x86/aperfmperf: Make parts of the frequency invariance code unconditional

The frequency invariance support is currently limited to x86/64 and SMP,
which is the vast majority of machines.

arch_scale_freq_tick() is called every tick on all CPUs and reads the APERF
and MPERF MSRs. The CPU frequency getters function do the same via dedicated
IPIs.

While it could be argued that on systems where frequency invariance support
is disabled (32bit, !SMP) the per tick read of the APERF and MPERF MSRs can
be avoided, it does not make sense to keep the extra code and the resulting
runtime issues of mass IPIs around.

As a first step split out the non frequency invariance specific
initialization code and the read MSR portion of arch_scale_freq_tick(). The
rest of the code is still conditional and guarded with a static key.

Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Rafael J. Wysocki <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Acked-by: Paul E. McKenney <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/include/asm/cpu.h

···
 #endif
 #endif
 
+extern void ap_init_aperfmperf(void);
+
 int mwait_usable(const struct cpuinfo_x86 *);
 
 unsigned int x86_family(unsigned int sig);

arch/x86/include/asm/topology.h

···
 
 extern void arch_set_max_freq_ratio(bool turbo_disabled);
 extern void freq_invariance_set_perf_ratio(u64 ratio, bool turbo_disabled);
-extern void bp_init_freq_invariance(void);
-extern void ap_init_freq_invariance(void);
 #else
 static inline void arch_set_max_freq_ratio(bool turbo_disabled) { }
 static inline void freq_invariance_set_perf_ratio(u64 ratio, bool turbo_disabled) { }
-static inline void bp_init_freq_invariance(void) { }
-static inline void ap_init_freq_invariance(void) { }
 #endif
 
 #ifdef CONFIG_ACPI_CPPC_LIB

arch/x86/kernel/cpu/aperfmperf.c

···
 #include <linux/smp.h>
 #include <linux/syscore_ops.h>
 
+#include <asm/cpu.h>
 #include <asm/cpu_device_id.h>
 #include <asm/intel-family.h>
 
···
 	smp_call_function_single(cpu, aperfmperf_snapshot_khz, NULL, 1);
 
 	return per_cpu(samples.khz, cpu);
+}
+
+static void init_counter_refs(void)
+{
+	u64 aperf, mperf;
+
+	rdmsrl(MSR_IA32_APERF, aperf);
+	rdmsrl(MSR_IA32_MPERF, mperf);
+
+	this_cpu_write(cpu_samples.aperf, aperf);
+	this_cpu_write(cpu_samples.mperf, mperf);
 }
 
 #if defined(CONFIG_X86_64) && defined(CONFIG_SMP)
···
 	return true;
 }
 
-static void init_counter_refs(void)
-{
-	u64 aperf, mperf;
-
-	rdmsrl(MSR_IA32_APERF, aperf);
-	rdmsrl(MSR_IA32_MPERF, mperf);
-
-	this_cpu_write(cpu_samples.aperf, aperf);
-	this_cpu_write(cpu_samples.mperf, mperf);
-}
-
 #ifdef CONFIG_PM_SLEEP
 static struct syscore_ops freq_invariance_syscore_ops = {
 	.resume = init_counter_refs,
···
 	freq_invariance_enable();
 }
 
-void __init bp_init_freq_invariance(void)
+static void __init bp_init_freq_invariance(void)
 {
-	if (!cpu_feature_enabled(X86_FEATURE_APERFMPERF))
-		return;
-
-	init_counter_refs();
-
 	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
 		return;
 
 	if (intel_set_max_freq_ratio())
 		freq_invariance_enable();
-}
-
-void ap_init_freq_invariance(void)
-{
-	if (cpu_feature_enabled(X86_FEATURE_APERFMPERF))
-		init_counter_refs();
 }
 
 static void disable_freq_invariance_workfn(struct work_struct *work)
···
 static void scale_freq_tick(u64 acnt, u64 mcnt)
 {
 	u64 freq_scale;
+
+	if (!arch_scale_freq_invariant())
+		return;
 
 	if (check_shl_overflow(acnt, 2*SCHED_CAPACITY_SHIFT, &acnt))
 		goto error;
···
 	pr_warn("Scheduler frequency invariance went wobbly, disabling!\n");
 	schedule_work(&disable_freq_invariance_work);
 }
+#else
+static inline void bp_init_freq_invariance(void) { }
+static inline void scale_freq_tick(u64 acnt, u64 mcnt) { }
+#endif /* CONFIG_X86_64 && CONFIG_SMP */
 
 void arch_scale_freq_tick(void)
 {
 	struct aperfmperf *s = this_cpu_ptr(&cpu_samples);
 	u64 acnt, mcnt, aperf, mperf;
 
-	if (!arch_scale_freq_invariant())
+	if (!cpu_feature_enabled(X86_FEATURE_APERFMPERF))
 		return;
 
 	rdmsrl(MSR_IA32_APERF, aperf);
···
 
 	scale_freq_tick(acnt, mcnt);
 }
-#endif /* CONFIG_X86_64 && CONFIG_SMP */
+
+static int __init bp_init_aperfmperf(void)
+{
+	if (!cpu_feature_enabled(X86_FEATURE_APERFMPERF))
+		return 0;
+
+	init_counter_refs();
+	bp_init_freq_invariance();
+	return 0;
+}
+early_initcall(bp_init_aperfmperf);
+
+void ap_init_aperfmperf(void)
+{
+	if (cpu_feature_enabled(X86_FEATURE_APERFMPERF))
+		init_counter_refs();
+}

arch/x86/kernel/smpboot.c

···
 	 */
 	set_cpu_sibling_map(raw_smp_processor_id());
 
-	ap_init_freq_invariance();
+	ap_init_aperfmperf();
 
 	/*
 	 * Get our bogomips.
···
 {
 	smp_prepare_cpus_common();
 
-	bp_init_freq_invariance();
 	smp_sanity_check();
 
 	switch (apic_intr_mode) {