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

[zhanghailiang]

On 2015/5/29 9:29, Wen Congyang 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.

Oops, i will fix them. thank you for pointing out this low grade mistake. :)

> >    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?

> >      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?

> >         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,
zhanghailiang

> > Thanks,
> >
> > Dave
> >
> > >
>
>
> .