Re: [RFC PATCH v1 0/9] freezer: Introduce freeze priority model to address process dependency issues

[Michal Hocko <mhocko@suse.com>]

On Thu 07-08-25 20:14:09, Zihuan Zhang wrote:
> The Linux task freezer was designed in a much earlier era, when userspace was relatively simple and flat.
> Over the years, as modern desktop and mobile systems have become increasingly complex—with intricate IPC,
> asynchronous I/O, and deep event loops—the original freezer model has shown its age.

A modern userspace might be more complex or convoluted but I do not
think the above statement is accurate or even correct.

> ## Background
> 
> Currently, the freezer traverses the task list linearly and attempts to freeze all tasks equally.
> It sends a signal and waits for `freezing()` to become true. While this model works well in many cases, it has several inherent limitations:
> 
> - Signal-based logic cannot freeze uninterruptible (D-state) tasks
> - Dependencies between processes can cause freeze retries 
> - Retry-based recovery introduces unpredictable suspend latency
> 
> ## Real-world problem illustration
> 
> Consider the following scenario during suspend:
> 
> Freeze Window Begins
> 
>     [process A] - epoll_wait()
>         │
>         ▼
>     [process B] - event source (already frozen)
> 
> → A enters D-state because of waiting for B

I thought opoll_wait was waiting in interruptible sleep.

> → Cannot respond to freezing signal
> → Freezer retries in a loop
> → Suspend latency spikes
> 
> In such cases, we observed that a normal 1–2ms freezer cycle could balloon to **tens of milliseconds**. 
> Worse, the kernel has no insight into the root cause and simply retries blindly.
> 
> ## Proposed solution: Freeze priority model
> 
> To address this, we propose a **layered freeze model** based on per-task freeze priorities.
> 
> ### Design
> 
> We introduce 4 levels of freeze priority:
> 
> 
> | Priority | Level             | Description                       |
> |----------|-------------------|-----------------------------------|
> | 0        | HIGH              | D-state TASKs                     |
> | 1        | NORMAL            | regular  use space TASKS          |
> | 2        | LOW               | not yet used                      |
> | 4        | NEVER_FREEZE      | zombie TASKs , PF_SUSPNED_TASK    |
> 
> 
> The kernel will freeze processes **in priority order**, ensuring that higher-priority tasks are frozen first.
> This avoids dependency inversion scenarios and provides a deterministic path forward for tricky cases.
> By freezing control or event-source threads first, we prevent dependent tasks from entering D-state prematurely — effectively avoiding dependency inversion.

I really fail to see how that is supposed to work to be honest. If a
process is running in the userspace then the priority shouldn't really
matter much. Tasks will get a signal, freeze themselves and you are
done. If they are running in the userspace and e.g. sleeping while not
TASK_FREEZABLE then priority simply makes no difference. And if they are
TASK_FREEZABLE then the priority doens't matter either.

What am I missing?
-- 
Michal Hocko
SUSE Labs