On Thu, Mar 03, 2016 at 05:28:29PM +0100, Peter Zijlstra wrote:

+void arch_scale_freq_tick(void)
+{
+	u64 aperf, mperf;
+	u64 acnt, mcnt;
+
+	if (!static_cpu_has(X86_FEATURE_APERFMPERF))
+		return;
+
+	aperf = rdmsrl(MSR_IA32_APERF);
+	mperf = rdmsrl(MSR_IA32_APERF);

Actually reading MPERF increases the chances of this working.

+
+	acnt = aperf - this_cpu_read(arch_prev_aperf);
+	mcnt = mperf - this_cpu_read(arch_prev_mperf);
+
+	this_cpu_write(arch_prev_aperf, aperf);
+	this_cpu_write(arch_prev_mperf, mperf);
+
+	this_cpu_write(arch_cpu_freq, div64_u64(acnt * SCHED_CAPACITY_SCALE, mcnt));
+}