linux-kernel - Re: [revert] mysql+oltp regression

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Date:	Mon, 11 Aug 2008 09:03:34 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Mike Galbraith <efault@....de>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [revert] mysql+oltp regression

Ingo Molnar wrote:
> * Gregory Haskins <ghaskins@...ell.com> wrote:
>
>   
>> Ingo Molnar wrote:
>>     
>>> * Mike Galbraith <efault@....de> wrote:
>>>
>>>   
>>>       
>>>> Greetings,
>>>>
>>>> During regression testing of tip/sched/clock fixes, a regression in  
>>>> low client count throughput turned up, which I traced this back to 
>>>> the commit below.  I don't see anything wrong with it, but suspect 
>>>> that it is preventing client/server pairs from staying together on 
>>>> the same CPU as buddies, which mysql definitely likes quite a lot.  
>>>> (I suspect that this is the case, because I've seen this same 
>>>> performance curve while tinkering with wakeup affinity and breaking 
>>>> it all to pieces;)
>>>>
>>>> Changelog and test results below in case nobody sees a problem with  
>>>> the commit itself.
>>>>     
>>>>         
>>> i've applied your fix to tip/sched/urgent for the time being, thanks  
>>> Mike for tracking it down. We can re-try newer iterations of Greg's  
>>> patch in tip/sched/devel.
>>>
>>>   
>>>       
>> Hmm..  The patch still looks correct afaict.  I fear we are just 
>> papering over some other issue by reverting it, but I will try to see 
>> if I can track this down.  We will, of course, now be skipping trying 
>> to balance the (effectively random) last task in the queue which may 
>> or may not result in better performance on sheer luck instead of 
>> algorithmic intelligence.  This makes me nervous.
>>     
>
> yeah - but we had that behavior for quite some time.
>
> This is how the patch cycle works normally: we had a fair chance to 
> discover this problem in your testing then in -tip testing and then in 
> linux-next or -mm but we didnt find it at any stage.
>
> Now we are in the upstream release cycle so unless there's some 
> immediate fix available (or there are _really_ strong reasons against 
> the revert) doing the revert is the right approach.
>
> A revert is not necessarily the indicator of the quality of the change 
> in question, it is a tester-driven exception event that guarantees that 
> the kernel improves in a monotonic way. (for all testers who opt to help 
> us in doing so)
>
> And given that the problem was readily reproducible for Mike, it should 
> be reproducible for you as well - so we dont actually make the bug 
> harder to fix by doing the revert.
>
> Perhaps we should introduce the notion of "Defer-to-next-release" 
> reverts - which this really is - in contrast to "Revert-because-bad", 
> which your change definitely is not.
>   

Hi Ingo,
  Understood, and a totally reasonable stance.  I mostly wanted to make 
sure it was understood that I don't think I can "fix" that particular 
patch since I think it was already correct.  Rather, I will have to try 
to identify some other area (presumably the load balancer) to harmonize 
with it.  I think we are on the same page, though. :)


>   
>> Speaking of this: Another patch I submitted to you Ingo (had to do 
>> with updating the load_weight inside task_setprio) seems to also have 
>> this phenomenon: e.g. its technically correct but further testing has 
>> revealed negative repercussions elsewhere.  So please ignore that 
>> patch (or revert if you already pulled in, but I don't think you 
>> have).  Ill try to look into this issue as well.
>>     
>
> ok, under which thread/subject is that? Not queued in tip/sched/* yet, 
> correct?
>   
Here is the original thread:

http://lkml.org/lkml/2008/7/3/416

I do not believe you have queued it anywhere (public anyway) yet.

Note I have already invalidated 1/2, and now I am retracting 2/2 as 
well.  (1/2 is actually a bogus patch, 2/2 is "technically correct" but 
causes ripples in the load balancer that need to be sorted out first.

Thanks!
-Greg



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