linux-kernel - Re: [patch] sched: Fix smp nice induced group scheduling load distribution woes

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Date:	Thu, 28 Apr 2016 14:29:33 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	Jeff Merkey <linux.mdb@...il.com>
Subject: Re: [patch] sched: Fix smp nice induced group scheduling load
 distribution woes

On Thu, 2016-04-28 at 11:11 +0200, Peter Zijlstra wrote:
> On Wed, Apr 27, 2016 at 09:09:51AM +0200, Mike Galbraith wrote:
> > On even a modest sized NUMA box any load that wants to scale
> > is essentially reduced to SCHED_IDLE class by smp nice scaling.
> > Limit niceness to prevent cramming a box wide load into a too
> > small space.  Given niceness affects latency, give the user the
> > option to completely disable box wide group fairness as well.
> 
> Have you tried the (obvious) ?

Duh, nope.

> I suppose we really should just do this (and yuyang's cleanup patches I
> suppose). Nobody has ever been able to reproduce those increased power
> usage claims and Google is running with this enabled.

Yup, works, and you don't have to carefully blink as you skim past it.

> ---
> 
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 69da6fcaa0e8..968f573413de 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -53,7 +53,7 @@ static inline void cpu_load_update_active(struct rq
> *this_rq) { }
>   * when BITS_PER_LONG <= 32 are pretty high and the returns do not
> justify the
>   * increased costs.
>   */
> -#if 0 /* BITS_PER_LONG > 32 -- currently broken: it increases power
> usage under light load  */
> +#ifdef CONFIG_64BIT
>  # define SCHED_LOAD_RESOLUTION	10
>  # define scale_load(w)		((w) << SCHED_LOAD_RESOLUTION)
>  # define scale_load_down(w)	((w) >> SCHED_LOAD_RESOLUTION)