title: Perf doesn't add up to 100%
body: <p>I am trying to measure the cpu usage of my Android mobile phone using perf tool. Since my phone (Redmi Note 3) doesn't have perf tool. I used simpleperf tool provided by google. I recorded system wide cpu usage using <code>perf record -e cpu-cycles -a</code>. The cpu usage overhead reported by perf must add to 100% but cpu usage report I got from my phone only adds up to 12% after 10 minutes of profiling. I profiled while putting my phone on idle with all applications killed. </p>
  
  <p>And also when I profiled for 1 minute it added to about 70%.</p>
  
  <p>Is it abnormal? or doesn't perf report idle time of cpu so I only have 12% of cpu usage left?</p>