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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* zhanghailiang (zhang.zhanghailiang@huawei.com) wrote:
> On 2015/5/29 9:29, Wen Congyang wrote:
> >On 05/29/2015 12:24 AM, Dr. David Alan Gilbert wrote:
> >>* zhanghailiang (zhang.zhanghailiang@huawei.com) 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.
> 
> Oops, i will fix them. thank you for pointing out this low grade mistake. :)

No problem; we all make them.

> >>   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.
> 
> Have you disabled DEBUG for colo proxy? I turned it on in default, is this related?

Yes, I've turned that off, I still get the big spikes; not looked why yet.

> >>     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
> 
> Failed comprison ? Do you mean the net packets in these two sides are always inconsistent?

Yes.

> >>        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.
> >
> 
> Yes, it is a bug, the callback function colonl_close_event() is called when holding
> rcu lock:
> netlink_release
>     ->atomic_notifier_call_chain
>          ->rcu_read_lock();
>          ->notifier_call_chain
>             ->ret = nb->notifier_call(nb, val, v);
> And here it is wrong to call synchronize_rcu which will lead to sleep.
> Besides, there is another function might lead to sleep, kthread_stop which is called
> in destroy_notify_cb.
> 
> >>
> >>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.
> 
> That is a good solution, i will fix both of the above problems.

Thanks,

Dave

> 
> Thanks,
> zhanghailiang
> 
> >
> >>
> >>Thanks,
> >>
> >>Dave
> >>
> >>>
> >
> >
> >.
> >
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK