Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Wed, 7 Oct 2020 16:34:44 +0200 | Subject | Re: [PATCH 0/2]cpufreq,topology,arm: disable FI for BL_SWITCHER |
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On Thu, Sep 24, 2020 at 2:30 PM Ionela Voinescu <ionela.voinescu@arm.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?
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