linux-kernel - Re: [PATCH] sched/fair: Optimize sum computation with a lookup table

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Message-ID: <20160408105423.GN3448@twins.programming.kicks-ass.net>
Date:	Fri, 8 Apr 2016 12:54:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Joe Perches <joe@...ches.com>
Cc:	Yuyang Du <yuyang.du@...el.com>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, bsegall@...gle.com, pjt@...gle.com,
	morten.rasmussen@....com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com
Subject: Re: [PATCH] sched/fair: Optimize sum computation with a lookup table

On Fri, Apr 08, 2016 at 03:31:41AM -0700, Joe Perches wrote:
> On Fri, 2016-04-08 at 10:07 +0800, Yuyang Du wrote:
> > __compute_runnable_contrib() uses a loop to compute sum, whereas a
> > table lookup can do it faster in a constant time.
> 
> Perhaps this becomes rather fragile code overly dependent on the
> current #define values of LOAD_AVG_MAX_N and LOAD_AVG_PERIOD.

Too late for that, these tables already heavily depend on these values.

Commit 5b51f2f80b3b ("sched: Make __update_entity_runnable_avg() fast"),
which introduced these tables, includes a program to re-compute the
values if we ever need to change them.

It might maybe make sense for Yuyang to update that program to also
compute this new table and include the whole new program in the
Changelog of this patch.