Re: [PATCH v4] PM / Freezer: Skip zombie/dead processes to reduce freeze latency

[Zihuan Zhang]

Hi Peter,

在 2025/7/17 02:36, Peter Zijlstra 写道:
> On Wed, Jul 16, 2025 at 06:38:55PM +0200, Oleg Nesterov wrote:
> > On 07/16, Zihuan Zhang wrote:
> > > @@ -51,7 +51,15 @@ static int try_to_freeze_tasks(bool user_only)
> > >   		todo = 0;
> > >   		read_lock(&tasklist_lock);
> > >   		for_each_process_thread(g, p) {
> > > -			if (p == current || !freeze_task(p))
> > > +			/*
> > > +			 * Zombie and dead tasks are not running anymore and cannot enter
> > > +			 * the __refrigerator(). Skipping them avoids unnecessary freeze attempts.
> > > +			 *
> > > +			 * TODO: Consider using PF_NOFREEZE instead, which may provide
> > > +			 * a more generic exclusion mechanism for other non-freezable tasks.
> > > +			 * However, for now, exit_state is sufficient to skip user processes.
> > I don't really understand the comment... The freeze_task() paths already
> > consider PF_NOFREEZE, although we can check it earlier as Peter suggests.
> Right; I really don't understand why we should special case
> ->exit_state. Why not DTRT and optimize NOFREEZE if all this really
> matters (smalls gains from what ISTR from the previous discussion).

The main reason we didn’t rely directly on PF_NOFREEZE is that it’s a 
mutable flag — in some cases, it can be cleared later, which makes early 
skipping potentially unsafe.

In contrast, exit_state is stable and skipping tasks based on it is safe.


Also, the previous version of the patch you shared might allow some 
paths to bypass lock_system_sleep(), which could break the intended 
protection.