This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
Home Products Support Community News


Re: [Xen-devel] [PATCH] time-xen : Reset monotonic time when sync up tim

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] time-xen : Reset monotonic time when sync up time from dom0 to domU
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Wed, 13 Oct 2010 09:02:14 -0700
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Keir Fraser <keir@xxxxxxx>, Shunli Yi <syi@xxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, Saipu Liu <saliu@xxxxxxxxxxxx>, Hang Du <hdu@xxxxxxxxxxxx>
Delivery-date: Wed, 13 Oct 2010 09:03:38 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <0b6cc1b3-f838-482e-89dc-6f19cc8c47cc@default>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <9C56B6AEEB2D60488B29B880DF7E53BE03C1B2C6B6@xxxxxxxxxxxxxxxxxxxxx> <4CB4672E020000780001C752@xxxxxxxxxxxxxxxxxx> <9C56B6AEEB2D60488B29B880DF7E53BE03C1B2C8F6@xxxxxxxxxxxxxxxxxxxxx 4CB5C43C020000780001CD39@xxxxxxxxxxxxxxxxxx> <0b6cc1b3-f838-482e-89dc-6f19cc8c47cc@default>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100921 Fedora/3.1.4-1.fc13 Lightning/1.0b3pre Thunderbird/3.1.4
 On 10/13/2010 08:56 AM, Dan Magenheimer wrote:
> (bringing the topic up to a more theoretical level and including
> Keir and Jeremy)
> I wonder if the "bug" here is that "dependent wall clock" is
> an incredibly poor replacement for NTP (or a Windows Time Server)
> and a hack and really shouldn't even exist?  I suppose one could
> argue that it makes some sense in a XCP environment, and maybe
> in a server environment where a single physical server is extremely
> isolated, but does it ever make sense in a real world server
> environment?
> In other words, I'm arguing that the correct "fix" here is:
> Don't use dependent wallclock.

Yes, that's always been my view.  pvops kernels don't implement it.


>> -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxxxx]
>> Sent: Wednesday, October 13, 2010 6:38 AM
>> To: Hang Du
>> Cc: Saipu Liu; Shunli Yi; xen-devel@xxxxxxxxxxxxxxxxxxx
>> Subject: RE: [Xen-devel] [PATCH] time-xen : Reset monotonic time when
>> sync up time from dom0 to domU
>>>>> On 13.10.10 at 05:24, "Du, Hang" <hdu@xxxxxxxxxxxx> wrote:
>>> Sorry for my brief description in previous mail and missing
>>> is_initial_xendomain check. The kernel I submit this patch is
>>> linux-2.6.18-xen-3.4.2, I submit the patch again with in_initial-
>> xendomain
>>> check.
>> Actually, I didn't expect you to blindly insert that check, but rather
>> either explain why only DomU-s need the changed behavior (as your
>> original description suggested), or refine the description of the
>> problem you're trying to solve.
>>> In this patch, we support the backward time changing sync to all
>> domUs which
>>> configured to use "dependent wall clock".
>>> Currently, without the backward time syncing, when we change the time
>>> backward in Dom0, the time in DomU would be froze.
>>> And this cause some commands hang and don't executed until the time
>> catch up
>>> with the domU time.
>>> For example:
>>> "rpm -q kernel-xen"
>>> "sleep 1"
>>> Monotonic time should be reset when sync up time from dom0 to domU to
>>> support domU backward time syncing.
>> I can see your point, however ...
>>> diff -urN a/arch/i386/kernel/time-xen.c   b/arch/i386/kernel/time-
>> xen.c
>>> --- a/arch/i386/kernel/time-xen.c   2010-10-11 10:41:06.000000000
>> +0800
>>> +++ b/arch/i386/kernel/time-xen.c   2010-10-11 10:43:32.000000000
>> +0800
>>> @@ -715,6 +715,8 @@
>>>     }
>>>     if (shadow_tv_version != HYPERVISOR_shared_info->wc_version) {
>>> +        if (!is_initial_xendomain() && !independent_wallclock)
>>> +            monotonic_reset();
>>>         update_wallclock();
>> ... you definitely need to call monotonic_reset() *after* the
>> update_wallclock() (or else another vCPU, preempted in
>> do_gettimeofday() between the end of the xtime read loop
>> and the acquiring of the monotonic lock, would be able to
>> overwrite monotonic.tv with values obtained before the wall
>> clock update - similar to the secondary problem addressed by
>> c/s 1021).
>> Further, blindly running a reset here doesn't seem nice in
>> the (general) case of the time getting updated forwards.
>>>         schedule_clock_was_set_work = 1;
>>>     }
>> You'll also want to address Dan's concerns, and you will want to
>> get your patch formatted so that it would actually apply (read:
>> no tab -> space conversion) if you expect it to be committed
>> by Keir at some point.
>> Jan
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxxxxxxxx
>> http://lists.xensource.com/xen-devel

Xen-devel mailing list

<Prev in Thread] Current Thread [Next in Thread>