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
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