[PATCH v1 2/3] arm64: idle: Cache AMU counters before entering idle

Vanshidhar Konda
2024-02-29T16:25:34Z

AMU counters do not increment while a CPU is in idle. Saving the value
of the core and constant counters prior to invoking WFI allows FIE to
compute the frequency of a CPU that is idle.

Signed-off-by: Vanshidhar Konda <vanshikonda@os.amperecomputing.com>
---
arch/arm64/kernel/idle.c | 10 ++++++++++
arch/arm64/kernel/topology.c | 14 ++++++++------
2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/idle.c b/arch/arm64/kernel/idle.c
index 05cfb347ec26..5ed2e57188a8 100644
--- a/arch/arm64/kernel/idle.c
+++ b/arch/arm64/kernel/idle.c
@@ -26,6 +26,16 @@ void __cpuidle cpu_do_idle(void)

arm_cpuidle_save_irq_context(&context);

+#ifdef CONFIG_ARM64_AMU_EXTN
+ /* Update the AMU counters before entering WFI. The cached AMU counter
+ * value is used to determine CPU frequency while the CPU is idle
+ * without needing to wake up the CPU.
+ */
+
+ if (cpu_has_amu_feat(smp_processor_id()))
+ update_freq_counters_refs();
+#endif
+
dsb(sy);
wfi();

diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index db8d14525cf4..8905eb0c681f 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -240,13 +240,15 @@ unsigned int arch_freq_get_on_cpu(int cpu)
} while (read_seqcount_retry(&cpu_sample->seq, seq));

/*
- * Bail on invalid count and when the last update was too long ago,
- * which covers idle and NOHZ full CPUs.
+ * Bail on invalid count and when the last update was too long ago.
+ * This covers idle, NOHZ full and isolated CPUs.
+ *
+ * Idle CPUs don't need to be measured because AMU counters stop
+ * incrementing during WFI/WFE.
*/
- if (!delta_const_cnt || ((jiffies - last) > MAX_SAMPLE_AGE)) {
- if (!(housekeeping_cpu(cpu, HK_TYPE_TICK) && idle_cpu(cpu)))
- goto fallback;
- }
+ if (!delta_const_cnt ||
+ ((jiffies - last) > MAX_SAMPLE_AGE && !idle_cpu(cpu)))
+ goto fallback;

/*
* CPU frequency = reference perf (in Hz) * (/\ delivered) / (/\ reference)
--
2.43.1