linux-kernel - Re: v5.7: new core kernel option missing help text

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Date:   Wed, 3 Jun 2020 20:48:01 -0400
From:   Thara Gopinath <thara.gopinath@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: v5.7: new core kernel option missing help text



On 6/3/20 4:25 PM, Valentin Schneider wrote:
> 
> On 03/06/20 20:58, Russell King - ARM Linux admin wrote:
>> On Wed, Jun 03, 2020 at 09:24:56PM +0200, Vincent Guittot wrote:
>>> On Wed, 3 Jun 2020 at 20:45, Russell King - ARM Linux admin
>>> <linux@...linux.org.uk> wrote:
>>>> It's a start.  I'm still wondering whether I should answer yes or no
>>>> for the platforms I'm building for.
>>>>
>>>> So far, all I've found is:
>>>>
>>>> arch/arm/include/asm/topology.h:#define arch_scale_thermal_pressure topology_get_thermal_pressure
>>>>
>>>> which really doesn't tell me anything about this.  So I'm still in
>>>> the dark.
>>>>
>>>> I guess topology_get_thermal_pressure is provided by something in
>>>> drivers/ which will be conditional on some driver or something.
>>>
>>> You need cpufreq_cooling device to make it useful and only for SMP
>>> I don't think that this should not be user configurable because even
>>> with the description above, it is not easy to choose.
>>> This should be set by the driver that implement the feature which is
>>> only cpufreq cooling device for now it
>>
>> As I have CONFIG_CPU_FREQ_THERMAL=y in my config, I'm guessing (and it's
>> only a guess) that I should say y to SCHED_THERMAL_PRESSURE ?
>>
> 
> arm and arm64 implement arch_scale_thermal_pressure(); the actual
> implementation is in the arch_topology "driver" (GENERIC_ARCH_TOPOLOGY).
> 
> Then, the caller of arch_set_thermal_pressure() is cpufreq_cooling (see
> below); that'll only get called if you have thermal zones using CPU
> cooling devices.
> 
> AFAICT the current state of things imply we should have something like
> 
>          depends on (ARM || ARM64) && GENERIC_ARCH_TOPOLOGY
> 
> for that option.

Hi Russel/Valentin

The feature itself like Valentin explained below allows scheduler to be 
aware of cpu capacity reduced due to thermal throttling. 
arch_set_thermal_pressure feeds the capped capacity to the scheduler and 
hence the feature makes sense only if arch_set_thermal_pressure is 
implemented. Having said that arch_set_thermal_pressure is implemented 
in arch_topology driver for arm and arm64 platforms. But the feature 
itself is not bound to arm/arm64 platforms. So it would make it wrong to 
add a "depends on (ARM || ARM64) option."

I agree with Vincent that allowing user to choose this option is 
probably not the best. IMO, this should be enabled by default in arm64 
defconfig considering both GENERIC_ARCH_TOPOLOGY and CPU_FREQ_THERMAL 
are enabled by default.
So if it is acceptable three things to be done are:
1. Add the help text.
2. Don't allow SCHED_THERMAL_PRESSURE configurable by user
3. Enable it by default in arm64 defconfig

> 
>>>>> +     help
>>>>> +       This option allows the scheduler to be aware of CPU thermal throttling
>>>>> +       (i.e. thermal pressure), providing arch_scale_thermal_pressure() is
>>>>> +       implemented.
>>
>> Is this feature documented in terms of what it does?  Do I assume that
>> as the thermal trip points start tripping, that has an influence on
>> the scheduler?  Or is it the case that the scheduler is wanting to
>> know when the cpu frequency changes?
>>
>> Grepping for "thermal" in Documentation/scheduler brings up nothing.
> 
> The former; changing a CPU cooling device's state (IOW changing its max
> allowed frequency for thermal reasons) leads to a call to
> arch_set_thermal_pressure() (see
> cpufreq_cooling.c::cpufreq_set_cur_state()).
> 
> It's somewhat interesting to have, at least in theory. On plain SMP that
> would let the scheduler see if some CPUs are more throttled that others,
> which would be leveraged when doing load balancing. It's more
> interesting for big.LITTLE & co, where in the worst cases we can have
> things like capacity inversion, i.e. the bigs are so thermally throttled
> that they give less oomf than a LITTLE.
> 

-- 
Warm Regards
Thara