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

[Ryan Thibodeaux <thibodux@gmail.com>]

On Tue, Mar 26, 2019 at 07:21:31PM -0400, Boris Ostrovsky wrote:
> On 3/26/19 5:13 AM, Dario Faggioli wrote:
> > On Mon, 2019-03-25 at 09:43 -0400, Boris Ostrovsky wrote:
> >> On 3/25/19 8:05 AM, luca abeni wrote:
> >>> 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?
> >>
> > So, AFAIUI, the thing is as follows. In clockevents_program_event(), we
> > keep the delta between now and the next timer event within
> > dev->max_delta_ns and dev->min_delta_ns:
> >
> >   delta = min(delta, (int64_t) dev->max_delta_ns);
> >   delta = max(delta, (int64_t) dev->min_delta_ns);
> >
> > For Xen (well, for the Xen clock) we have:
> >
> >   .max_delta_ns = 0xffffffff,
> >   .min_delta_ns = TIMER_SLOP,
> >
> > which means a guest can't ask for a timer to fire earlier than 100us
> > ahead, which is a bit too coarse, especially on contemporary hardware.
> >
> > For "lapic_deadline" (which was what was in use in KVM guests, in our
> > experiments) we have:
> >
> >   lapic_clockevent.max_delta_ns = clockevent_delta2ns(0x7FFFFF, &lapic_clockevent);
> >   lapic_clockevent.min_delta_ns = clockevent_delta2ns(0xF, &lapic_clockevent);
> >
> > Which means max is 0x7FFFFF device ticks, and min is 0xF.
> > clockevent_delta2ns() does the conversion from ticks to ns, basing on
> > the results of the APIC calibration process. It calls cev_delta2ns()
> > which does some scaling, shifting, divs, etc, and, at the very end,
> > this:
> >
> >   /* Deltas less than 1usec are pointless noise */
> >   return clc > 1000 ? clc : 1000;
> >
> > So, as Ryan is also saying, the actual minimum, in this case, depends
> > on hardware, with a sanity check of "never below 1us" (which is quite
> > smaller than 100us!)
> >
> > Of course, the actual granularity depends on hardware in the Xen case
> > as well, but that is handled in Xen itself. And we have mechanisms in
> > place in there to avoid timer interrupt storms (like, ahem, the Xen's
> > 'timer_slop' boot parameter... :-P)
> >
> > And this is basically why I was also thinking we can/should lower the
> > default value of TIMER_SLOP, here in the Xen clock implementation in
> > Linux.
> 
> What do you think would be a sane value? 10us? Should we then still keep
> this patch?
> 
> My concern would be that if we change the current value and it turns out
> to be very wrong we'd then have no recourse.
> 
> 
> -boris
> 

Speaking out of turn but as a participant in this thread, I would not
assume to change the default value for all cases without significant
testing by the community, touching a variety of configurations.

It feels like changing the default has a non-trivial amount of 
unknowns that would need to be addressed.

Not surprisingly, I am biased to the approach of my patch which
does not change the default but offers flexibility to all.

- Ryan