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

[Andrey Ryabinin]

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()

> 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.

> 
>    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.
> 
>> ---