Re: [RFC 00/20] ns: Introduce Time Namespace

[Andrei Vagin <avagin@gmail.com>]

On Mon, Oct 29, 2018 at 09:33:14PM +0100, Thomas Gleixner wrote:
> 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?

We didn't find any proc files where boot or monotonic time is reported,
but we will double check this.

> 
> 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.

It is a good point. We will work on it.

> 
> 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.

Thomas, there is no rush at all. This functionality is critical for
CRUI, but we have enough time to solve it properly.

The only thing what I want is that this functionality continues moving
forward and will not be put in the back burner.

> 
> 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?

Dmitry and I are going to be there.

Thanks!
Andrei

> 
> Thanks,
> 
> 	tglx
> 
>