Re: [PATCH v3 1/2] PM / devfreq: Generic CPU frequency to device frequency mapping governor

[skannan]

On 2018-08-02 14:00, skannan@xxxxxxxxxxxxxx wrote:
> On 2018-08-02 02:56, MyungJoo Ham wrote:
> > > Many CPU architectures have caches that can scale independent of the 
> > > CPUs.
> > > Frequency scaling of the caches is necessary to make sure the cache 
> > > is not
> > > a performance bottleneck that leads to poor performance and power. 
> > > The same
> > > idea applies for RAM/DDR.
> > >
> > > To achieve this, this patch adds a generic devfreq governor that 
> > > takes the
> > > current frequency of each CPU frequency domain and then adjusts the
> > > frequency of the cache (or any devfreq device) based on the frequency 
> > > of
> > > the CPUs. It listens to CPU frequency transition notifiers to keep 
> > > itself
> > > up to date on the current CPU frequency.
> > >
> > > To decide the frequency of the device, the governor does one of the
> > > following:
> >
> > This exactly has the same purpose with "passive" governor except for 
> > the
> > single part: passive governor depends on another devfreq driver, not
> > cpufreq driver.
> >
> > If both governors have many features in common, can you merge them 
> > into one?
> > Passive governor also has "get_target_freq", which allows driver 
> > authors
> > to define the mapping.
>
> Thanks for the review and pointing me to the passive governor. I agree
> that at a high level they are both doing the same. I can certainly
> stuff this CPU freq to dev freq mapping into passive governor, but I
> think it'll just make one huge set of code that's harder to understand
> and maintain because it trying to do different things under the hood.
>
> There are also a bunch of complexities and optimizations that come
> with the cpufreq-map governor that don't fit with the passive
> governor.
>
> 1. It's not one CPU who's frequency we have to listen to. There are
> multiple CPUs/policies we have to aggregate across.
> 2. We have to deal with big vs little having different needs/mappings.
> 3. Since it's always just CPUfreq, I can optimize the handling in the
> transition notifiers. If I have 4 different devices that are scaled
> based on CPU freq, I still use only 1 transition notifier. It becomes
> a bit harder to do with the passive governor.
> 4. It requires that the device explicitly support the passive governor
> and pick a mapping function. With cpufreq-map governor, the device
> drivers don't need to make any changes. Whoever is making a
> device/board can choose what devices to scale up base on CPU freq
> based on their board and their needs. Even an end user can say, scale
> the GPU based on my CPU based on interpolation if they choose to.
> 5. If a device has some other use for the private data, it can't work
> with passive governor, but can work with cpufreq-map governor.
> 6. I also want to improve cpufreq-map governor to handle hotplug
> correctly in later patches (needs more discussion) and that'll add
> more complexity.
>
> I think for these reasons we shouldn't combine these two governors.
> Let me know what you think.

Friendly reminder.

Thanks,
Saravana