Re: 2.6.12-rc6-mm1

["J.A. Magallon" <jamagallon@able.es>]

On 06.10, Con Kolivas wrote:

> The priority biasing was off by mutliplying the total load by the total 
> priority bias and this ruins the ratio of loads between runqueues. This
> patch should correct the ratios of loads between runqueues to be proportional
> to overall load.
> 

2.6.12-rc6-mm1 + this patch just oopses nicely on boot.
I did not had a digital camera handy, but the first oops that fit in the
screen was this call chain:

kernel_thread_helper
init
init
do:base_setup
usermodehelper_init
__create_workqueue
EIP in try_to_wake_up

After this, there was another with some do_div_error calls...

Something looks un-initialized the first time, or the integer arithmetic
is wrong. I really dont like a*(b/c), I really prefer (a*b)/c. It is more
common b/c == 0 (because b<c), than the possibility of overflowing (a*b).

So I tried both. With this, it boots again:

--- linux-2.6.11-jam24/kernel/sched.c.orig	2005-06-11 00:59:44.000000000 +0200
+++ linux-2.6.11-jam24/kernel/sched.c	2005-06-11 01:03:32.000000000 +0200
@@ -987,9 +987,10 @@
 		 * prevent idle rebalance from trying to pull tasks from a
 		 * queue with only one running task.
 		 */
-		unsigned long prio_bias = rq->prio_bias / rq->nr_running;
+		unsigned long prio_scale = (rq->nr_running > 0 ? 
+										rq->nr_running : 1);
 
-		source_load *= prio_bias;
+		source_load = (source_load*rq->prio_bias) / prio_scale;
 	}
 
 	return source_load;
@@ -1015,9 +1016,10 @@
 		target_load = max(cpu_load, load_now);
 
 	if (idle == NOT_IDLE || rq->nr_running > 1) {
-		unsigned long prio_bias = rq->prio_bias / rq->nr_running;
+		unsigned long prio_scale = (rq->nr_running > 0 ? 
+										rq->nr_running : 1);
 
-		target_load *= prio_bias;
+		target_load = (target_load*rq->prio_bias) / prio_scale;
 	}
 
 	return target_load;


Perhaps this:

	if (idle == NOT_IDLE || rq->nr_running > 1)

should be

	if (idle == NOT_IDLE && rq->nr_running > 1)

???

Hope this helps, thanks.

> Signed-off-by: Con Kolivas <kernel@kolivas.org>
> 
> Index: linux-2.6.12-rc6-mm1/kernel/sched.c
> ===================================================================
> --- linux-2.6.12-rc6-mm1.orig/kernel/sched.c	2005-06-10 23:56:56.000000000 +1000
> +++ linux-2.6.12-rc6-mm1/kernel/sched.c	2005-06-10 23:59:57.000000000 +1000
> @@ -978,7 +978,7 @@ static inline unsigned long __source_loa
>  	else
>  		source_load = min(cpu_load, load_now);
>  
> -	if (idle == NOT_IDLE || rq->nr_running > 1)
> +	if (idle == NOT_IDLE || rq->nr_running > 1) {
>  		/*
>  		 * If we are busy rebalancing the load is biased by
>  		 * priority to create 'nice' support across cpus. When
> @@ -987,7 +987,10 @@ static inline unsigned long __source_loa
>  		 * prevent idle rebalance from trying to pull tasks from a
>  		 * queue with only one running task.
>  		 */
> -		source_load *= rq->prio_bias;
> +		unsigned long prio_bias = rq->prio_bias / rq->nr_running;
> +
> +		source_load *= prio_bias;
> +	}
>  
>  	return source_load;
>  }
> @@ -1011,8 +1014,11 @@ static inline unsigned long __target_loa
>  	else
>  		target_load = max(cpu_load, load_now);
>  
> -	if (idle == NOT_IDLE || rq->nr_running > 1)
> -		target_load *= rq->prio_bias;
> +	if (idle == NOT_IDLE || rq->nr_running > 1) {
> +		unsigned long prio_bias = rq->prio_bias / rq->nr_running;
> +
> +		target_load *= prio_bias;
> +	}
>  
>  	return target_load;
>  }
> 

--
J.A. Magallon <jamagallon()able!es>     \               Software is like sex:
werewolf!able!es                         \         It's better when it's free
Mandriva Linux release 2006.0 (Cooker) for i586
Linux 2.6.11-jam24 (gcc 4.0.0 (4.0.0-3mdk for Mandriva Linux release 2006.0))