2004-11-09 10:58:23

by Oleg Nesterov

Subject: [PATCH] little schedule() cleanup: use cached current value

Hello.

schedule() can use prev/next instead of get_current().

Oleg.

Signed-off-by: Oleg Nesterov <[email protected]>

--- 2.6.10-rc1/kernel/sched.c~ 2004-11-08 19:43:29.000000000 +0300
+++ 2.6.10-rc1/kernel/sched.c 2004-11-08 22:52:26.547195920 +0300
@@ -2508,7 +2508,7 @@ need_resched:
* The idle thread is not allowed to schedule!
* Remove this check after it has been exercised a bit.
*/
- if (unlikely(current == rq->idle) && current->state != TASK_RUNNING) {
+ if (unlikely(prev == rq->idle) && prev->state != TASK_RUNNING) {
printk(KERN_ERR "bad: scheduling from the idle thread!\n");
dump_stack();
}
@@ -2531,8 +2531,8 @@ need_resched:

spin_lock_irq(&rq->lock);

- if (unlikely(current->flags & PF_DEAD))
- current->state = EXIT_DEAD;
+ if (unlikely(prev->flags & PF_DEAD))
+ prev->state = EXIT_DEAD;
/*
* if entering off of a kernel preemption go straight
* to picking the next task.
@@ -2636,7 +2636,7 @@ switch_tasks:
} else
spin_unlock_irq(&rq->lock);

- reacquire_kernel_lock(current);
+ reacquire_kernel_lock(next);
preempt_enable_no_resched();
if (unlikely(test_thread_flag(TIF_NEED_RESCHED)))
goto need_resched;

2004-11-09 11:44:02

by Ingo Molnar

Subject: Re: [PATCH] little schedule() cleanup: use cached current value

* Oleg Nesterov <[email protected]> wrote:

> @@ -2636,7 +2636,7 @@ switch_tasks:
> } else
> spin_unlock_irq(&rq->lock);
>
> - reacquire_kernel_lock(current);
> + reacquire_kernel_lock(next);
> preempt_enable_no_resched();
> if (unlikely(test_thread_flag(TIF_NEED_RESCHED)))
> goto need_resched;

nack. We switch the kernel stack in switch_to() so 'next' here is the
old task we switched to before we went off the CPU. The value of 'next'
doesnt get carried over a context-switch. (we do carry 'prev' over via
assembly trickery because we need it, but not 'next'.) This change would
most likely result in rare, hard-to-track-down BKL-related crashes all
around the place.

the other bits are OK, please resend them.

Ingo

2004-11-09 13:26:33

by Oleg Nesterov

Subject: Re: [PATCH] little schedule() cleanup: use cached current value

Ingo Molnar wrote:

> nack. We switch the kernel stack in switch_to() so 'next' here
> is the old task we switched to before we went off the CPU.

yes, sorry. here is corrected patch.

schedule() can use prev instead of get_current().

Signed-off-by: Oleg Nesterov <[email protected]>

--- 2.6.10-rc1/kernel/sched.c~ 2004-11-09 18:57:45.000000000 +0300
+++ 2.6.10-rc1/kernel/sched.c 2004-11-09 19:16:22.798486040 +0300
@@ -2508,7 +2508,7 @@ need_resched:
* The idle thread is not allowed to schedule!
* Remove this check after it has been exercised a bit.
*/
- if (unlikely(current == rq->idle) && current->state != TASK_RUNNING) {
+ if (unlikely(prev == rq->idle) && prev->state != TASK_RUNNING) {
printk(KERN_ERR "bad: scheduling from the idle thread!\n");
dump_stack();
}
@@ -2531,8 +2531,8 @@ need_resched:

spin_lock_irq(&rq->lock);

- if (unlikely(current->flags & PF_DEAD))
- current->state = EXIT_DEAD;
+ if (unlikely(prev->flags & PF_DEAD))
+ prev->state = EXIT_DEAD;
/*
* if entering off of a kernel preemption go straight
* to picking the next task.

2004-11-09 14:30:27

by Ingo Molnar

Subject: Re: [PATCH] little schedule() cleanup: use cached current value

* Oleg Nesterov <[email protected]> wrote:

> Ingo Molnar wrote:
>
> > nack. We switch the kernel stack in switch_to() so 'next' here
> > is the old task we switched to before we went off the CPU.
>
> yes, sorry. here is corrected patch.
>
> schedule() can use prev instead of get_current().
>
> Signed-off-by: Oleg Nesterov <[email protected]>

Acked-by: Ingo Molnar <[email protected]>

Ingo