[PATCH] eliminate a truckload of context switches

[PATCH] eliminate a truckload of context switches

[Mike Galbraith]

Greetings,

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.

Before:
user  :       0:08:02.33  58.7%  page in :   763148
nice  :       0:00:00.00   0.0%  page out:   569132
system:       0:02:39.83  19.4%  swap in :   125546
idle  :       0:02:59.73  21.9%  swap out:   133831
uptime:       0:13:41.88         context :   685149

After:
user  :       0:08:01.93  59.8%  page in :   768532
nice  :       0:00:00.00   0.0%  page out:   578009
system:       0:02:17.27  17.0%  swap in :   127100
idle  :       0:03:06.34  23.1%  swap out:   136101
uptime:       0:13:25.53         context :   391966

Instrumentation sample:
c0185cf0  submit_bh +<10/80> (0.34) pid(4)
c0185bb3  generic_make_request +<13/140> (0.32) pid(4)
c0184e03  blk_get_queue +<f/4c> (0.24) pid(4)
c01b2195  ide_get_queue +<d/38> (0.39) pid(4)
c0185527  __make_request +<13/68c> (0.54) pid(4)
c0186ac7  elevator_linus_merge +<13/130> (7.72) pid(4)
c0133977  __wait_on_buffer +<13/9c> (1.32) pid(4)
c01141c3  add_wait_queue +<f/38> (0.38) pid(4)
c011323b  schedule +<13/398> (4.66) pid(4->6)
...
c011323b  schedule +<13/398> (3.75) pid(381->4)
c01057e3  __switch_to +<13/cc> (2.66) pid(4)
c011323b  schedule +<13/398> (2.49) pid(4->426)
...
c011323b  schedule +<13/398> (4.97) pid(426->4)
c01057e3  __switch_to +<13/cc> (3.77) pid(4)
c011323b  schedule +<13/398> (1.76) pid(4->419)
...
c011323b  schedule +<13/398> (1.47) pid(400->4)
c01057e3  __switch_to +<13/cc> (0.66) pid(4)
c011323b  schedule +<13/398> (0.65) pid(4->400)
...
c011323b  schedule +<13/398> (1.46) pid(400->4)
c01057e3  __switch_to +<13/cc> (1.66) pid(4)
c011323b  schedule +<13/398> (0.73) pid(4->400)
...zzz


--- 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);
 	}
 }

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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Re: [PATCH] eliminate a truckload of context switches

[J . A . Magallon]

On 05.13 Rik van Riel wrote:
> 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...
> 

If it matters, I have been running it on 2.4.4-ac8 and works fine...

-- 
J.A. Magallon                           #  Let the source be with you...        
mailto:jamagallon@able.es
Linux Mandrake release 8.1 (Cooker) for i586
Linux werewolf 2.4.4-ac8 #1 SMP Sat May 12 01:16:37 CEST 2001 i686