# CPU frequency and idle states

This data source is available on Linux and Android (Since P).
It records changes in the CPU power management scheme through the
Linux kernel ftrace infrastructure.
It involves three aspects:

#### Frequency scaling

Records changes in the frequency of a CPU. An event is emitted every time the
scaling governor scales the CPU frequency up or down.

On most Android devices the frequency scaling is per-cluster (group of
big/little cores) so it's not unusual to see groups of four CPUs changing
frequency at the same time.

#### idle states

When no threads are eligible to be executed (e.g. they are all in sleep states)
the kernel sets the CPU into an idle state, turning off some of the circuitry
to reduce idle power usage. Most modern CPUs have more than one idle state:
deeper idle states use less power but also require more time to resume from.

Note that idle transitions are relatively fast and cheap, a CPU can enter and
leave idle states hundreds of times in a second.
Idle-ness must not be confused with full device suspend, which is a stronger and
more invasive power saving state (See below). CPUs can be idle even when the
screen is on and the device looks operational.

The details about how many idle states are available and their semantic is
highly CPU/SoC specific. At the trace level, the idle state 0 means not-idle,
values greater than 0 represent increasingly deeper power saving states
(e.g., single core idle -> full package idle).

Note that most Android devices won't enter idle states as long as the USB
cable is plugged in (the USB driver stack holds wakelocks). It is not unusual
to see only one idle state in traces collected through USB.

On most SoCs the frequency has little value when the CPU is idle, as the CPU is
typically clock-gated in idle states. In those cases the frequency in the trace
happens to be the last frequency the CPU was running at before becoming idle.

Known issues:

* The event is emitted only when the frequency changes. This might
  not happen for long periods of times. In short traces
  it's possible that some CPU might not report any event, showing a gap on the
  left-hand side of the trace, or none at all. Perfetto doesn't currently record
  the initial cpu frequency when the trace is started.

* Currently the UI doesn't render the cpufreq track if idle states (see below)
  are not captured. This is a UI-only bug, data is recorded and query-able
  through trace processor even if not displayed.

### UI

In the UI, CPU frequency and idle-ness are shown on the same track. The height
of the track represents the frequency, the coloring represents the idle
state (colored: not-idle, gray: idle). Hovering or clicking a point in the
track will reveal both the frequency and the idle state:
  
![](/docs/images/cpu-frequency.png "CPU frequency and idle states in the UI")

### SQL

At the SQL level, both frequency and idle states are modeled as counters,
Note that the cpuidle value 0xffffffff (4294967295) means _back to not-idle_.

```sql
select ts, t.name, cpu, value from counter as c
left join cpu_counter_track as t on c.track_id = t.id
where t.name = 'cpuidle' or t.name = 'cpufreq'
```

ts | name | cpu | value
---|------|------|------
261187013242350 | cpuidle | 1 | 0
261187013246204 | cpuidle | 1 | 4294967295
261187013317818 | cpuidle | 1 | 0
261187013333027 | cpuidle | 0 | 0
261187013338287 | cpufreq | 0 | 1036800
261187013357922 | cpufreq | 1 | 1036800
261187013410735 | cpuidle | 1 | 4294967295
261187013451152 | cpuidle | 0 | 4294967295
261187013665683 | cpuidle | 1 | 0
261187013845058 | cpufreq | 0 | 1900800

### TraceConfig

```protobuf
data_sources: {
    config {
        name: "linux.ftrace"
        ftrace_config {
            ftrace_events: "power/cpu_frequency"
            ftrace_events: "power/cpu_idle"
            ftrace_events: "power/suspend_resume"
        }
    }
}
```

### Full-device suspend

Full device suspend happens when a laptop is put in "sleep" mode (e.g. by
closing the lid) or when a smartphone display is turned off for enough time.

When the device is suspended, most of the hardware units are turned off entering
the highest power-saving state possible (other than full shutdown).

Note that most Android devices don't suspend immediately after dimming the
display but tend to do so if the display is forced off through the power button.
The details are highly device/manufacturer/kernel specific.

Known issues:

* The UI doesn't display clearly the suspended state. When an Android device
  suspends it looks like as if all CPUs are running the kmigration thread and
  one CPU is running the power HAL.