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

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Date:   Thu, 13 Feb 2020 20:08:02 +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 Thu, Feb 13, 2020 at 7:35 PM Thara Gopinath
<thara.gopinath@...aro.org> wrote:
>
> 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.

One reason to do this, IMHO, is to keep platform conversions separate
from the core infrastructure in a series, so the core can get merged
while platform maintainers can take their time to decide if, when, how
to merge this.