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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Andrei Vagin <avagin@gmail.com>,
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Date:   Mon, 29 Oct 2018 16:21:57 -0500
In-Reply-To: <alpine.DEB.2.21.1810292026040.5984@nanos.tec.linutronix.de>
        (Thomas Gleixner's message of "Mon, 29 Oct 2018 21:33:14 +0100 (CET)")
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Subject: Re: [RFC 00/20] ns: Introduce Time Namespace
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Thomas Gleixner <tglx@linutronix.de> writes:

> Andrei,
>
> On Sat, 20 Oct 2018, Andrei Vagin wrote:
>> When a container is migrated to another host, we have to restore its
>> monotonic and boottime clocks, but we still expect that the container
>> will continue using the host real-time clock.
>> 
>> Before stating this series, I was thinking about this, I decided that
>> these cases can be solved independently. Probably, the full isolation of
>> the time sub-system will have much higher overhead than just offsets for
>> a few clocks. And the idea that isolation of the real-time clock should
>> be optional gives us another hint that offsets for monotonic and
>> boot-time clocks can be implemented independently.
>> 
>> Eric and Tomas, what do you think about this? If you agree that these
>> two cases can be implemented separately, what should we do with this
>> series to make it ready to be merged?
>> 
>> I know that we need to:
>> 
>> * look at device drivers that report timestamps in CLOCK_MONOTONIC base.
>
> and CLOCK_BOOTTIME and that's quite a few.
>
>> * forbid changing offsets after creating timers
>
> There are more things to think about. What about interfaces which expose
> boot time or monotonic time in /proc?
>
> Aside of that (I finally came around to look at the series in more detail)
> I'm really unhappy about the unconditional overhead once the Time namespace
> config switch is enabled. This applies especially to the VDSO. We spent
> quite some time recently to squeeze a few cycles out of those functions and
> it would be a pity to pointlessly waste cycles for the !namespace case.
>
> I can see the urge for this, but please let us think it through properly
> before rushing anything in which we are going to regret once we want to do
> more sophisticated time domain management, e.g. support for isolated clock
> real time. I'm worried, that without a clear plan about the overall
> picture, we end up with duct tape which is hard to distangle after the
> fact.
>
> There have been a few other things brought up versus time management in
> general, like the TSN folks utilizing grand clock masters which expose
> random time instead of proper TAI. Plus some requirements for exposing some
> sort of 'monotonic' clocks which are derived from external synchronization
> mechanisms, but should not affect the regular time keeping clocks.
>
> While different issues, these all fall into the category of separate time
> domains, so taking a step back to the drawing board is probably the best
> thing what we can do now.
>
> There are certainly a few things which can be looked at independently,
> e.g. the VDSO mechanics or general mechanisms to avoid plastering the whole
> kernel with these name space functions applying offsets left and right. I
> rather have dedicated core functionality which replaces/amends existing
> timer functions to become time namespace aware.
>
> I'll try to find some time in the next weeks to look deeper into that, but
> I can't promise anything before returning from LPC. Btw, LPC would be a
> great opportunity to discuss that. Are you and the other name space wizards
> there by any chance?

I will be and there are going to be both container and CRIU
mini-conferences.  So there should at least some of us around.

Eric