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From: "Jan Beulich" <JBeulich@suse.com>
To: Joao Martins <joao.m.martins@oracle.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
	Keir Fraser <keir@xen.org>,
	xen-devel@lists.xen.org
Subject: Re: [PATCH 2/5] x86/time: implement tsc as clocksource
Date: Tue, 22 Mar 2016 06:46:10 -0600	[thread overview]
Message-ID: <56F14CA202000078000DF28B@prv-mh.provo.novell.com> (raw)
In-Reply-To: <56F13D92.1030704@oracle.com>

>>> On 22.03.16 at 13:41, <joao.m.martins@oracle.com> wrote:

> 
> On 03/18/2016 08:21 PM, Andrew Cooper wrote:
>> On 17/03/16 16:12, Joao Martins wrote:
>>> Introduce support for using TSC as platform time which is the highest
>>> resolution time and most performant to get (~20 nsecs).  Though there
>>> are also several problems associated with its usage, and there isn't a
>>> complete (and architecturally defined) guarantee that all machines
>>> will provide reliable and monotonic TSC across all CPUs, on different
>>> sockets and different P/C states.  I believe Intel to be the only that
>>> can guarantee that. For this reason it's set with less priority when
>>> compared to HPET unless adminstrator changes "clocksource" boot option
>>> to "tsc". Upon initializing it, we also check for time warps and
>>> appropriate CPU features i.e.  TSC_RELIABLE, CONSTANT_TSC and
>>> NONSTOP_TSC. And in case none of these conditions are met, we fail in
>>> order to fallback to the next available clocksource.
>>>
>>> It is also worth noting that with clocksource=tsc there isn't any
>>> need to synchronise with another clocksource, and I could verify that
>>> great portion the time skew was eliminated and seeing much less time
>>> warps happening. With HPET I observed ~500 warps in the period
>>> of 1h of around 27 us, and with TSC down to 50 warps in the same
>>> period having each warp < 100 ns. The warps still exist though but
>>> are only related to cross CPU calibration (being how much it takes to
>>> rendezvous with master), in which a later patch in this series aims to
>>> solve.
>>>
>>> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
>>> ---
>>> Cc: Keir Fraser <keir@xen.org>
>>> Cc: Jan Beulich <jbeulich@suse.com>
>>> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
>> 
>> Some style corrections, but no functional problems.
>> 
>> Reviewed-by Andrew Cooper <andrew.cooper3@citrix.com>
>> 
> I found out one issue in the tsc clocksource initialization path with respect to
> the reliability check. This check is running when initializing platform timer,
> but not all CPUS are up at that point (init_xen_time()) which means that the
> check will always succeed. So for clocksource=tsc I need to defer initialization
> to a later point (on verify_tsc_reliability()) and if successful switch to that
> clocksource. Unless you disagree, v2 would have this and just requires one
> additional preparatory change prior to this patch.

Hmm, that's suspicious when thinking about CPU hotplug: What
do you intend to do when a CPU comes online later, failing the
check? So far we don't do runtime switching of the clock source,
and I'm not sure that's a good idea to do when there are running
guests.

Jan


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  reply	other threads:[~2016-03-22 12:46 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-17 16:12 [PATCH 0/5] x86/time: PVCLOCK_TSC_STABLE_BIT support Joao Martins
2016-03-17 16:12 ` [PATCH 1/5] public/xen.h: add flags field to vcpu_time_info Joao Martins
2016-03-18 20:12   ` Andrew Cooper
2016-03-21 11:42     ` Joao Martins
2016-03-21 11:43       ` Andrew Cooper
2016-03-21 11:51         ` Joao Martins
2016-03-21 15:10   ` Jan Beulich
2016-03-21 15:27     ` Andrew Cooper
2016-03-21 15:40       ` Joao Martins
2016-03-17 16:12 ` [PATCH 2/5] x86/time: implement tsc as clocksource Joao Martins
2016-03-18 20:21   ` Andrew Cooper
2016-03-21 11:43     ` Joao Martins
2016-03-22 12:41     ` Joao Martins
2016-03-22 12:46       ` Jan Beulich [this message]
2016-03-22 15:51         ` Joao Martins
2016-03-22 16:02           ` Jan Beulich
2016-03-22 20:40             ` Joao Martins
2016-03-23  7:28               ` Jan Beulich
2016-03-23 12:05                 ` Joao Martins
2016-03-23 14:05                   ` Jan Beulich
2016-03-17 16:12 ` [PATCH 3/5] x86/time: streamline platform time init on plt_init() Joao Martins
2016-03-18 20:32   ` Andrew Cooper
2016-03-21 11:45     ` Joao Martins
2016-03-17 16:12 ` [PATCH 4/5] x86/time: refactor read_platform_stime() Joao Martins
2016-03-18 20:34   ` Andrew Cooper
2016-03-21 11:45     ` Joao Martins
2016-03-21 13:08       ` Andrew Cooper
2016-03-17 16:12 ` [PATCH 5/5] x86/time: implement PVCLOCK_TSC_STABLE_BIT Joao Martins
2016-03-18 20:58   ` Andrew Cooper
2016-03-21 11:50     ` Joao Martins

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