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120 lines
4.6 KiB
120 lines
4.6 KiB
# CPU frequency and idle states
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This data source is available on Linux and Android (Since P).
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It records changes in the CPU power management scheme through the
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Linux kernel ftrace infrastructure.
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It involves three aspects:
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#### Frequency scaling
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Records changes in the frequency of a CPU. An event is emitted every time the
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scaling governor scales the CPU frequency up or down.
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On most Android devices the frequency scaling is per-cluster (group of
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big/little cores) so it's not unusual to see groups of four CPUs changing
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frequency at the same time.
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#### idle states
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When no threads are eligible to be executed (e.g. they are all in sleep states)
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the kernel sets the CPU into an idle state, turning off some of the circuitry
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to reduce idle power usage. Most modern CPUs have more than one idle state:
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deeper idle states use less power but also require more time to resume from.
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Note that idle transitions are relatively fast and cheap, a CPU can enter and
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leave idle states hundreds of times in a second.
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Idle-ness must not be confused with full device suspend, which is a stronger and
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more invasive power saving state (See below). CPUs can be idle even when the
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screen is on and the device looks operational.
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The details about how many idle states are available and their semantic is
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highly CPU/SoC specific. At the trace level, the idle state 0 means not-idle,
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values greater than 0 represent increasingly deeper power saving states
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(e.g., single core idle -> full package idle).
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Note that most Android devices won't enter idle states as long as the USB
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cable is plugged in (the USB driver stack holds wakelocks). It is not unusual
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to see only one idle state in traces collected through USB.
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On most SoCs the frequency has little value when the CPU is idle, as the CPU is
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typically clock-gated in idle states. In those cases the frequency in the trace
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happens to be the last frequency the CPU was running at before becoming idle.
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Known issues:
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* The event is emitted only when the frequency changes. This might
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not happen for long periods of times. In short traces
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it's possible that some CPU might not report any event, showing a gap on the
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left-hand side of the trace, or none at all. Perfetto doesn't currently record
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the initial cpu frequency when the trace is started.
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* Currently the UI doesn't render the cpufreq track if idle states (see below)
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are not captured. This is a UI-only bug, data is recorded and query-able
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through trace processor even if not displayed.
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### UI
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In the UI, CPU frequency and idle-ness are shown on the same track. The height
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of the track represents the frequency, the coloring represents the idle
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state (colored: not-idle, gray: idle). Hovering or clicking a point in the
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track will reveal both the frequency and the idle state:
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
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### SQL
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At the SQL level, both frequency and idle states are modeled as counters,
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Note that the cpuidle value 0xffffffff (4294967295) means _back to not-idle_.
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```sql
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select ts, t.name, cpu, value from counter as c
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left join cpu_counter_track as t on c.track_id = t.id
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where t.name = 'cpuidle' or t.name = 'cpufreq'
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```
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ts | name | cpu | value
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---|------|------|------
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261187013242350 | cpuidle | 1 | 0
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261187013246204 | cpuidle | 1 | 4294967295
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261187013317818 | cpuidle | 1 | 0
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261187013333027 | cpuidle | 0 | 0
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261187013338287 | cpufreq | 0 | 1036800
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261187013357922 | cpufreq | 1 | 1036800
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261187013410735 | cpuidle | 1 | 4294967295
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261187013451152 | cpuidle | 0 | 4294967295
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261187013665683 | cpuidle | 1 | 0
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261187013845058 | cpufreq | 0 | 1900800
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### TraceConfig
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```protobuf
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data_sources: {
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config {
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name: "linux.ftrace"
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ftrace_config {
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ftrace_events: "power/cpu_frequency"
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ftrace_events: "power/cpu_idle"
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ftrace_events: "power/suspend_resume"
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}
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}
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}
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```
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### Full-device suspend
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Full device suspend happens when a laptop is put in "sleep" mode (e.g. by
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closing the lid) or when a smartphone display is turned off for enough time.
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When the device is suspended, most of the hardware units are turned off entering
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the highest power-saving state possible (other than full shutdown).
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Note that most Android devices don't suspend immediately after dimming the
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display but tend to do so if the display is forced off through the power button.
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The details are highly device/manufacturer/kernel specific.
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Known issues:
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* The UI doesn't display clearly the suspended state. When an Android device
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suspends it looks like as if all CPUs are running the kmigration thread and
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one CPU is running the power HAL.
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