sched/core: Fix arch_scale_freq_tick() on tickless systems
authorYair Podemsky <ypodemsk@redhat.com>
Wed, 30 Nov 2022 12:51:21 +0000 (14:51 +0200)
committerIngo Molnar <mingo@kernel.org>
Sat, 7 Jan 2023 11:25:50 +0000 (12:25 +0100)
commit7fb3ff22ad8772bbf0e3ce1ef3eb7b09f431807f
treed21a763963b1c316fc8de5048d47458d2239158b
parent0a71553536d270e988580a3daa9fc87535908221
sched/core: Fix arch_scale_freq_tick() on tickless systems

In order for the scheduler to be frequency invariant we measure the
ratio between the maximum CPU frequency and the actual CPU frequency.

During long tickless periods of time the calculations that keep track
of that might overflow, in the function scale_freq_tick():

  if (check_shl_overflow(acnt, 2*SCHED_CAPACITY_SHIFT, &acnt))
          goto error;

eventually forcing the kernel to disable the feature for all CPUs,
and show the warning message:

   "Scheduler frequency invariance went wobbly, disabling!".

Let's avoid that by limiting the frequency invariant calculations
to CPUs with regular tick.

Fixes: e2b0d619b400 ("x86, sched: check for counters overflow in frequency invariant accounting")
Suggested-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Signed-off-by: Yair Podemsky <ypodemsk@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Acked-by: Giovanni Gherdovich <ggherdovich@suse.cz>
Link: https://lore.kernel.org/r/20221130125121.34407-1-ypodemsk@redhat.com
kernel/sched/core.c