Re: [RFC PATCH 00/19] cpufreq locking cleanups and documentation

[Rafael J. Wysocki]

On Monday, January 11, 2016 05:35:41 PM Juri Lelli wrote:
> Hi all,
> 
> In the context of the ongoing discussion about introducing a simple platform
> energy model to guide scheduling decisions (Energy Aware Scheduling [1])
> concerns have been expressed by Peter about the component in charge of driving
> clock frequency selection (Steve recently posted an update of such component
> [2]): https://lkml.org/lkml/2015/8/15/141.
> 
> The problem is that, with this new approach, cpufreq core functions need to be
> accessed from scheduler hot-paths and the overhead associated with the current
> locking scheme might result to be unsustainable. 
> 
> Peter's proposed approach of using RCU logic to reduce locking overhead seems
> reasonable, but things may not be so straightforward as originally thought. The
> very first thing I actually realized when I started looking into this is that
> it was hard for me to understand which locking mechanism was protecting which
> data structure. As mostly a way to build a better understanding of the current
> cpufreq locking scheme and also as preparatory work for implementing RCU logic,
> I came up with this set of patches. In fact, at this stage, I would like each
> patch to be considered as a question I'm asking rather than a proposed change,
> thus the RFC tag for the series; with the intent of documenting current locking
> scheme and modifying it a bit in order to make RCU logic implementation easier.
> Actually, as you'll soon notice, I didn't really start from scratch. Mike
> shared with me some patches he has been developing while looking at the same
> problem. I've given Mike attribution for the patches that I took unchanged from
> him, with thanks for sharing his findings with me.
> 
> High level description of patches:
> 
>  o [01-04] cleanup and move code around to make things (hopefully) cleaner
>  o [05-14] insert lockdep assertions and fix uncovered erroneous situations
>  o [15-18] remove overkill usage of locking mechanism
>  o 19      adds documentation for the cleaned up locking scheme
> 
> With Viresh' tests [3] on both arm TC2 and arm64 Juno boards I'm not seeing
> anything bad happening. However, coverage is really small (as is my personal
> confidence of not breaking things for other confs :-)).
> 
> This set is based on top of linux-pm/linux-next as of today and it is also
> available from here:

Due to the merge window in progress I have more urgent things to do than
looking at this material right now.  Sorry about that.

I may be able to look at it towards the end of the week.

Thanks,
Rafael