Re: [RFC PATCH] PM: Optionally block user fork during freeze to improve performance

[Zihuan Zhang]

Hi David,
Thanks for your advice!

在 2025/6/10 18:50, David Hildenbrand 写道:
> 　　　 　 　 　　 　 　 　 　 　 　 　 　 　 　　
> Can't this problem be mitigated by simply not scheduling the new fork'ed
> process while the system is frozen?
>
> Or what exact scenario are you worried about?

Let me revisit the core issue for clarity. Under normal conditions, most 
processes in the system are in a sleep state, and only a few are 
runnable. So even with thousands of processes, the freezer generally 
works reliably and completes within a reasonable time
However, in our fork-based test scenario, we observed repeated freeze 
retries. This is not due to process count directly, but rather due to a 
scheduling behavior during the freeze phase. Specifically, the freezer 
logic contains the following snippet:
Here is the relevant freezer code that introduces the yield:
* We need to retry, but first give the freezing tasks some * time to 
enter the refrigerator. Start with an initial * 1 ms sleep followed by 
exponential backoff until 8 ms. */ usleep_range(sleep_usecs / 2, 
sleep_usecs); if (sleep_usecs < 8 * USEC_PER_MSEC) sleep_usecs *= 2;
This mechanism is usually effective because most tasks are sleeping and 
quickly enter the frozen state. But with concurrent fork() bombs, we 
observed that this CPU relinquish gives new child processes a chance to 
run, delaying or blocking the freezer's progress.
When only a single fork loop is running, it’s often frozen before the 
next retry. But when multiple forkers compete for CPU, we observed an 
increase in the todo count and repeated retries.
So while preventing the scheduling of newly forked processes would solve 
the problem at its root, it would require deeper architectural changes 
(e.g., task-level flags or restrictions at the scheduler level).
We initially considered whether replacing usleep_range() with a 
non-yielding wait might reduce this contention window. However, this 
approach turned out to be counterproductive — it starves other normal 
user tasks that need CPU time to reach their try_to_freeze() checkpoint, 
ultimately making the freeze process slower .
You’re right — blocking fork() is quite intrusive, so it’s worth 
exploring alternatives. We’ll try implementing your idea of preventing 
the newly forked process from being scheduled while the system is 
freezing, rather than failing the fork() call outright.

This may allow us to maintain compatibility with existing userspace 
while avoiding interference with the freezer traversal. We’ll evaluate 
whether this approach can reliably mitigate the issue (especially the 
scheduling race window between copy_process() and freeze_task()), and 
report back with results.

Best regards,
Zihuan Zhang