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

[David Hildenbrand]

On 18.06.25 13:30, Zihuan Zhang wrote:
> Hi David,
>
> 在 2025/6/16 15:45, David Hildenbrand 写道:
> > > > [...]
> > > In our test scenario, although new processes can indeed be created
> > > during the usleep_range() intervals between freeze iterations, it’s
> > > actually difficult to make the freezer fail outright. This is because
> > > user processes are forcibly frozen: when they return to user space and
> > > check for pending signals, they enter try_to_freeze() and transition
> > > into the refrigerator.
> > >
> > > However, since the scheduler is fair by design, it gives both newly
> > > forked tasks and yet-to-be-frozen tasks a chance to run. This
> > > competition for CPU time can slightly delay the overall freeze process.
> > > While this typically doesn’t lead to failure, it does cause more retries
> > > than necessary, especially under CPU pressure.
> >
> > I think that goes back to my original comment: why are we even
> > allowing fork children to run at all when we are currently freezing
> > all tasks?
> >
> > I would imagine that try_to_freeze_tasks() should force any new
> > processes (forked children) to start in the frozen state directly and
> > not get scheduled in the first place.
> Thanks again for your comments and suggestion.
>
> We understand the motivation behind your idea: ideally, newly forked
> tasks during freezing should either be immediately frozen or prevented
> from running at all, to avoid unnecessary retries and delays. That makes
> perfect sense.
>
> However, implementing this seems non-trivial under the current freezer
> model, as it relies on voluntary transitions and lacks a mechanism to
> block forked children from being scheduled.
>
> Any insights or pointers would be greatly appreciated.

I'm afraid I can't provide too much guidance on scheduler logic.

Apparently we have this freezer_active global that forces existing 
frozen pages to enter the freezing_slow_path().

There, we perform multiple checks, including "pm_freezing && !(p->flags 
& PF_KTHREAD)".

I would have thought that we would want to make fork()/clone() children 
while freezing also result in freezing_slow_path()==true, and stop them 
from getting scheduled in the first place.

Again, no scheduler expert, but that's something I would look into.

--
Cheers,

David / dhildenb