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: <CAHLCerMEieWMyk8RcM-y8c3Usq_e5CTYJ4AqhCQOzihRTUWbTg@mail.gmail.com>
Date:   Thu, 13 Feb 2020 17:55:47 +0530
From:   Amit Kucheria <amit.kucheria@...durent.com>
To:     Thara Gopinath <thara.gopinath@...aro.org>
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 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.

>
> 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.

>  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>