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: <1288001573.15336.52.camel@twins>
Date:	Mon, 25 Oct 2010 12:12:53 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Damien Wyart <damien.wyart@...e.fr>,
	Chase Douglas <chase.douglas@...onical.com>,
	Ingo Molnar <mingo@...e.hu>, tmhikaru@...il.com,
	Thomas Gleixner <tglx@...utronix.de>,
	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)

On Fri, 2010-10-22 at 16:03 -0700, Venkatesh Pallipadi wrote:
> I started making small changes to the code, but none of the change helped much.
> I think the problem with the current code is that, even though idle CPUs
> update load, the fold only happens when one of the CPU is busy
> and we end up taking its load into global load.
> 
> So, I tried to simplify things and doing the updates directly from idle loop.
> This is only a test patch, and eventually we need to hook it off somewhere
> else, instead of idle loop and also this is expected work only as x86_64
> right now.
> 
> Peter: Do you think something like this will work? loadavg went
> quite on two of my test systems after this change (4 cpu and 24 cpu).

Not really, CPUs can stay idle for _very_ long times (!x86 cpus that
don't have crappy timers like HPET which roll around every 2-4 seconds).

But all CPUs staying idle for a long time is exactly the scenario you
fix before using the decay_load_misses() stuff, except that is for the
load-balancer per-cpu load numbers not the global cpu load avg. Won't a
similar approach work here?
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