Re: [PATCH] x86/xen: Add "xen_timer_slop" command line option

[Ryan Thibodeaux]

On Mon, Mar 25, 2019 at 09:43:20AM -0400, Boris Ostrovsky wrote:
> On 3/25/19 8:05 AM, luca abeni wrote:
> > Hi all,
> >
> > On Sat, 23 Mar 2019 11:41:51 +0100
> > luca abeni <luca.abeni@xxxxxxxxxxxxxxx> wrote:
> > [...]
> >>>> Is there any data that shows effects of using this new parameter?
> >>>>     
> >>> Yes, I've done some research and experiments on this. I did it
> >>> together with a friend, which I'm Cc-ing, as I'm not sure we're
> >>> ready/capable to share the results, yet (Luca?).  
> >> I think we can easily share the experimental data (cyclictest output
> >> and plots).
> >>
> >> Moreover, we can share the scripts and tools for running the
> >> experiments (so, everyone can easily reproduce the numbers by simply
> >> typing "make" and waiting for some time :)
> >>
> >>
> >> I'll try to package the results and the scripts/tools this evening,
> >> and I'll send them.
> > Sorry for the delay. I put some quick results here:
> > http://retis.santannapisa.it/luca/XenTimers/
> > (there also is a link to the scripts to be used for reproducing the
> > results). The latencies have been measured by running cyclictest in the
> > guest (see the scripts for details).
> >
> > The picture shows the latencies measured with an unpatched guest kernel
> > and with a guest kernel having TIMER_SLOP set to 1000 (arbitrary small
> > value :).
> > All the experiments have been performed booting the hypervisor with a
> > small timer_slop (the hypervisor's one) value. So, they show that
> > decreasing the hypervisor's timer_slop is not enough to measure low
> > latencies with cyclictest.
> 
> 
> 
> I have a couple of questions:
> * Does it make sense to make this a tunable for other clockevent devices
> as well?

I gather that would be on a case-by-case basis for very specific 
ones.

For many timers in the kernel, the minimums are determined by the
actual hardware  backing the timer, and the minimum can be
adjusted by the clockevent code. This case is special since it 
is entirely a software-based implementation in the kernel, where 
the actual timer implementation is in the Xen hypervisor.

> * This patch adjusts min value. Could max value (ever) need a similar
> adjustment?

I cannot think of such a case where that would be helpful, but I
cannot rule that out or speak as an authority.

- Ryan