linux-kernel - Re: [PATCH 0/2]cpufreq,topology,arm: disable FI for BL

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Message-ID: <CAJZ5v0hr+MZzokObNq5L0q1Fd0M5EXc6QmLXDv9b85P5b4yp4g@mail.gmail.com>
Date:   Wed, 7 Oct 2020 16:34:44 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Ionela Voinescu <ionela.voinescu@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Sudeep Holla <sudeep.holla@....com>
Cc:     Will Deacon <will@...nel.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2]cpufreq,topology,arm: disable FI for BL_SWITCHER

On Thu, Sep 24, 2020 at 2:30 PM Ionela Voinescu <ionela.voinescu@....com> wrote:
>
> This series is the result of the discussions ([1], [2]) around the
> complications that the BL_SWITCHER poses when it comes to Frequency
> Invariance (FI) and it aims to restart the discussions.
>
> To properly scale its per-entity load-tracking signals, the task
> scheduler needs to be given a frequency scale factor, i.e. some image of
> the current frequency the CPU is running at, relative to its maximum
> frequency.
>
> But (reiterating the message in the changelog of patch 2/2), big.LITTLE
> switching complicates the setting of a correct cpufreq-based frequency
> invariance scale factor due to (as observed in
> drivers/cpufreq/vexpress-spc-cpufreq.c):
>  - Incorrect current and maximum frequencies as a result of the
>    exposure of a virtual frequency table to the cpufreq core,
>  - Missed updates as a result of asynchronous frequency adjustments
>    caused by frequency changes in other CPU pairs.
> More information on this feature can be found at [3].
>
> Given that its functionality is atypical in regards to FI and that this
> is an old technology, patch 2/2 disable FI for when big.LITTLE switching
> is configured in to prevent incorrect scale setting.
>
> For this purpose patch 1/2 changes the way arch_set_freq_scale() is
> defined in architecture code which brings it in line with the logic of
> other architectural function definitions while allowing for less invasive
> filtering of FI support.
>
> In the discussions at [2], three possible solutions were suggested:
>  - (1) conditioning FI by !CONFIG_BL_SWITCHER
>  - (2) leave as is with note in driver specifying this FI broken
>    functionality
>  - (3) removing full BL_SWITCHER support
>
> This series restructures the solution at (1). The reason for it is that
> the new patch limits the ifdef filtering to the arm topology include file,
> a location where frequency invariance functions are defined. Therefore,
> this seems more appropriate given that the b.L switcher is an arm
> technology and that the new FI filtering location seems more natural for
> conditioned FI disabling.
>
> Solutions (2) and (3) were not implemented given that there might be some
> remaining users of this technology (Samsung Chromebook 2 - Samsung Exynos
> 5 Octa 5420, Samsung Exynos 5 Octa 5800) and therefore leaving this
> broken (2) seems equally bad to removing support for it (3).
>
> [1] https://lore.kernel.org/lkml/20200701090751.7543-5-ionela.voinescu@arm.com/
> [2] https://lore.kernel.org/lkml/20200722093732.14297-4-ionela.voinescu@arm.com/
> [3] https://lwn.net/Articles/481055/

I can take this set with the ACKs from Viresh if that's fine by
everyone.  Catalin?  Sudeep?