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

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Message-ID: <87y2jej8mp.fsf@nanos.tec.linutronix.de>
Date:   Sat, 07 Nov 2020 01:47:10 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Carlos O'Donell <carlos@...hat.com>,
        Zack Weinberg <zackw@...ix.com>, Cyril Hrubis <chrubis@...e.cz>
Cc:     Dmitry Safonov <dima@...sta.com>, Andrei Vagin <avagin@...il.com>,
        GNU C Library <libc-alpha@...rceware.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Y2038][time namespaces] Question regarding CLOCK_REALTIME support plans in Linux time namespaces

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.

> * 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.

Thanks,

        tglx