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

[ebiederm@xmission.com (Eric W. Biederman)]

Thomas Gleixner <tglx@linutronix.de> writes:

> Eric,
>
> On Fri, 28 Sep 2018, Eric W. Biederman wrote:
>> Thomas Gleixner <tglx@linutronix.de> writes:
>> > On Wed, 26 Sep 2018, Eric W. Biederman wrote:
>> >> At the same time using the techniques from the nohz work and a little
>> >> smarts I expect we could get the code to scale.
>> >
>> > You'd need to invoke the update when the namespace is switched in and
>> > hasn't been updated since the last tick happened. That might be doable, but
>> > you also need to take the wraparound constraints of the underlying
>> > clocksources into account, which again can cause walking all name spaces
>> > when they are all idle long enough.
>> 
>> The wrap around constraints being how long before the time sources wrap
>> around so you have to read them once per wrap around?  I have not dug
>> deeply enough into the code to see that yet.
>
> It's done by limiting the NOHZ idle time when all CPUs are going into deep
> sleep for a long time, i.e. we make sure that at least one CPU comes back
> sufficiently _before_ the wraparound happens and invokes the update
> function.
>
> It's not so much a problem for TSC, but not every clocksource the kernel
> supports has wraparound times in the range of hundreds of years.
>
> But yes, your idea of keeping track of wraparounds might work. Tricky, but
> looks feasible on first sight, but we should be aware of the dragons.

Oh.  Yes.  Definitely.  A key enabler of any namespace implementation is
figuring out how to tame the dragons.

>> Please pardon me for thinking out load.
>> 
>> There are one or more time sources that we use to compute the time
>> and for each time source we have a conversion from ticks of the
>> time source to nanoseconds.
>> 
>> Each time source needs to be sampled at least once per wrap-around
>> and something incremented so that we don't loose time when looking
>> at that time source.
>> 
>> There are several clocks presented to userspace and they all share the
>> same length of second and are all fundamentally offsets from
>> CLOCK_MONOTONIC.
>
> Yes. That's the readout side. This one is doable. But now look at timers.
>
> If you arm the timer from a name space, then it needs to be converted to
> host time in order to sort it into the hrtimer queue and at some point arm
> the clockevent device for it. This works as long as host and name space
> time have a constant offset and the same skew.
>
> Once the name space time has a different skew this falls apart because the
> armed timer will either expire late or early.
>
> Late might be acceptable, early violates the spec. You could do an extra
> check for rescheduling it, if it's early, but that requires to store the
> name space time accessor in the hrtimer itself because not every timer
> expiry happens so that it can be checked in the name space context (think
> signal based timers). We need to add this extra magic right into
> __hrtimer_run_queues() which is called from the hard and soft interrupt. We
> really don't want to touch all relevant callbacks or syscalls. The latter
> is not sufficient anyway for signal based timer delivery.
>
> That's going to be interesting in terms of synchronization and might also
> cause substantial overhead at least for the timers which belong to name
> spaces.
>
> But that also means that anything which is early can and probably will
> cause rearming of the timer hardware possibly for a very short delta. We
> need to think about whether this can be abused to create interrupt storms.
>
> Now if you accept a bit late, which I'm not really happy about, then you
> surely won't accept very late, i.e. hours, days. But that can happen when
> settimeofday() comes into play. Right now with a single time domain, this
> is easy. When settimeofday() or adjtimex() makes time jump, we just go and
> reprogramm the hardware timers accordingly, which might also result in
> immediate expiry of timers.
>
> But this does not help for time jumps in name spaces because the timer is
> enqueued on the host time base.
>
> And no, we should not think about creating per name space hrtimer queues
> and then have to walk through all of them for finding the first expiring
> timer in order to arm the hardware. That cannot scale.
>
> Walking all hrtimer bases on all CPUs and check all queued timers whether
> they belong to the affected name space does not scale either.
>
> So we'd need to keep track of queued timers belonging to a name space and
> then just handle them. Interesting locking problem and also a scalability
> issue because this might need to be done on all online CPUs. Haven't
> thought it through, but it makes me shudder.

Yes.  I can see how this is a dragon that we need to figure out how to
tame.  It already exist somewhat for CLOCK_MONOTONIC vs CLOCK_REALTIME
but still.

>> I see two fundamental driving cases for a time namespace.
>
> <SNIP>
>
> I completely understand the problem you are trying to solve and yes, the
> read out of time should be a solvable problem.

There is simplified subproblem that I want to ask about but I will reply
separately for that.

>> Not that I think a final implementation would necessary look like what I
>> have described. I just think it is possible with extreme care to evolve
>> the current code base into something that can efficiently handle
>> multiple time domains with slightly different lenghts of second.
>
> Yes, it really needs some serious thoughts and timekeeping is a really
> complex place especially with NTP/PTP in play. We had quite some quality
> time to make it work correctly and reliably, now you come along and want to
> transform it into a multidimensional puzzle. :)

I thought it was Einstein who pointed out what a puzzle timekeeping is,
with the rest of us just playing catch up. ;-)

>> It does though sound like it is going to take some serious digging
>> through the code to understand how what everything does and how and why
>> everthing works the way it does.  Not something grafted on top with just
>> a cursory understanding of how the code works.
>
> I fully agree and I'm happy to help with explanations and ideas and being
> the one who shoots holes into yours.

Sounds good.

Eric