Mastering Linux PM QoS: How to Optimize Power Management with Constraints

Background and Motivation

QoS (Quality of Service) originally describes network performance guarantees; in Linux power management it is treated as a service whose goal is to minimize power consumption while satisfying latency, throughput, and other performance constraints.

PM QoS Overview and Framework

The kernel provides two QoS frameworks: a system‑level class framework (defined in kernel/power/qos.c) and a per‑device framework (defined in drivers/base/power/qos.c). System‑level constraints historically included cpu&dma latency, network latency, network throughput, and memory bandwidth; in kernel‑5.4 they were reduced to only cpu&dma latency with an added cpufreq constraint.

Typical user‑space interfaces are exposed via misc devices such as /dev/cpu_dma_latency, /dev/network_latency, /dev/network_throughput, and /dev/memory_bandwidth.

Core Data Structures

struct pm_qos_request – handle used for add, update, and remove operations.

struct pm_qos_constraints – represents a specific QoS class and its limits.

struct pm_qos_object – contains a misc device that provides user‑space access to the class.

Important fields: node – a plist_node that stores the request value as a priority, automatically sorted in ascending order so the smallest (or largest) value is at the front of the list. pm_qos_class – enumerates the QoS class. target_value and default_value – the desired constraint value and the initial value (usually 0, meaning no limit). type – indicates whether the class uses minimum, maximum, or sum aggregation. notifiers – callback list invoked when the constraint value changes. list – linked list of all requests for the class.

Primary API Functions

void pm_qos_add_request(struct pm_qos_request *req, int pm_qos_class, s32 value)

– inserts a new request node into the class list and updates the target value.

void pm_qos_update_request(struct pm_qos_request *req, s32 new_value)

– changes the value of an existing request.

void pm_qos_update_request_timeout(struct pm_qos_request *req, s32 new_value, unsigned long timeout_us)

– same as update but with a timeout for delayed changes. void pm_qos_remove_request(struct pm_qos_request *req) – removes a request from the list. void pm_qos_add_notifier(...) and void pm_qos_remove_notifier(...) – register or unregister callbacks that are invoked when a class’s target value changes.

Usage Example

A camera driver requests a CPU_DMA_LATENCY of 50 µs (path: drivers/media/platform/via-camera.c). This forces the cpuidle governor to stay at C2 level (where exit latency is ≤57 µs) instead of deeper C3 states, ensuring the DMA transfer is not corrupted while still saving power.

References

http://www.wowotech.net/tag/pm_qos

Kernel 5.4 source: https://android.googlesource.com/kernel/common/+/refs/heads/android11-5.4

Kernel 4.19 source: https://android.googlesource.com/kernel/common/+/refs/heads/android-4.19-stable