x86/aperfmperf: Erase stale arch_freq_scale values when disabling frequency invariance readings

Once disable_freq_invariance_work is called the scale_freq_tick function
will not compute or update the arch_freq_scale values.
However the scheduler will still read these values and use them.
The result is that the scheduler might perform unfair decisions based on stale
values.

This patch adds the step of setting the arch_freq_scale values for all
cpus to the default (max) value SCHED_CAPACITY_SCALE, Once all cpus
have the same arch_freq_scale value the scaling is meaningless.

Signed-off-by: Yair Podemsky <ypodemsk@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230110160206.75912-1-ypodemsk@redhat.com

Author: Yair Podemsky

arch/x86/kernel/cpu/aperfmperf.c

@@ -330,7 +330,16 @@ static void __init bp_init_freq_invariance(void)
 
 static void disable_freq_invariance_workfn(struct work_struct *work)
 {
+	int cpu;
+
 	static_branch_disable(&arch_scale_freq_key);
+
+	/*
+	 * Set arch_freq_scale to a default value on all cpus
+	 * This negates the effect of scaling
+	 */
+	for_each_possible_cpu(cpu)
+		per_cpu(arch_freq_scale, cpu) = SCHED_CAPACITY_SCALE;
 }
 
 static DECLARE_WORK(disable_freq_invariance_work,