Re: [PATCH] sched: Fix adverse effects of NFS client on interactive response

[Peter Williams <pwil3058@bigpond.net.au>]

Mike Galbraith wrote:
> At 10:13 AM 1/6/2006 +1100, Peter Williams wrote:
> 
>> Mike Galbraith wrote:
>>
>>> At 10:31 PM 1/5/2006 +1100, Peter Williams wrote:
>>>
>>>> Mike Galbraith wrote:
>>>>
>>>>> At 08:51 AM 1/5/2006 +1100, Peter Williams wrote:
>>>>>
>>>>>> I think that some of the harder to understand parts of the 
>>>>>> scheduler code are actually attempts to overcome the undesirable 
>>>>>> effects (such as those I've described) of inappropriately 
>>>>>> identifying tasks as interactive.  I think that it would have been 
>>>>>> better to attempt to fix the inappropriate identifications rather 
>>>>>> than their effects and I think the prudent use of 
>>>>>> TASK_NONINTERACTIVE is an important tool for achieving this.
>>>>>
>>>>>
>>>>>
>>>>> IMHO, that's nothing but a cover for the weaknesses induced by 
>>>>> using exclusively sleep time as an information source for the 
>>>>> priority calculation.  While this heuristic does work pretty darn 
>>>>> well, it's easily fooled (intentionally or otherwise).  The 
>>>>> challenge is to find the right low cost informational component, 
>>>>> and to stir it in at O(1).
>>>>
>>>>
>>>>
>>>> TASK_NONINTERACTIVE helps in this regard, is no cost in the code 
>>>> where it's used and probably decreases the costs in the scheduler 
>>>> code by enabling some processing to be skipped.  If by its judicious 
>>>> use the heuristic is only fed interactive sleep data the heuristics 
>>>> accuracy in identifying interactive tasks should be improved.  It 
>>>> may also allow the heuristic to be simplified.
>>>
>>>
>>> I disagree.  You can nip and tuck all the bits of sleep time you 
>>> want, and it'll just shift the lumpy spots around (btdt).
>>
>>
>> Yes, but there's a lot of (understandable) reluctance to do any major 
>> rework of this part of the scheduler so we're stuck with nips and 
>> tucks for the time being.  This patch is a zero cost nip and tuck.
> 
> 
> Color me skeptical, but nonetheless, it looks to me like the mechanism 
> might need the attached.

Is that patch complete?  (This is all I got.)

--- linux-2.6.15/kernel/sched.c.org	Fri Jan  6 08:44:09 2006
+++ linux-2.6.15/kernel/sched.c	Fri Jan  6 08:51:03 2006
@@ -1353,7 +1353,7 @@

  out_activate:
  #endif /* CONFIG_SMP */
-	if (old_state == TASK_UNINTERRUPTIBLE) {
+	if (old_state & TASK_UNINTERRUPTIBLE) {
  		rq->nr_uninterruptible--;
  		/*
  		 * Tasks on involuntary sleep don't earn
@@ -3010,7 +3010,7 @@
  				unlikely(signal_pending(prev))))
  			prev->state = TASK_RUNNING;
  		else {
-			if (prev->state == TASK_UNINTERRUPTIBLE)
+			if (prev->state & TASK_UNINTERRUPTIBLE)
  				rq->nr_uninterruptible++;
  			deactivate_task(prev, rq);
  		}

In the absence of any use of TASK_NONINTERACTIVE in conjunction with 
TASK_UNINTERRUPTIBLE it will have no effect.  Personally, I think that 
all TASK_UNINTERRUPTIBLE sleeps should be treated as non interactive 
rather than just be heavily discounted (and that TASK_NONINTERACTIVE 
shouldn't be needed in conjunction with it) BUT I may be wrong 
especially w.r.t. media streamers such as audio and video players and 
the mechanisms they use to do sleeps between cpu bursts.

> 
> On the subject of nip and tuck, take a look at the little proggy posted 
> in thread [SCHED] wrong priority calc - SIMPLE test case.  That testcase 
> was the result of Paolo Ornati looking into a real problem on his 
> system.  I just 'fixed' that nanosleep() problem by judicious 
> application of TASK_NONINTERACTIVE to the schedule_timeout().  Sure, it 
> works, but it doesn't look like anything but a bandaid (tourniquet in 
> this case:) to me.
> 
>         -Mike

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce