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: <20101020132732.GA30024@brouette>
Date:	Wed, 20 Oct 2010 15:27:32 +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)

> > 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)

In fact, after several hours of uptime, I also came into a situation of
the load being around 0.80 or 0.60 when idle with the commit reverted.
So IMHO, just reverting is not a better option than keeping the
offending commit, and a real rework of the code is needed to clean up
the situation.

Should'nt we enlarge the list of CC, because for now, responsivity has
been close to 0 and it seems we will get a 2.6.36 with buggy load avg
calculation. Even if it is only statistics, many supervision tools rely
on the load avg, so for production environments, this is not a good
thing.


Best,
-- 
Damien
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