Re: [PATCH] eliminate a truckload of context switches

[Rik van Riel]

On Tue, 8 May 2001, Mike Galbraith wrote:

> While running a ktrace enabled kernel (IKD), I noticed many useless
> context switches.  The problem is that we continually pester kswapd/
> kflushd at times when they can't do anything other than go back to
> sleep.  As you'll see below, we do this quite a bit under heavy load.

I agree, both with your analysis that the context switches
aren't needed and with the patch you sent to fix it.

Linus, if it isn't in the kernel yet, please put it in...

thanks,

Rik

> --- linux-2.4.4.ikd/mm/vmscan.c.org	Tue May  8 10:54:33 2001
> +++ linux-2.4.4.ikd/mm/vmscan.c	Tue May  8 11:16:17 2001
> @@ -964,7 +964,7 @@
> 
>  void wakeup_kswapd(void)
>  {
> -	if (current != kswapd_task)
> +	if (waitqueue_active(&kswapd_wait))
>  		wake_up_process(kswapd_task);
>  }
> 
> --- linux-2.4.4.ikd/fs/buffer.c.org	Tue May  8 10:55:33 2001
> +++ linux-2.4.4.ikd/fs/buffer.c	Tue May  8 11:15:22 2001
> @@ -2578,16 +2578,16 @@
>  	return flushed;
>  }
> 
> +DECLARE_WAIT_QUEUE_HEAD(kflushd_wait);
>  struct task_struct *bdflush_tsk = 0;
> 
>  void wakeup_bdflush(int block)
>  {
> -	if (current != bdflush_tsk) {
> +	if (waitqueue_active(&kflushd_wait))
>  		wake_up_process(bdflush_tsk);
> 
> -		if (block)
> -			flush_dirty_buffers(0);
> -	}
> +	if (block)
> +		flush_dirty_buffers(0);
>  }
> 
>  /*
> @@ -2711,14 +2711,10 @@
>  		 * skip the sleep and flush some more. Otherwise, we
>  		 * go to sleep waiting a wakeup.
>  		 */
> -		set_current_state(TASK_INTERRUPTIBLE);
>  		if (!flushed || balance_dirty_state(NODEV) < 0) {
>  			run_task_queue(&tq_disk);
> -			schedule();
> +			interruptible_sleep_on(&kflushd_wait);
>  		}
> -		/* Remember to mark us as running otherwise
> -		   the next schedule will block. */
> -		__set_current_state(TASK_RUNNING);
>  	}
>  }
> 
> 
> -
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Rik
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