Re: [PATCH v2 2/3] rt-tests: cyclictest: Support idle state disabling via libcpupower

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2024-12-06 13:14:02 [+0100], Tomas Glozar wrote:
> pá 6. 12. 2024 v 12:52 odesílatel Sebastian Andrzej Siewior
> <bigeasy@linutronix.de> napsal:
> > What is the default behaviour and what is the intended behaviour?
> > Couldn't we somehow avoid adding yet another option?
> 
> The default behavior is to hold /dev/cpu_dma_latency at zero, which
> disables all idle states on all CPUs, not only those on which
> cyclictest measurements are running. This has the disadvantage of a
> higher power consumption than needed in most cases. With
> --deepest-idle-state, idle states are limited only on CPUs cyclictest
> is running on.

This is a real use case? /dev/cpu_dma_latency is a big hammer to disable
everything that might cause latency.
So you have 4 CPUs, CPU0 is getting idle from time to time and CPU1-3 is
doing RT work so it can't take sleep?

> There are two reasons why we can't just use the latter and have to
> have options for both. Firstly, some hardware does not support
> disabling idle states on individual CPUs via the cpuidle sysfs
> interface, which is used by --deepest-idle-state. Secondly, latencies
> measured with --deepest-idle-state might still be higher in some cases
> compared to holding /dev/cpu_dma_latency.

Yes. Especially if you find power management thingy that is covered by
cpu_dma_latency but not by this new switch.
 
> Tomas

Sebastian