linux-kernel - Re: [PATCH v3 6/6] sched/fair: Move (inactive) option from code to config

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Message-ID: <20160428093733.GX3430@twins.programming.kicks-ass.net>
Date:	Thu, 28 Apr 2016 11:37:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org, bsegall@...gle.com,
	pjt@...gle.com, morten.rasmussen@....com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	lizefan@...wei.com, umgwanakikbuti@...il.com
Subject: Re: [PATCH v3 6/6] sched/fair: Move (inactive) option from code to
 config

On Tue, Apr 05, 2016 at 12:12:31PM +0800, Yuyang Du wrote:
> The option of increased load resolution (fixed point arithmetic range) is
> unconditionally deactivated with #if 0. But since it may still be used
> somewhere (e.g., in Google), we want to keep this option.
> 
> Regardless, there should be a way to express this option. Considering the
> current circumstances, the reconciliation is we define a config
> CONFIG_CFS_INCREASE_LOAD_RANGE and it depends on FAIR_GROUP_SCHED and
> 64BIT and BROKEN.
> 
> Suggested-by: Ingo Molnar <mingo@...nel.org>

So I'm very tempted to simply, unconditionally, reinstate this larger
range for everything CONFIG_64BIT && CONFIG_FAIR_GROUP_SCHED.

There was but the single claim on increased power usage, nobody could
reproduce / analyze and Google has been running with this for years now.

Furthermore, it seems to be leading to the obvious problems on bigger
machines where we basically run out of precision by the sheer number of
cpus (nr_cpus ~ SCHED_LOAD_SCALE and stuff comes apart quickly).