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sched: Simplify the reacquire_kernel_lock() logic 

- Contrary to what 6d558c3a says, there is no need to reload
  prev = rq->curr after the context switch. You always schedule
  back to where you came from, prev must be equal to current
  even if cpu/rq was changed.

- This also means reacquire_kernel_lock() can use prev instead
  of current.

- No need to reassign switch_count if reacquire_kernel_lock()
  reports need_resched(), we can just move the initial assignment
  down, under the "need_resched_nonpreemptible:" label.

- Try to update the comment after context_switch().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100519125711.GA30199@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Oleg Nesterov 2010-05-19 14:57:11 +02:00 committed by Ingo Molnar
parent c676329abb
commit 246d86b518
1 changed files with 6 additions and 7 deletions
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13
kernel/sched.c
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@ -3636,7 +3636,6 @@ need_resched:
rq = cpu_rq(cpu);
rcu_note_context_switch(cpu);
prev = rq->curr;
switch_count = &prev->nivcsw;
release_kernel_lock(prev);
need_resched_nonpreemptible:
@ -3649,6 +3648,7 @@ need_resched_nonpreemptible:
raw_spin_lock_irq(&rq->lock);
clear_tsk_need_resched(prev);
switch_count = &prev->nivcsw;
if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
if (unlikely(signal_pending_state(prev->state, prev))) {
prev->state = TASK_RUNNING;
@ -3689,8 +3689,10 @@ need_resched_nonpreemptible:
context_switch(rq, prev, next); /* unlocks the rq */
/*
* the context switch might have flipped the stack from under
* us, hence refresh the local variables.
* The context switch have flipped the stack from under us
* and restored the local variables which were saved when
* this task called schedule() in the past. prev == current
* is still correct, but it can be moved to another cpu/rq.
*/
cpu = smp_processor_id();
rq = cpu_rq(cpu);
@ -3699,11 +3701,8 @@ need_resched_nonpreemptible:
post_schedule(rq);
if (unlikely(reacquire_kernel_lock(current) < 0)) {
prev = rq->curr;
switch_count = &prev->nivcsw;
if (unlikely(reacquire_kernel_lock(prev)))
goto need_resched_nonpreemptible;
}
preempt_enable_no_resched();
if (need_resched())