Re: [RFC] Interrupt latency measurement technique

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

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?

One thing to watch out for is the virtualised offset of dom0's timer, I
can't remember if this is expected to be zero or non-zero, if the latter
then you will need to subtract it out.

>  Can this technique be used?

One thing which it misses is the time from the IRQ line going high to
reaching gic_interrupt. I don't know if that is a concern for you.

Not sure what embedded folks usually do here (you probably know better
than me). One technique I've heard of is to inject an interrupt via an
external GPIO input and toggle another GPIO in response, then you can
use a scope to observe the difference.

Ian.