Re: [PATCH 2/5] x86/time: implement tsc as clocksource

["Jan Beulich" <JBeulich@suse.com>]

>>> 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:
>> 
>>>
>>> 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?
> 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.

>> 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.

Jan

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