Re: [RFC] Interrupt latency measurement technique

[Ian Campbell <ian.campbell@citrix.com>]

On Thu, 2014-08-28 at 10:58 +0300, Andrii Tseglytskyi wrote:
> Hi Ian
> 
> On Thu, Aug 28, 2014 at 4:07 AM, Ian Campbell <ian.campbell@citrix.com> wrote:
> > On Wed, 2014-08-27 at 12:14 +0300, Andrii Tseglytskyi wrote:
> >> Hi,
> >>
> >> I need to measure IRQ latency introduced by Xen: (GIC -> Xen IRQ
> >> handler -> Dom0 IRQ handler)
> >> I need to know how many time I spend in Xen IRQ handler.
> >>
> >> Can someone comment - is the following algorithm works:
> >>
> >> - in function xen/arch/arm/gic.c: gic_interrupt() store timer counter value:
> >> xen_timer_val = READ_SYSREG64(CNTPCT_EL0) - READ_SYSREG64(CNTVOFF_EL2)
> >>
> >> - in any IRQ handler in dom0 store timer counter value:
> >> dom0_timer_val = READ_SYSREG64(CNTPCT_EL0)
> >>
> >> - calculate time diff in nanoseconds:
> >> time_diff_ns = ticks_to_ns(dom0_timer_val - xen_timer_val)
> >>
> >> Using this technique I measured display IRQ latency and got about
> >> - 20 to 30 usec latency on 1 GHz MPU frequency
> >> - 10 to 20 usec latencyon 1.5 GHz MPU frequency
> >>
> >> Are these numbers expectable?
> >
> > How do they compare to just native Linux?
> >
> 
> Never measured native Linux. I just need to measure overhead, introduced by Xen.
> So I take a time difference between git_interrupt() in Xen and
> gic_interrupt() in Linux.

OK. I'm afraid I don't have much of a reference point for these numbers
so I don't know if 30usec is e.g .1% additional overhead or 100%.