Zombie processes are not subject to freezing, but they are still part of
the global task list. During freeze traversal, tasks are examined for
priority and eligibility, which may involve unnecessary locking even for
non-freezable tasks like zombies.

This patch assigns a default freeze priority to zombie tasks during exit,
so that the freezer can skip priority setup and locking for them in
subsequent iterations.

This helps reduce overhead during freeze traversal, especially when many
zombie processes exist in the system.

Signed-off-by: Zihuan Zhang <zhangzihuan@kylinos.cn>
---
 kernel/sched/core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index be00629f0ba4..5a26d7511047 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -42,6 +42,7 @@
 #include <linux/context_tracking.h>
 #include <linux/cpuset.h>
 #include <linux/delayacct.h>
+#include <linux/freezer.h>
 #include <linux/init_task.h>
 #include <linux/interrupt.h>
 #include <linux/ioprio.h>
@@ -6980,6 +6981,7 @@ void __noreturn do_task_dead(void)
 	current->flags |= PF_NOFREEZE;
 
 	__schedule(SM_NONE);
+	freeze_set_default_priority(current, FREEZE_PRIORITY_NEVER);
 	BUG();
 
 	/* Avoid "noreturn function does return" - but don't continue if BUG() is a NOP: */
-- 
2.25.1

On 08/07, Zihuan Zhang wrote:
>
> @@ -6980,6 +6981,7 @@ void __noreturn do_task_dead(void)
>  	current->flags |= PF_NOFREEZE;
>
>  	__schedule(SM_NONE);
> +	freeze_set_default_priority(current, FREEZE_PRIORITY_NEVER);
>  	BUG();

But this change has no effect?

Firstly, this last __schedule() should not return, note the BUG() we have.

Secondly, this zombie is already PF_NOFREEZE, freeze_task() will return
false anyway.

Oleg.

在 2025/8/8 22:29, Oleg Nesterov 写道:
> On 08/07, Zihuan Zhang wrote:
>> @@ -6980,6 +6981,7 @@ void __noreturn do_task_dead(void)
>>   	current->flags |= PF_NOFREEZE;
>>
>>   	__schedule(SM_NONE);
>> +	freeze_set_default_priority(current, FREEZE_PRIORITY_NEVER);
>>   	BUG();
> But this change has no effect?
>
> Firstly, this last __schedule() should not return, note the BUG() we have.
>
> Secondly, this zombie is already PF_NOFREEZE, freeze_task() will return
> false anyway.

Thanks for pointing that out.
Indeed, I’ve noticed that in the current position the code has no effect.
If we move this code to a more appropriate place, it should improve both 
safety and usefulness compared to the previous implementation.

> Oleg.
>

在 2025/8/8 22:29, Oleg Nesterov 写道:
> On 08/07, Zihuan Zhang wrote:
>> @@ -6980,6 +6981,7 @@ void __noreturn do_task_dead(void)
>>   	current->flags |= PF_NOFREEZE;
>>
>>   	__schedule(SM_NONE);
>> +	freeze_set_default_priority(current, FREEZE_PRIORITY_NEVER);
>>   	BUG();
> But this change has no effect?
>
> Firstly, this last __schedule() should not return, note the BUG() we have.
>
> Secondly, this zombie is already PF_NOFREEZE, freeze_task() will return
> false anyway.
Sorry, but in our tests with a large number of zombie tasks, returning 
early reduced the overhead. Even though freeze_task() would return false 
for PF_NOFREEZE, skipping the extra path still saved time in our 
suspend/freezer loop.
> Oleg.
>

On 08/11, Zihuan Zhang wrote:
> 
> 在 2025/8/8 22:29, Oleg Nesterov 写道:
> >On 08/07, Zihuan Zhang wrote:
> >>@@ -6980,6 +6981,7 @@ void __noreturn do_task_dead(void)
> >>  	current->flags |= PF_NOFREEZE;
> >>
> >>  	__schedule(SM_NONE);
> >>+	freeze_set_default_priority(current, FREEZE_PRIORITY_NEVER);
> >>  	BUG();
> >But this change has no effect?
> >
> >Firstly, this last __schedule() should not return, note the BUG() we have.
> >
> >Secondly, this zombie is already PF_NOFREEZE, freeze_task() will return
> >false anyway.
> Sorry, but in our tests with a large number of zombie tasks, returning early
> reduced the overhead. Even though freeze_task() would return false for
> PF_NOFREEZE, skipping the extra path still saved time in our suspend/freezer

https://lore.kernel.org/all/20250707084214.GD1613200@noisy.programming.kicks-ass.net/

Anyway the patch makes no sense in its current form, see my note
about __schedule() above.

Oleg.