Re: [Y2038][time namespaces] Question regarding CLOCK_REALTIME support plans in Linux time namespaces

[Carlos O'Donell <carlos@redhat.com>]

On 11/6/20 7:47 PM, Thomas Gleixner wrote:
> On Thu, Nov 05 2020 at 12:25, Carlos O'Donell wrote:
>> On 10/30/20 9:38 PM, Thomas Gleixner wrote:
>> If kata grows up quickly perhaps this entire problem becomes solved, but until
>> then I continue to have a testing need for a distinct CLOCK_REALTIME in a
>> time namespace (and it need not be unconditional, if I have to engage magic
>> then I'm happy to do that).
> 
> Conditional, that might be a way to go.
> 
> Would CONFIG_DEBUG_DISTORTED_CLOCK_REALTIME be a way to go? IOW,
> something which is clearly in the debug section of the kernel which wont
> get turned on by distros (*cough*) and comes with a description that any
> bug reports against it vs. time correctness are going to be ignored.

Yes. I would be requiring CONFIG_DEBUG_DISTORTED_CLOCK_REALTIME.

Let me be clear though, the distros have *+debug kernels for which this
CONFIG_DEBUG_* could get turned on? In Fedora *+debug kernels we enable all
sorts of things like CONFIG_DEBUG_OBJECTS_* and CONFIG_DEBUG_SPINLOCK etc.
etc. etc.

I would push Fedora/RHEL to ship this in the *+debug kernels. That way I can have
this on for local test/build cycle. Would you be OK with that?

We could have it disabled by default but enabled via proc like
unprivileged_userns_clone was at one point? I want to avoid accidental use in
Fedora *+debug kernels unless the developer is actively going to run tests that
require time manipulation e.g. thousands of DNSSEC tests with timeouts [1].

I also need a way to determine the feature is enabled or disabled so I can XFAIL
the tests and tell the developer they need to turn on the feature in the host
kernel (and not to complain when CLOCK_REALTIME is wrong). A proc interface solves
this in a straight forward way.

I could then also tell my hardware partners to turn it on during certain test/build
cycles. It violates "ship what you test" but increases test coverage and can be
run as a distinct test cycle. I could also have our internal builders turn this
feature on so we can run rpm %check phases with this feature enabled (operations
might refuse, but in that case my day-to-day developer testing still helps by
orders of magnitude).

Notes:
[1] Petr Špaček commented on DNSSEC and expiration testing as another real-world testing
scenario: https://sourceware.org/pipermail/libc-alpha/2020-November/119785.html
Still a testing scenario, but an example outside of glibc for networking, where they
have a need to execute thousands of tests with accelerated timeout. If vm+containers
catches up, and I think they will, we'll have a solution in a few years.

>> * Adding CLOCK_REALTIME to the kernel is a lot of work given the expected
>>   guarantees for a local system.
> 
> Correct.
> 
>> * CLOCK_REALTIME is an expensive resource to maintain, even more expensive
>>   than other resources where the kernel can balance their usage.
> 
> Correct.
> 
>> * On balance it would be better to use vm or vm+containers e.g. kata as a
>>   solution to having CLOCK_REALTIME distinct in the container.
> 
> That'd be the optimal solution, but the above might be a middle ground.

Agreed.

-- 
Cheers,
Carlos.