linux-kernel - v5.8+ powersave governor breakage?

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Date:   Sun, 01 Nov 2020 17:23:59 +0100
From:   Mike Galbraith <efault@....de>
To:     lkml <linux-kernel@...r.kernel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Peter Zijlstra <peterz@...radead.org>
Subject: v5.8+ powersave governor breakage?

Greetings,

As you can see in the data below, my i4790 box used to default to the
powersave governor despite CONFIG_CPU_FREQ_DEFAULT_GOV_ONDEMAND=y, and
disallowed switching to ondemand.  Post 5.8, powersave locks in at the
lowest freq available, and while ondemand becomes the default, it takes
a cross core scheduling performance hit compared to previous powersave.

On a positive note, schedutil, which I used to have cause to avoid,
seems to have improved a lot while I wasn't looking, and now actually
improving cross core performance a wee bit.  Seems I should configure
stable 5.[89] kernels to use it as well.

taskset 0xc pipe-test 1

CONFIG_CPU_FREQ_DEFAULT_GOV_ONDEMAND=y                         defaults to
5.4.74          2.183264 usecs/loop -- avg  2.179480 917.7 KHz powersave, ondemand not allowed
5.5.19          2.184331 usecs/loop -- avg  2.179776 917.5 KHz powersave, ondemand not allowed
5.6.19          2.198156 usecs/loop -- avg  2.204380 907.3 KHz powersave, ondemand not allowed
5.7.19          2.230582 usecs/loop -- avg  2.231981 896.1 KHz powersave, ondemand not allowed
5.8.17          3.203949 usecs/loop -- avg  3.207272 623.6 KHz ondemand
5.8.17         12.840411 usecs/loop -- avg 12.898500 155.1 KHz ondemand -> powersave
5.9.2           3.024442 usecs/loop -- avg  3.040023 657.9 KHz ondemand
5.9.2          12.348519 usecs/loop -- avg 12.299751 162.6 KHz ondemand -> powersave
CONFIG_CPU_FREQ_DEFAULT_GOV_SCHEDUTIL=y due to db865272d9c4
5.10.gc2dc4c0   2.121057 usecs/loop -- avg  2.121487 942.7 KHz schedutil                note: 4.0 GHz perfect!
5.10.gc2dc4c0   2.987989 usecs/loop -- avg  2.988658 669.2 KHz schedutil -> ondemand    note: 2.9 GHz not so perfect
5.10.gc2dc4c0  12.447348 usecs/loop -- avg 12.475363 160.3 KHz schedutil -> powersave   note: 800 MHz heeelp meee
5.10.gc2dc4c0   2.135548 usecs/loop -- avg  2.137868 935.5 KHz schedutil -> performance note: 4.0 GHz (obviously)