linux-kernel - Re: [PATCH] sched: Folding nohz load accounting more accurate

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Date:	Fri, 15 Jun 2012 22:27:14 +0800
From:	Charles Wang <muming.wq@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Charles Wang <muming.wq@...il.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>, Tao Ma <tm@....ma>,
	含黛 <handai.szj@...bao.com>,
	Doug Smythies <dsmythies@...us.net>
Subject: Re: [PATCH] sched: Folding nohz load accounting more accurate

On Tuesday, June 12, 2012 05:56 PM, Peter Zijlstra wrote:

> Also added Doug to CC, hopefully we now have everybody who pokes at this
> stuff.
> 
> On Tue, 2012-06-12 at 17:34 +0800, Charles Wang wrote:
>> consider following case:
>>
>> 5HZ+1
>> | cpu0_load     cpu1    cpu2    cpu3    calc_load_tasks
>> |    1           1       1       1      
>> |  -->calc_load                             1
>> |    1           1       1       1      
>> |              -->calc_load                 2
>> |    0           0       1       0
>> |                      -->calc_load         2+1-3=1
> 
> Not sure but last time I did the math 2+1-3 ended up being 0.
> 
>> |    1           1       0       1      
>> |                            -->calc_load   1-1=0
>> V
>> 5HZ+11     -->calc_global_load              0
>>
>> actually the load should be around 3, but shows nearly 0.
>>
>> 1 tick is much long for some workloads. 
> 
> Yes, one tick is long for some stuff, but seeing we sample once every 5
> seconds a little fuzz around sampling the nr_running+nr_uninterruptible
> thing shouldn't be too bad.
> 
> But I think I see what you're getting at.. lemme get more tea and ponder
> this a bit.
> .
> 


In our mind per-cpu sampling for cpu idle and non-idle is equal. But
actually may not. For non-idle cpu sampling, it's right the load when
sampling. But for idle, cause of nohz, the sampling will be delayed to
nohz exit(less than 1 tick after nohz exit). Nohz exit is always caused
by processes woken up--non-idle model. It's not fair here, idle
calculated to non-idle.

     time-expect-sampling
                   |    time-do-sampling
                   |         |
                   V         V
-|-------------------------|--
start_nohz              stop_nohz


This may explain why using my patch the load shows higher, also may
explain some reports about high load for current.

I tried a experiments, results showed better. Now i need more experiments.

Peter, is this right as i thought?
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