linux-kernel - Re: High CPU load when machine is idle (related to PROBLEM: Unusually high load average when idle in 2.6.35, 2.6.35.1 and later)

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Message-ID: <20101014145813.GA2185@brouette>
Date:	Thu, 14 Oct 2010 16:58:13 +0200
From:	Damien Wyart <damien.wyart@...e.fr>
To:	Peter Zijlstra <peterz@...radead.org>,
	Chase Douglas <chase.douglas@...onical.com>,
	Ingo Molnar <mingo@...e.hu>, tmhikaru@...il.com,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: High CPU load when machine is idle (related to PROBLEM:
 Unusually high load average when idle in 2.6.35, 2.6.35.1 and later)

Hello,

> With 2.6.36-rc6, I'm seeing a load of around 0.60 when the machine is
> completely idle. This is similar to what someone reported for the latest
> 2.6.35.x stables. This is on a core i7 machine, but I've no time to
> bisect or test earlier versions right now, but I guess this is easy to
> reproduce on the same plateform.

After further investigation and cross-checking with the thread "PROBLEM:
Unusually high load average when idle in 2.6.35, 2.6.35.1 and later",
I came to the following results:

- the commit 74f5187ac873042f502227701ed1727e7c5fbfa9 isolated by Tim
  seems to be the culprit;
- reverting it solves the problem with 2.6.36-rc7 in NOHZ mode: the load
  when idle goes down to 0.00 (which it never does with the patch
  applied)
- using nohz=no with the commit reverted still gives correct behaviour
  (tested just in case)

- vanilla 2.6.36-rc7 with this commit applied has the problem (load is
  around 0.60 when machine idle, sometimes less after several hours
  of uptime, but never 0.00), and rebooting with nohz=no makes the
  problem disappear: load goes down to 0.00 quickly after boot process
  has finished.

I hope this answers the questions raised in the Tim's thread.

Could someone with knowledge of the commit take a look at the problem?
It would be a bit annoying to have this problem in 2.6.36, since Tim's
initial report dates back to 2 weeks ago...

I can help for further testing if needed.


Thanks in advance,
-- 
Damien
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