Re: [PATCH] kernel <linux-2.6.11.10> kernel/sched.c

[Nick Piggin <nickpiggin@yahoo.com.au>]

chen Shang wrote:

> I minimized my patch and against to 2.6.12-rc4 this time, see below.
>
> The new schedstat fields are for the test propose only, so I removed
> them completedly from patch. Theoritically, requeue_task() is always
> cheaper than dequeue_task() followed by enqueue_task(). So, if 99% of
> priority recalculation trigger requeue_task(), it will save.
>
> In addition, my load is to build the kernel, which took around 30
> minutes with around 30% CPU usage on 2x2 processors (duel processors
> with HT enable).
> Here is the statistics:
>         
> CPU0: priority_changed (669 times), priority_unchanged(335,138 times)
> CPU1: priority_changed (784 times), priority_unchanged(342,419 times)
> CPU2: priority_changed (782 times), priority_unchanged(283,494 times)
> CPU3: priority_changed (872 times), priority_unchanged(365,865 times)

OK that gives you a good grounds to look at the patch, but _performance_
improvement is what is needed to get it included.

> Thanks,
> -chen
>
>
> /*=====Patch=====*/
> --- linux-2.6.12-rc4.orig/kernel/sched.c	2005-05-19 14:57:55.000000000 -0700
> +++ linux-2.6.12-rc4/kernel/sched.c	2005-05-19 23:47:22.000000000 -0700
> @@ -2613,7 +2613,7 @@
> 	struct list_head *queue;
> 	unsigned long long now;
> 	unsigned long run_time;
> -	int cpu, idx;
> +	int cpu, idx, prio;
>
> 	/*
> 	 * Test if we are atomic.  Since do_exit() needs to call into
> @@ -2735,9 +2735,17 @@
> 			delta = delta * (ON_RUNQUEUE_WEIGHT * 128 / 100) / 128;
>
> 		array = next->array;
> -		dequeue_task(next, array);
> +		prio = next->prio;
> +		
> 		recalc_task_prio(next, next->timestamp + delta);
> -		enqueue_task(next, array);
> +
> +		if (unlikely(prio != next->prio))
> +		{
> +			dequeue_task(next, array);
> +			enqueue_task(next, array);
> +		}
> +		else
> +			requeue_task(next, array);

Coding style says
if (unlikely()) {
   ...
} else
   ...


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