x86/aperfmperf: Replace aperfmperf_get_khz()

The frequency invariance infrastructure provides the APERF/MPERF samples
already. Utilize them for the cpu frequency display in /proc/cpuinfo.

The sample is considered valid for 20ms. So for idle or isolated NOHZ full
CPUs the function returns 0, which is matching the previous behaviour.

This gets rid of the mass IPIs and a delay of 20ms for stabilizing observed
by Eric when reading /proc/cpuinfo.

Reported-by: Eric Dumazet <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Tested-by: Eric Dumazet <[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/kernel/cpu/aperfmperf.c

···
 	return time_delta <= APERFMPERF_STALE_THRESHOLD_MS;
 }
 
-unsigned int aperfmperf_get_khz(int cpu)
-{
-	if (!cpu_khz)
-		return 0;
-
-	if (!boot_cpu_has(X86_FEATURE_APERFMPERF))
-		return 0;
-
-	if (!housekeeping_cpu(cpu, HK_TYPE_MISC))
-		return 0;
-
-	if (rcu_is_idle_cpu(cpu))
-		return 0; /* Idle CPUs are completely uninteresting. */
-
-	aperfmperf_snapshot_cpu(cpu, ktime_get(), true);
-	return per_cpu(samples.khz, cpu);
-}
-
-void arch_freq_prepare_all(void)
-{
-	ktime_t now = ktime_get();
-	bool wait = false;
-	int cpu;
-
-	if (!cpu_khz)
-		return;
-
-	if (!boot_cpu_has(X86_FEATURE_APERFMPERF))
-		return;
-
-	for_each_online_cpu(cpu) {
-		if (!housekeeping_cpu(cpu, HK_TYPE_MISC))
-			continue;
-		if (rcu_is_idle_cpu(cpu))
-			continue; /* Idle CPUs are completely uninteresting. */
-		if (!aperfmperf_snapshot_cpu(cpu, now, false))
-			wait = true;
-	}
-
-	if (wait)
-		msleep(APERFMPERF_REFRESH_DELAY_MS);
-}
-
 unsigned int arch_freq_get_on_cpu(int cpu)
 {
 	struct aperfmperf_sample *s = per_cpu_ptr(&samples, cpu);
···
 	raw_write_seqcount_end(&s->seq);
 
 	scale_freq_tick(acnt, mcnt);
+}
+
+/*
+ * Discard samples older than the define maximum sample age of 20ms. There
+ * is no point in sending IPIs in such a case. If the scheduler tick was
+ * not running then the CPU is either idle or isolated.
+ */
+#define MAX_SAMPLE_AGE	((unsigned long)HZ / 50)
+
+unsigned int aperfmperf_get_khz(int cpu)
+{
+	struct aperfmperf *s = per_cpu_ptr(&cpu_samples, cpu);
+	unsigned long last;
+	unsigned int seq;
+	u64 acnt, mcnt;
+
+	if (!cpu_feature_enabled(X86_FEATURE_APERFMPERF))
+		return 0;
+
+	do {
+		seq = raw_read_seqcount_begin(&s->seq);
+		last = s->last_update;
+		acnt = s->acnt;
+		mcnt = s->mcnt;
+	} while (read_seqcount_retry(&s->seq, seq));
+
+	/*
+	 * Bail on invalid count and when the last update was too long ago,
+	 * which covers idle and NOHZ full CPUs.
+	 */
+	if (!mcnt || (jiffies - last) > MAX_SAMPLE_AGE)
+		return 0;
+
+	return div64_u64((cpu_khz * acnt), mcnt);
 }
 
 static int __init bp_init_aperfmperf(void)

fs/proc/cpuinfo.c

···
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>
 
-__weak void arch_freq_prepare_all(void)
-{
-}
-
 extern const struct seq_operations cpuinfo_op;
+
 static int cpuinfo_open(struct inode *inode, struct file *file)
 {
-	arch_freq_prepare_all();
 	return seq_open(file, &cpuinfo_op);
 }
 

include/linux/cpufreq.h

···
 			struct cpufreq_governor *old_gov) { }
 #endif
 
-extern void arch_freq_prepare_all(void);
 extern unsigned int arch_freq_get_on_cpu(int cpu);
 
 #ifndef arch_set_freq_scale