[Devel] [PATCH rh7 1/2] vdso: virtualized monotonic gettime through vdso

[Dmitry Safonov]

On 05/24/2017 07:34 PM, Andrey Ryabinin wrote:
> 
> 
> On 05/24/2017 07:29 PM, Dmitry Safonov wrote:
>> On 05/24/2017 06:21 PM, Andrey Ryabinin wrote:
>>> We already have infrastructure for virtualized vdso, however we use
>>> it only to change LINUX_VERSION_NAME in container. Simply store container's
>>> start time - ve->start_timespec in vdso variable - VDSO64_ve_start_timespec,
>>> and use it in __vdso_clock_gettime() to calculate container's monotonic time.
>>>
>>> Make uts_arch_setup_additional_pages()/uts_prep_vdso_pages_locked() to always
>>> setup new vdso, since previous policy to setup vdso only if uts_ns->name.release
>>> wouldn't work for virtualized __vdso_clock_gettime()
>>>
>>> https://jira.sw.ru/browse/PSBM-66451
>>> Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
>>
>> Well, that's all looks good, but I've two questions:
>>
>> 1. Where VDSO64_ve_start_timespec is set? I do not see that part.
>>
> 
> in uts_arch_setup_additional_pages() and uts_prep_vdso_pages_locked()

Uhm, yes, but that means that ve_ts_write() is broken for
running CT.

> 
>> 2. The part with unconditional creation of vdso pages array in
>>     uts_arch_setup_additional_pages()/uts_prep_vdso_pages_locked()
>>     will result in each exec() creating new vdso for all tasks in CT.
>>     So, that will result in n*8kB additional memory, where n is
>>     number of exec()'ed tasks in CTs.
>>     I'm not sure how much can be n, so that may be OK.
>>     But can we find a way to make it p*8kB, where p - is nr. of CTs?
> 
> Nope, vdso pages created only once, and attached to uts_ns. Once pages created
> all follow up execs will reuse them. So it's one vdso per uts_ns.

Oh, good - I've just misread that code.
So, that should be fine.

> 
>>
>>     As far as I can see, vdso pages array was also copied for UTS
>>     virtualization where it was needed previously, so (2) question may be
>>     not related to this patches set, but for the way how it is done
>>     already. FWIW, what I mean here - it would be worth to have one copy
>>     of vdso pages per-CT.
>>
>>     May be I'm trying to rearrange the deck chairs on the Titanic and
>>     that increase is just insignificant.
>>
>>> ---


-- 
              Dmitry