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SubjectRe: [PATCH 2/2] sched: Consider task_struct::saved_state in wait_task_inactive().
Date
On 26/07/22 08:17, Sebastian Andrzej Siewior wrote:
> On 2022-07-25 18:47:58 [+0100], Valentin Schneider wrote:
>> > --- a/kernel/sched/core.c
>> > +++ b/kernel/sched/core.c
>> > @@ -3257,6 +3257,40 @@ int migrate_swap(struct task_struct *cur
>> > }
>> > #endif /* CONFIG_NUMA_BALANCING */
>> >
>> > +#ifdef CONFIG_PREEMPT_RT
>>
>> Would something like the below be useful?
>>
>> /*
>> * If p->saved_state is anything else than TASK_RUNNING, then p blocked on an
>> * rtlock *before* voluntarily calling into schedule() after setting its state
>> * to X. For things like ptrace (X=TASK_TRACED), the task could have more work
>> * to do upon acquiring the lock before whoever called wait_task_inactive()
>> * should return. IOW, we have to wait for:
>> *
>> * p.saved_state = TASK_RUNNING
>> * p.__state = X
>> *
>> * which implies the task isn't blocked on an RT lock and got to schedule() by
>> * itself.
>> *
>> * Also see comments in ttwu_state_match().
>> */
>
> This sums up the code. I would s/schedule/schedule_rtlock/ since there
> are two entrypoints.

Right, this any better?

/*
* Consider:
*
* set_special_state(X);
*
* do_things()
* // Somewhere in there is an rtlock that can be contended:
* current_save_and_set_rtlock_wait_state();
* [...]
* schedule_rtlock(); (A)
* [...]
* current_restore_rtlock_saved_state();
*
* schedule(); (B)
*
* If p->saved_state is anything else than TASK_RUNNING, then p blocked on an
* rtlock (A) *before* voluntarily calling into schedule() (B) after setting its
* state to X. For things like ptrace (X=TASK_TRACED), the task could have more
* work to do upon acquiring the lock in do_things() before whoever called
* wait_task_inactive() should return. IOW, we have to wait for:
*
* p.saved_state = TASK_RUNNING
* p.__state = X
*
* which implies the task isn't blocked on an RT lock and got to schedule() (B).
*
* Also see comments in ttwu_state_match().
*/

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