Re: [PATCH] schedule: simplify the reacquire_kernel_lock() logic

[Yong Zhang <yong.zhang0@gmail.com>]

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@redhat.com>

This make it more clear now. Thank you Oleg.

Acked-by: Yong Zhang <yong.zhang0@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())