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: <1287662999.3488.117.camel@twins>
Date:	Thu, 21 Oct 2010 14:09:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Damien Wyart <damien.wyart@...e.fr>
Cc:	Chase Douglas <chase.douglas@...onical.com>,
	Ingo Molnar <mingo@...e.hu>, tmhikaru@...il.com,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Venkatesh Pallipadi <venki@...gle.com>
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 Wed, 2010-10-20 at 19:26 +0200, Peter Zijlstra wrote:

> -static void calc_load_account_idle(struct rq *this_rq)
> +void calc_load_account_idle(void)
>  {
> +	struct rq *this_rq = this_rq();
>  	long delta;
>  
>  	delta = calc_load_fold_active(this_rq);
> +	this_rq->calc_load_inactive = delta;
> +	this_rq->calc_load_seq = atomic_read(&calc_load_seq);
> +
>  	if (delta)
>  		atomic_long_add(delta, &calc_load_tasks_idle);
>  }
>  
> +void calc_load_account_nonidle(void)
> +{
> +	struct rq *this_rq = this_rq();
> +
> +	if (atomic_read(&calc_load_seq) == this_rq->calc_load_seq) {
> +		atomic_long_sub(this_rq->calc_load_inactive, &calc_load_tasks_idle);
> +		/*
> +		 * Undo the _fold_active() from _account_idle(). This
> +		 * avoids us loosing active tasks and creating a negative
> +		 * bias
> +		 */
> +		this_rq->calc_load_active -= this_rq->calc_load_inactive;
> +	}
> +}

Ok, so while trying to write a changelog on this patch I got myself
terribly confused again..

calc_load_active_fold() is a relative operation and simply gives delta
values since the last time it got called. That means that the sum of
multiple invocations in a given time interval should be identical to a
single invocation.

Therefore, the going idle multiple times during LOAD_FREQ hypothesis
doesn't really make sense. 

Even if it became idle but wasn't idle at the LOAD_FREQ turn-over it
shouldn't matter, since the calc_load_account_active() call will simply
fold the remaining delta with the accrued idle delta and the total
should all match up once we fold into the global calc_load_tasks.

So afaict its should all have worked and this patch is a big NOP,.
except it isn't.. 

Damn I hate this bug.. ;-) Anybody?
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