[linux 6.12-y] uosste feature - Add wmark-step

[just-essential]

Summary by Sourcery

Add a configurable watermark stepping mechanism to memory cgroups and the page allocator to allow fine-tuned reserve adjustments per cgroup and task context.

New Features:

• Introduce CONFIG_WMARK_STEP option to enable watermark stepping across the kernel.
• Expose a new memory.wmark_step cgroup file for per-cgroup watermark step configuration.
• Add a new /sys/kernel/mm/wmark_step sysfs group with attributes for enabling the feature and tuning kthread step size.

Enhancements:

• Propagate wmark_step values in mem_cgroup allocation and inheritance.
• Adjust zone watermarks in __zone_watermark_ok based on wmark_step and a new ALLOC_WMARK_STEP flag.
• Implement get_wmark_step_value helper to select the correct step value for the current task.

Build:

• Add Kconfig entries for CONFIG_WMARK_STEP and related parameters.

[sourcery-ai]

Reviewer's Guide

This patch adds a configurable watermark-step mechanism that lets each memcg and the core allocator adjust zone watermark minima by a programmable step size, exposing controls via cgroup files and a sysfs group, and weaving the new ALLOC_WMARK_STEP flag into the watermark and allocation logic.

ER diagram for mem_cgroup and wmark_step sysfs/cgroup files

erDiagram
    MEM_CGROUP ||--o{ MEMORY_FILES : exposes
    MEMORY_FILES {
        string name
        int flags
        function seq_show
        function write
    }
    MEM_CGROUP {
        int swappiness
        int wmark_step
    }
    SYSFS_GROUP ||--o{ WMARK_STEP_ATTRS : contains
    WMARK_STEP_ATTRS {
        string name
        function show
        function store
    }
    SYSFS_GROUP {
        string name
        attribute[] attrs
    }
    MEM_CGROUP ||--o{ SYSFS_GROUP : has

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Class diagram for mem_cgroup and wmark_step integration

classDiagram
    class mem_cgroup {
        int swappiness
        #ifdef CONFIG_WMARK_STEP
        int wmark_step
        #endif
        struct cgroup_file events_file
        struct cgroup_file events_local_file
    }

    class cgroup_subsys_state
    mem_cgroup --|> cgroup_subsys_state

    class kernfs_open_file
    class seq_file

    mem_cgroup : +memory_wmark_step_show(seq_file*, void*)
    mem_cgroup : +memory_wmark_step_write(kernfs_open_file*, char*, size_t, loff_t)
    mem_cgroup : +get_wmark_step_value()

    class kobject
    class kobj_attribute
    kobject : +wmark_step_kthread_show()
    kobject : +wmark_step_kthread_store()
    kobject : +wmark_step_enabled_show()
    kobject : +wmark_step_enabled_store()

    class attribute_group
    attribute_group : +wmark_step_attrs[]
    attribute_group : +wmark_step_attr_group

    class page_alloc
    page_alloc : +__zone_watermark_ok()
    page_alloc : +gfp_to_alloc_flags()
    page_alloc : +__setup_per_zone_wmarks()

    class internal_h
    internal_h : +ALLOC_WMARK_STEP
    internal_h : +wmark_step_enable
    internal_h : +wmark_step_max
    internal_h : +wmark_step_irq
    internal_h : +wmark_step_kthread
    internal_h : +wmark_step_default
    internal_h : +wmark_step_size
    internal_h : +get_wmark_step_value()

Loading

File-Level Changes

Change	Details	Files
Extend mem_cgroup to carry a wmark_step value	Add int wmark_step field to struct mem_cgroup Inherit parent’s wmark_step or initialize default in mem_cgroup_css_alloc Guard additions under CONFIG_WMARK_STEP	mm/memcontrol.c include/linux/memcontrol.h
Expose per-cgroup wmark_step via cgroupfs	Implement memory_wmark_step_show sequence file Implement memory_wmark_step_write with parsing and tree propagation Add wmark_step entry in memory_files cftype array	mm/memcontrol.c
Provide dynamic lookup of wmark_step for current task	Add get_wmark_step_value() handling IRQ, kthread, disabled cases Look up mem_cgroup via RCU and return its wmark_step	mm/memcontrol.c mm/internal.h
Create sysfs group under mm_kobj for global wmark_step settings	Define wmark_step_enabled and wmark_step_kthread kobj_attributes Assemble wmark_step_attr_group Invoke sysfs_create_group in mem_cgroup_init	mm/memcontrol.c
Define and expose wmark_step parameters in the allocator	Declare globals wmark_step_enable, wmark_step_max, wmark_step_irq, wmark_step_kthread, wmark_step_default, wmark_step_size Add externs to internal.h	mm/page_alloc.c mm/internal.h
Integrate ALLOC_WMARK_STEP into watermark and allocation logic	Modify __zone_watermark_ok to subtract step reserve when ALLOC_WMARK_STEP is set Introduce ALLOC_WMARK_STEP flag and add it in gfp_to_alloc_flags if enabled Include wmark_step reserve in __setup_per_zone_wmarks for ZONE_NORMAL	mm/page_alloc.c

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[Copilot]

Pull Request Overview

This PR adds a configurable watermark stepping mechanism to memory cgroups and the page allocator to enable fine-tuned reserve adjustments per cgroup and task context.

• Introduces new global watermark step parameters and a CONFIG_WMARK_STEP option.
• Modifies allocation logic to adjust watermarks based on context (e.g., ALLOC_RESERVES vs. ALLOC_WMARK_STEP).
• Adds a sysfs group (wmark_step) and new cgroup file to allow runtime configuration of watermark stepping.

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
mm/page_alloc.c	Adds watermark step global variables and adjusts __zone_watermark_ok logic.
mm/memcontrol.c	Introduces get_wmark_step_value and sysfs support for configuring watermark step.
mm/internal.h	Declares watermark step symbols and ALLOC_WMARK_STEP flag.
mm/Kconfig	Adds configuration option for watermark stepping.
include/linux/memcontrol.h	Exposes wmark_step field within mem_cgroup.
arch//configs/_defconfig	Enables CONFIG_WMARK_STEP in various architecture specific default configs.

Comments suppressed due to low confidence (3)

mm/page_alloc.c:3180

• Consider adding a comment here to explain the fallback behavior when free pages (min) are insufficient to apply the watermark step (min <= pages_wmark_step).

		if (unlikely(min <= pages_wmark_step)) {

mm/memcontrol.c:4292

• It would be beneficial to add a function comment for get_wmark_step_value to explain its context-specific behavior (IRQ, kthread, and process) and the rationale behind the chosen return values.

int get_wmark_step_value(void)

mm/memcontrol.c:4991

• [nitpick] Consider adding a logging statement or more detailed documentation in this sysfs store function to clarify the behavior when an invalid wmark_step value is provided, which could aid in debugging configuration issues.

	if (value > wmark_step_max)