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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Date:	Mon, 25 Oct 2010 09:29:12 -0700
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Mon, Oct 25, 2010 at 3:12 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 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?
>

Yes. Thought about that. One problem there is that works with nohz_idle_balance,
which will not be called if all the CPUs are idle for example.
As this is once in 5 secs, probably doing nr_running() and
nr_uninterruptible() should
be OK even on huge systems. But, that was the original code here, except that
it was inside xtime_lock.

Thanks,
Venki
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