[ltt-dev] Interpreting tracing results

[Mathieu Desnoyers]

* Akyurek, Ali (EXT) (ali.akyurek.ext at siemens.com) wrote:
> 
> Hi all,
> 
> In my arm machine, context switching takes 1220 or 1221 nanoseconds. 
> i thought this time as (start of sched_schedule - start of
> whatever_previous_event), Actually that is the time between different
> PID values in rows.am i right?
> 
> and
> 
> Handling of interrupts (i take only the top handlers, not soft ones.)
> takes 2441 nanoseconds.
> i thought this time as (end of irq_exit - start of irq_entry), am i
> right?
> 
> Why are these values multiple of 1220 nanoseconds? What is related to? 
> 

Hi,

LTTng uses the generic trace clock for ARM by default. See
include/asm-generic/trace-clock.h.

This clock has only the precision of HZ frequency, and I use an atomic
counter in the LSBs to keep the events ordered.

Please have a look at the ARM-related messages in the past 2 weeks for
implementations of sub-arch specific clock sources for ARM. I will
integrate this kind of work soon after making sure it's smp-safe when it
needs to.

Mathieu

> Thanks all.
> 
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-- 
Mathieu Desnoyers
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