linux-kernel - RE: Time keeps on slipping... on Hyper-V

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Message-ID: <CD2CABCB2C0A0D4682C5F8AD84014154097C10@HKXPRD3002MB006.064d.mgd.msft.net>
Date:	Fri, 26 Sep 2014 09:46:27 +0000
From:	Thomas Shao <huishao@...rosoft.com>
To:	Mike Surcouf <mps.surcouf.lkml@...il.com>
CC:	Sitsofe Wheeler <sitsofe@...il.com>, Olaf Hering <olaf@...fle.de>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"driverdev-devel@...uxdriverproject.org" 
	<driverdev-devel@...uxdriverproject.org>,
	"apw@...onical.com" <apw@...onical.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	"KY Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>
Subject: RE: Time keeps on slipping... on Hyper-V



> -----Original Message-----
> From: Mike Surcouf [mailto:mps.surcouf.lkml@...il.com]
> Sent: Friday, September 26, 2014 5:24 PM
> To: Thomas Shao
> Cc: Sitsofe Wheeler; Olaf Hering; gregkh@...uxfoundation.org; linux-
> kernel@...r.kernel.org; driverdev-devel@...uxdriverproject.org;
> apw@...onical.com; jasowang@...hat.com; KY Srinivasan; Haiyang Zhang
> Subject: Re: Time keeps on slipping... on Hyper-V
> 
> > We still recommend user to configure NTP in the guest VM. With the new
> > time sync feature in this patch, you could have one more option to enable
> the guest-host sync, if the NTP didn't work in the environment.
> > For example the guest VM didn't have network connection.
> 
> Microsoft used to use host time-samples in their older drivers but this was
> dropped (when I don't know).
> Now we must use NTP to correct hyperv_clocksource which suffers from the
> usual problems associated with virtual environment  cpu loading.
> 
> Host time-samples in conjunction with an effective clock stability algorithm
> with slews rather than steps should be the default.
> NTP is a workaround and should not be the primary solution.
> 
> I have seen a lots of posts for RHEL CENTOS  Linux that have a
> FAST.hyper_clocksource confirmed by myself on CENTOS 6.5 and 7.0 (also
> confirmed by Olaf on SLES).
> 
> If this patch steps the clock once in a while without any form of slewing then
> it has the potential to break things as files and logs travel BACK in time.
> 
> Apologies Thomas could you send me the patch via email as lkml.org does
> not have it in the archives.
> 
Sure. I'll send the package again. It's strange that this patch didn't in the archives. And in my patch, it will slew the time, if the time drift is less than 1 s. It only steps the clock if the drift is larger than 1 s.

> Regards Mike