linux-kernel - Re: [PATCH v2 2/3] sched: Cleanup SCHED_THERMAL

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Date:   Mon, 27 Jul 2020 19:45:49 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qian Cai <cai@....pw>,
        Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-pm@...r.kernel.org, Russell King <linux@...linux.org.uk>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Amit Daniel Kachhap <amit.kachhap@...il.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v2 2/3] sched: Cleanup SCHED_THERMAL_PRESSURE kconfig
 entry

On 27/07/2020 16:18, Qian Cai wrote:
> On Sun, Jul 12, 2020 at 05:59:16PM +0100, Valentin Schneider wrote:
>> As Russell pointed out [1], this option is severely lacking in the
>> documentation department, and figuring out if one has the required
>> dependencies to benefit from turning it on is not straightforward.
>>
>> Make it non user-visible, and add a bit of help to it. While at it, make it
>> depend on CPU_FREQ_THERMAL.
>>
>> [1]: https://lkml.kernel.org/r/20200603173150.GB1551@shell.armlinux.org.uk
>>
>> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
>> ---
>>  init/Kconfig | 15 ++++++++++++++-
>>  1 file changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/init/Kconfig b/init/Kconfig
>> index 0498af567f70..0a97d85568b2 100644
>> --- a/init/Kconfig
>> +++ b/init/Kconfig
>> @@ -492,8 +492,21 @@ config HAVE_SCHED_AVG_IRQ
>>  	depends on SMP
>>  
>>  config SCHED_THERMAL_PRESSURE
>> -	bool "Enable periodic averaging of thermal pressure"
>> +	bool
>>  	depends on SMP
>> +	depends on CPU_FREQ_THERMAL
>> +	help
>> +	  Select this option to enable thermal pressure accounting in the
>> +	  scheduler. Thermal pressure is the value conveyed to the scheduler
>> +	  that reflects the reduction in CPU compute capacity resulted from
>> +	  thermal throttling. Thermal throttling occurs when the performance of
>> +	  a CPU is capped due to high operating temperatures.
>> +
>> +	  If selected, the scheduler will be able to balance tasks accordingly,
>> +	  i.e. put less load on throttled CPUs than on non/less throttled ones.
>> +
>> +	  This requires the architecture to implement
>> +	  arch_set_thermal_pressure() and arch_get_thermal_pressure().
>>  
>>  config BSD_PROCESS_ACCT
>>  	bool "BSD Process Accounting"
>> -- 
> 
> On arm64 linux-next (20200727),
> 
> https://gitlab.com/cailca/linux-mm/-/blob/master/arm64.config
> 
> WARNING: unmet direct dependencies detected for SCHED_THERMAL_PRESSURE
>   Depends on [n]: SMP [=y] && CPU_FREQ_THERMAL [=n]
>   Selected by [y]:
>   - ARM64 [=y]

Not sure, but:

(1) do we wan to let people enable SCHED_THERMAL_PRESSURE for arm64 so
arm64 can potentially run w/o a CPU freq cooling device?

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 2d4abbc9f8d0..baffe8b66da2 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -192,7 +192,6 @@ config ARM64
        select PCI_SYSCALL if PCI
        select POWER_RESET
        select POWER_SUPPLY
-       select SCHED_THERMAL_PRESSURE
        select SPARSE_IRQ
        select SWIOTLB
        select SYSCTL_EXCEPTION_TRACE
diff --git a/init/Kconfig b/init/Kconfig
index 37b089f87804..8b36e07fb230 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -492,7 +492,7 @@ config HAVE_SCHED_AVG_IRQ
        depends on SMP

 config SCHED_THERMAL_PRESSURE
-       bool
+       bool "Thermal pressure accounting"
        depends on SMP
        depends on CPU_FREQ_THERMAL
        help

Or

(2) should SCHED_THERMAL_PRESSURE for arm64 be enabled by default?

But then it makes no sense to allow the removal of CONFIG_CPU_FREQ_THERMAL.

linux-next/master$ make ARCH=arm64 defconfig

// Remove CONFIG_CPU_FREQ_THERMAL
linux-next/master$ grep CPU_FREQ_THERMAL .config
# CONFIG_CPU_FREQ_THERMAL is not set

linux-next/master$ make
scripts/kconfig/conf  --syncconfig Kconfig

WARNING: unmet direct dependencies detected for SCHED_THERMAL_PRESSURE
  Depends on [n]: SMP [=y] && CPU_FREQ_THERMAL [=n]
  Selected by [y]:
  - ARM64 [=y]

WARNING: unmet direct dependencies detected for SCHED_THERMAL_PRESSURE
  Depends on [n]: SMP [=y] && CPU_FREQ_THERMAL [=n]
  Selected by [y]:
  - ARM64 [=y]

WARNING: unmet direct dependencies detected for SCHED_THERMAL_PRESSURE
  Depends on [n]: SMP [=y] && CPU_FREQ_THERMAL [=n]
  Selected by [y]:
  - ARM64 [=y]
  HOSTCC  scripts/dtc/dtc.o

---

There is a similar issue with arm.

I would prefer for (1).