linux-kernel - Re: load average always more then 1 on idle system with dyntick (just after boot)

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Message-ID: <20150414101904.GG25394@azat>
Date:	Tue, 14 Apr 2015 13:19:04 +0300
From:	Azat Khuzhin <a3at.mail@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linux Kernel ML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: load average always more then 1 on idle system with dyntick
 (just after boot)

On Tue, Apr 14, 2015 at 11:54:24AM +0200, Peter Zijlstra wrote:
> On Tue, Mar 31, 2015 at 10:52:48PM +0300, Azat Khuzhin wrote:
> > Hi all,
> > 
> > On one of machines [SUPERMICRO], after installing fresh kernel
> > (v4.0-rc5-25-g90a5a89), I noticed that loadavg always greater then 1.
> > 
> > I do a lot of digging and finally have more information on this issue:
> > CONFIG_NO_HZ_IDLE=y  # loadavg always > 1
> > CONFIG_HZ_PERIODIC=y # loadavg < 1
> > 
> > After this I tried to disable "nohz" at boot, to determine whether it is
> > statically added code under #ifdef during compilation or not, so I added
> > "nohz=off" to cmdline, and it helps!
> > 
> > Also if you enable preemption loadavg is also < 1:
> > CONFIG_PREEMPT_VOLUNTARY=y
> 
> So you need CONFIG_PREEMPT=n and CONFIG_NO_HZ_IDLE=y for this to happen?

Yep, CONFIG_PREEMPT_NONE=y and CONFIG_NO_HZ_IDLE=y

> That's somewhat odd (and uncommon I think, most everybody has at least
> VOLUNTARY enabled these days).

This is not a big problem for me, I could enable VOLUNTARY, but I don't
think that it will be useful for my workload. But I could be wrong.

> I'll try to look over the code to see if I can find a preempt relation,
> weird that.

Thanks!
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