[Xen-devel] [RFC] Interrupt latency measurement technique

[Andrii Tseglytskyi <andrii.tseglytskyi@xxxxxxxxxxxxxxx>]

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? Can this technique be used?

Regards,
Andrii

-- 

Andrii Tseglytskyi | Embedded Dev
GlobalLogic
www.globallogic.com

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