Re: [Qemu-devel] [PATCH COLO-Frame v5 00/29] COarse-grain LOck-stepping(COLO) Virtual Machines for Non-stop Service

[Dr. David Alan Gilbert]

* Wen Congyang (address@hidden) wrote:
> 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?

I've seen two ways:
   1) Shutdown the guest - when the guest exits and qemu exits, then I see this 
problem
   2) If there is a problem with the colo-proxy-script (I got the path wrong) 
so qemu
      quit.

> > [<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.

Thank you,

Dave

> > 
> > 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.
> 
> Thanks
> Wen Congyang
> 
> > 
> > Thanks,
> > 
> > Dave
> > 
> >>
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK