On 03/03/16 18:26, Peter Zijlstra wrote:

On Thu, Mar 03, 2016 at 05:28:55PM +0000, Dietmar Eggemann wrote:

quoted

quoted

+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);
+
+	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));

Wasn't there the problem that this ratio goes to zero if the cpu is idle
in the old power estimation approach on x86?

Yeah, there was something funky.

SDM says they only count in C0 (ie. !idle), so it _should_ work.

I see, back than the problem was 0 capacity in idle but this is about
frequency.