arm64: Update AMU-based freq scale factor on entering idle

JIRA: https://issues.redhat.com/browse/RHEL-80968

commit 39b1997
Author: Beata Michalska <beata.michalska@arm.com>
Date:   Fri Jan 31 16:24:39 2025 +0000

    arm64: Update AMU-based freq scale factor on entering idle

    Now that the frequency scale factor has been activated for retrieving
    current frequency on a given CPU, trigger its update upon entering
    idle. This will, to an extent, allow querying last known frequency
    in a non-invasive way. It will also improve the frequency scale factor
    accuracy when a CPU entering idle did not receive a tick for a while.
    As a consequence, for idle cores, the reported frequency will be the
    last one observed before entering the idle state.

    Suggested-by: Vanshidhar Konda <vanshikonda@os.amperecomputing.com>
    Signed-off-by: Beata Michalska <beata.michalska@arm.com>
    Reviewed-by: Prasanna Kumar T S M <ptsm@linux.microsoft.com>
    Reviewed-by: Sumit Gupta <sumitg@nvidia.com>
    Link: https://lore.kernel.org/r/20250131162439.3843071-5-beata.michalska@arm.com
    Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Signed-off-by: Jennifer Berringer <jberring@redhat.com>

Author: Jennifer Berringer

arch/arm64/kernel/topology.c

@@ -213,6 +213,19 @@ static __always_inline bool amu_fie_cpu_supported(unsigned int cpu)
 		cpumask_test_cpu(cpu, amu_fie_cpus);
 }
 
+void arch_cpu_idle_enter(void)
+{
+	unsigned int cpu = smp_processor_id();
+
+	if (!amu_fie_cpu_supported(cpu))
+		return;
+
+	/* Kick in AMU update but only if one has not happened already */
+	if (housekeeping_cpu(cpu, HK_TYPE_TICK) &&
+	    time_is_before_jiffies(per_cpu(cpu_amu_samples.last_scale_update, cpu)))
+		amu_scale_freq_tick();
+}
+
 #define AMU_SAMPLE_EXP_MS	20
 
 int arch_freq_get_on_cpu(int cpu)