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From: Yong Zhang on 19 May 2010 09:20
On Wed, May 19, 2010 at 02:57:11PM +0200, Oleg Nesterov wrote:
> - 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(a)redhat.com>

This make it more clear now. Thank you Oleg.

Acked-by: Yong Zhang <yong.zhang0(a)gmail.com>

> ---
>
> 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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