- 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@xxxxxxxxxx>
---
kernel/sched.c | 13 ++++++-------
1 file changed, 6 insertions(+), 7 deletions(-)
--- 34-rc1/kernel/sched.c~SCHEDULE_PREV_EQ_TO_CURRENT 2010-05-18 23:32:50.000000000 +0200
+++ 34-rc1/kernel/sched.c 2010-05-19 14:32:57.000000000 +0200
@@ -3679,7 +3679,6 @@ need_resched:
rq = cpu_rq(cpu);
rcu_sched_qs(cpu);
prev = rq->curr;
- switch_count = &prev->nivcsw;
release_kernel_lock(prev);
need_resched_nonpreemptible:
@@ -3693,6 +3692,7 @@ need_resched_nonpreemptible:
update_rq_clock(rq);
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;
@@ -3719,8 +3719,10 @@ 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);
@@ -3729,11 +3731,8 @@ 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())
--
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