a8e268acbe3e644a16c15bc79603cfe9eb084 from yesterday 
> and 
> older e58e5152b74945871b00a88164901c0d46e6365e tags on colo-proxy.
> I'm not sure of the right fix; perhaps it might be possible to replace the 
> synchronize_rcu in colo_node_release by a call_rcu that does the kfree later?

I agree with it.

Wen Congyang

> Thanks,
> Dave

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