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

There are two way to get CPU frequency data:

Enabling the power/cpu_frequency ftrace event. (See TraceConfig below). This will record an event every time the in-kernel cpufreq scaling driver changes the frequency. Note that this is not supported on all platforms. In our experience it works reliably on ARM-based SoCs but produces no data on most modern Intel-based platforms. This is because recent Intel CPUs use an internal DVFS which is directly controlled by the CPU, and that doesn‘t expose frequency change events to the kernel. Also note that even on ARM-based platforms, the event is emitted only when a CPU frequency changes. In many cases the CPU frequency won’t change for several seconds, which will show up as an empty block at the start of the trace. We suggest always combining this with polling (below) to get a reliable snapshot of the initial frequency.
Polling sysfs by enabling the linux.sys_stats data source and setting cpufreq_period_ms to a value > 0. This will periodically poll /sys/devices/system/cpu/cpu*/cpufreq/cpuinfo_cur_freq and record the current value in the trace buffer. Works on both Intel and ARM-based platforms.

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.

Available frequencies

It is possible to record one-off also the full list of frequencies supported by each CPU by enabling the linux.system_info data source. This will record /sys/devices/system/cpu/cpu*/cpufreq/scaling_available_frequencies when the trace recording start. This information is typically used to tell apart big/little cores by inspecting the cpu_freq table.

This is not supported on modern Intel platforms for the same aforementioned reasons of power/cpu_frequency.

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:

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.

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

The list of known CPU frequencies, can be queried using the cpu_freq table.

TraceConfig

// Event-driven recording of frequency and idle state changes.
data_sources: {
    config {
        name: "linux.ftrace"
        ftrace_config {
            ftrace_events: "power/cpu_frequency"
            ftrace_events: "power/cpu_idle"
            ftrace_events: "power/suspend_resume"
        }
    }
}

// Polling the current cpu frequency.
data_sources: {
    config {
        name: "linux.sys_stats"
        sys_stats_config {
            cpufreq_period_ms: 500
        }
    }
}

// Reporting the list of available frequency for each CPU.
data_sources {
    config {
        name: "linux.system_info"
    }
}

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.