Re: [Xen-devel] [PATCH] x86/time: use correct (local) time stamp in constant-TSC calibration fast path

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.06.16 at 20:19, <joao.m.martins@xxxxxxxxxx> wrote:
> [changing Dario address to citrix.com as it was bouncing for me ]
> 
> On 06/09/2016 04:52 PM, Jan Beulich wrote:
>>>>> On 09.06.16 at 17:00, <joao.m.martins@xxxxxxxxxx> wrote:
>>> On 06/09/2016 01:57 PM, Jan Beulich wrote:
>>>>>>> On 09.06.16 at 14:11, <joao.m.martins@xxxxxxxxxx> wrote:
>>>> So in effect for the fast path the patch
>>>> changes the situation from c->stime_local_stamp being effectively
>>>> unused to c->stime_master_stamp being so. In the former case, if
>>>> that really hadn't been a typo, deleting the write of that field from
>>>> time_calibration_std_rendezvous() would have made sense, as
>>>> get_s_time() certainly is more overhead than the simply memory
>>>> read and write needed for keeping c->stime_master_stamp up to
>>>> date (despite not being used).
>>> I agree, but what I meant previously was more of a concern meaning: CPU 0 
>>> is 
> 
>>> doing an
>>> expensive read_platform_time (e.g. HPET supposedly microseconds order, plus 
>>> a
>>> non-contented lock) to set stime_master_stamp that doesn't get used at all -
>>> effectively not using the clocksource set initially at boot.
>> 
>> Yeah, there's likely some cleanup potential here, but of course we
>> need to be pretty careful about not doing something that may be
>> needed by some code paths but not others. But if you think you
>> can help the situation without harming maintainability, feel free to
>> go ahead.
>> 
> OK, Makes sense. I'll likely do already so of it on my related series.

Actually, since local time gets seeded from platform time in
init_percpu_time(), I don't think we can do away with
maintaining platform time.

Jan


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