Re: i.MX28 milliseconds latencies, interrupts disabled in arch_cpu_idle?

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Le 17/04/2014 22:38, Stanislav Meduna a écrit :
> Hi,
> 
> on a i.MX28:
> 
> Linux nxt 3.12.15-rt25+ #83 PREEMPT RT Thu Apr 17 20:54:00 CEST 2014 armv5tejl GNU/Linux
>
> (...)
>
> Maybe it is just some tracing infrastructure interference - the effect
> of enabling the tracing is huge on this platform for some reason.
> Just enabling the irqsoff tracing makes the machine unsuitable
> for any serious load.

For the record, latencies on imx28 are high because:
1- it has an arm926 core, which uses a VIVT cache, and Linux chooses to
flush cache at each mm switch to avoid aliasing issues;
2- the arm926 idle routine, in order to work around a hardware bug
flushes the instruction cache before calling the "wfi" instruction.

#2 can be avoided by using nohlt
#1 can be avoided by using the ARM FCSE extension.

A patch exists for #1 which is currently integrated into the Adeos patch
but could exist independently. I could post the patch here for Linux
vanilla here if there is enough interest, and let you do the merge with
the preempt_rt patch.

Regards.

-- 
Gilles.
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