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From: Joao Martins <joao.m.martins@oracle.com>
To: Jan Beulich <JBeulich@suse.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: Wed, 23 Mar 2016 12:05:12 +0000	[thread overview]
Message-ID: <56F28678.5080004@oracle.com> (raw)
In-Reply-To: <56F2539D02000078000DF78F@prv-mh.provo.novell.com>



On 03/23/2016 07:28 AM, Jan Beulich wrote:
>>>> On 22.03.16 at 21:40, <joao.m.martins@oracle.com> wrote:
>> On 03/22/2016 04:02 PM, Jan Beulich wrote:
>>>>>> On 22.03.16 at 16:51, <joao.m.martins@oracle.com> wrote:
>>>> On 03/22/2016 12:46 PM, Jan Beulich wrote:
>>>>>>>> On 22.03.16 at 13:41, <joao.m.martins@oracle.com> wrote:
>>>>>>>
>>>>>> 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?
>>>> Good point, but I am not sure whether that would happen. The initcall
>>>> verify_tsc_reliability seems to be called only for the boot processor. Or are
>>>> you saying that it's case that initcalls are called too when hotplugging cpus
>>>> later on? If that's the case then my suggestion wouldn't work as you point out -
>>>> or rather without having runtime switching support as you point out below.
>>>
>>> Looks like I didn't express myself clearly enough: "failing the check"
>>> wasn't meant to imply the failure would actually occur, but rather
>>> that failure would occur if the check was run on that CPU. IOW the
>>> case of a CPU getting hotplugged which doesn't satisfy the needs
>>> for selecting TSC as the clock source.
>> Ah, I see. I believe this wouldn't be an issue with the current rendezvous
>> mechanism (std_rendezvous), as the delta is computed between local_tsc_stamp
>> taken in that (hotplugged) CPU in the calibration and the rdtsc() in the 
>> guest
>> same CPU, even though having CPU0 TSC (master) as system_time.
>>
>> However it can be a problem with PVCLOCK_TSC_STABLE_BIT as the hotplugged CPU
>> could have its TSC drifted, and since setting this flag relies on
>> synchronization of TSCs we would need to clear the flag enterily.
> 
> Except that we can't, after guests already got started, validly clear
> that flag afaics.
Correct.

> The only option I see here would be to never set
> this flag if CPU hotplug is possible - by looking at the hot pluggable
> CPU count and, if non-zero, perhaps allowing a command line
> override to indicate no hotplug is intended (it may well be that such
> is already implicitly available).
OK, will add this then to allow the flag only if the conditions above are met.
Thanks for the pointer!

>>>>> 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.
>>>> Totally agree, but I would be proposing to be at initialization phase where
>>>> there wouldn't be guests running. We would start with HPET, then only switch 
>>>> to
>>>> TSC if that check has passed (and would happen once).
>>>>
>>>> Simpler alternative would be to call init_xen_time after all CPUs are 
>>>> brought up
>>>> (and would also keep this patch as is), but I am not sure about the
>>>> repercussions of that.
>>>
>>> I don't see how either would help with the hotplug case.
>> This was in response to what I thought you meant with your earlier question
>> (which I misunderstood). But my question is still valid I believe. The 
>> reason
>> for choosing between my suggested approaches is that tsc_check_reliability()
>> requires all CPUs up so that the check is correctly performed.
> 
> Sure - it seems quite obvious that all boot time available CPUs
> should be checked.
Cool, so I will go with moving init_xen_time right after all CPUs are up but
before initcalls are invoked.

Joao

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  reply	other threads:[~2016-03-23 12:05 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
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 [this message]
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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