linux-kernel - Re: [Patch v9 3/8] arm,arm64,drivers:Add infrastructure to store and update instantaneous thermal pressure

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Message-ID: <5E4557B1.8020809@linaro.org>
Date:   Thu, 13 Feb 2020 09:05:37 -0500
From:   Thara Gopinath <thara.gopinath@...aro.org>
To:     Amit Kucheria <amit.kucheria@...durent.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, ionela.voinescu@....com,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Zhang Rui <rui.zhang@...el.com>, qperret@...gle.com,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Juri Lelli <juri.lelli@...hat.com>, corbet@....net,
        LKML <linux-kernel@...r.kernel.org>,
        Amit Daniel Kachhap <amit.kachhap@...il.com>,
        Javi Merino <javi.merino@...nel.org>
Subject: Re: [Patch v9 3/8] arm,arm64,drivers:Add infrastructure to store and
 update instantaneous thermal pressure

On 02/13/2020 07:25 AM, Amit Kucheria wrote:
> On Wed, Jan 29, 2020 at 4:06 AM Thara Gopinath
> <thara.gopinath@...aro.org> wrote:
>>
>> Add architecture specific APIs to update and track thermal pressure on a
>> per cpu basis. A per cpu variable thermal_pressure is introduced to keep
>> track of instantaneous per cpu thermal pressure. Thermal pressure is the
>> delta between maximum capacity and capped capacity due to a thermal event.
> 
> s/capped/decreased to have consistent use throughout the series e.g. in patch 1.
> 
> Though personally, I like "capped capacity"  in which case
> s/decreased/capped in patch 1 and elsewhere.

I will fix this
> 
>>
>> topology_get_thermal_pressure can be hooked into the scheduler specified
>> arch_cpu_thermal_capacity to retrieve instantaneous thermal pressure of a
>> cpu.
>>
>> arch_set_thermal_pressure can be used to update the thermal pressure.
>>
>> Considering topology_get_thermal_pressure reads thermal_pressure and
>> arch_set_thermal_pressure writes into thermal_pressure, one can argue for
>> some sort of locking mechanism to avoid a stale value.  But considering
>> topology_get_thermal_pressure can be called from a system critical path
>> like scheduler tick function, a locking mechanism is not ideal. This means
>> that it is possible the thermal_pressure value used to calculate average
>> thermal pressure for a cpu can be stale for upto 1 tick period.
>>
>> Signed-off-by: Thara Gopinath <thara.gopinath@...aro.org>
>> ---
>>
>> v6->v7:
>>         - Changed the input argument in arch_set_thermal_pressure from
>>           capped capacity to delta capacity(thermal pressure) as per
>>           Ionela's review comments.
>>
>>  arch/arm/include/asm/topology.h   |  3 +++
>>  arch/arm64/include/asm/topology.h |  3 +++
> 
> Any particular reason to enable this for arm/arm64 in this patch
> itself? I'd have enabled them in two separate patches after this one.

No reason. No reason not to as well as arch_topology is "Arm specific
cpu topology file" and changes are one-liners.

> 
>>  drivers/base/arch_topology.c      | 11 +++++++++++
>>  include/linux/arch_topology.h     | 10 ++++++++++
>>  4 files changed, 27 insertions(+)
>>
>> diff --git a/arch/arm/include/asm/topology.h b/arch/arm/include/asm/topology.h
>> index 8a0fae9..3a50a19 100644
>> --- a/arch/arm/include/asm/topology.h
>> +++ b/arch/arm/include/asm/topology.h
>> @@ -16,6 +16,9 @@
>>  /* Enable topology flag updates */
>>  #define arch_update_cpu_topology topology_update_cpu_topology
>>
>> +/* Replace task scheduler's default thermal pressure retrieve API */
>> +#define arch_cpu_thermal_pressure topology_get_thermal_pressure
>> +
>>  #else
>>
>>  static inline void init_cpu_topology(void) { }
>> diff --git a/arch/arm64/include/asm/topology.h b/arch/arm64/include/asm/topology.h
>> index a4d945d..a70896f 100644
>> --- a/arch/arm64/include/asm/topology.h
>> +++ b/arch/arm64/include/asm/topology.h
>> @@ -25,6 +25,9 @@ int pcibus_to_node(struct pci_bus *bus);
>>  /* Enable topology flag updates */
>>  #define arch_update_cpu_topology topology_update_cpu_topology
>>
>> +/* Replace task scheduler's default thermal pressure retrieve API */
>> +#define arch_cpu_thermal_pressure topology_get_thermal_pressure
>> +
>>  #include <asm-generic/topology.h>
>>
>>  #endif /* _ASM_ARM_TOPOLOGY_H */
>> diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
>> index 6119e11..68dfa49 100644
>> --- a/drivers/base/arch_topology.c
>> +++ b/drivers/base/arch_topology.c
>> @@ -42,6 +42,17 @@ void topology_set_cpu_scale(unsigned int cpu, unsigned long capacity)
>>         per_cpu(cpu_scale, cpu) = capacity;
>>  }
>>
>> +DEFINE_PER_CPU(unsigned long, thermal_pressure);
>> +
>> +void arch_set_thermal_pressure(struct cpumask *cpus,
>> +                              unsigned long th_pressure)
>> +{
>> +       int cpu;
> 
> <snip>
> 


-- 
Warm Regards
Thara