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

From: Peter Zijlstra <peterz@infradead.org>
To: Charles Wang <muming.wq@gmail.com>
Cc: linux-kernel@vger.kernel.org, "Ingo Molnar" <mingo@redhat.com>,
	"Charles Wang" <muming.wq@taobao.com>, "Tao Ma" <tm@tao.ma>,
	含黛 <handai.szj@taobao.com>, "Doug Smythies" <dsmythies@telus.net>
Subject: Re: [PATCH] sched: Folding nohz load accounting more accurate
Date: Tue, 12 Jun 2012 11:56:10 +0200	[thread overview]
Message-ID: <1339494970.31548.66.camel@twins> (raw)
In-Reply-To: <4FD70D12.5030404@gmail.com>

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.