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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.03.16 at 13:41, <joao.m.martins@xxxxxxxxxx> 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@xxxxxxxxxx>
>>> ---
>>> Cc: Keir Fraser <keir@xxxxxxx>
>>> Cc: Jan Beulich <jbeulich@xxxxxxxx>
>>> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> 
>> Some style corrections, but no functional problems.
>> 
>> Reviewed-by Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> 
> 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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