perf_counter: More aggressive frequency adjustment

Also employ the overflow handler to adjust the frequency, this results
in a stable frequency in about 40~50 samples, instead of that many ticks.

This also means we can start sampling at a sample period of 1 without
running head-first into the throttle.

It relies on sched_clock() to accurately measure the time difference
between the overflow NMIs.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/cpu/perf_counter.c

@@ -696,10 +696,11 @@ static int __hw_perf_counter_init(struct perf_counter *counter)
 	if (!attr->exclude_kernel)
 		hwc->config |= ARCH_PERFMON_EVENTSEL_OS;
 
-	if (!hwc->sample_period)
+	if (!hwc->sample_period) {
 		hwc->sample_period = x86_pmu.max_period;
+		atomic64_set(&hwc->period_left, hwc->sample_period);
+	}
 
-	atomic64_set(&hwc->period_left, hwc->sample_period);
 	counter->destroy = hw_perf_counter_destroy;
 
 	/*