linux-kernel - Re: [PATCH V2 RFC 3/3] kvm: Check system load and handle different commit cases accordingly

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Date:	Mon, 29 Oct 2012 18:54:40 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Avi Kivity <avi@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Jiannan Ouyang <ouyang@...pitt.edu>,
	Chegu Vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Gleb Natapov <gleb@...hat.com>,
	Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH V2 RFC 3/3] kvm: Check system load and handle different
 commit cases accordingly

On Mon, 2012-10-29 at 19:37 +0530, Raghavendra K T wrote:
> +/*
> + * A load of 2048 corresponds to 1:1 overcommit
> + * undercommit threshold is half the 1:1 overcommit
> + * overcommit threshold is 1.75 times of 1:1 overcommit threshold
> + */
> +#define COMMIT_THRESHOLD (FIXED_1)
> +#define UNDERCOMMIT_THRESHOLD (COMMIT_THRESHOLD >> 1)
> +#define OVERCOMMIT_THRESHOLD ((COMMIT_THRESHOLD << 1) -
> (COMMIT_THRESHOLD >> 2))
> +
> +unsigned long kvm_system_load(void)
> +{
> +       unsigned long load;
> +
> +       load = avenrun[0] + FIXED_1/200;
> +       load = load / num_online_cpus();
> +
> +       return load;
> +} 

ARGH.. no that's wrong.. very wrong.

 1) avenrun[] EXPORT_SYMBOL says it should be removed, that's not a
joke.

 2) avenrun[] is a global load, do not ever use a global load measure

 3) avenrun[] has nothing what so ever to do with runqueue lengths,
someone with a gazillion tasks in D state will get a huge load but the
cpu is very idle.


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