linux-kernel - Re: A question regarding TG load tracking functions in fair.c

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Message-ID: <20140228122922.GG9987@twins.programming.kicks-ass.net>
Date:	Fri, 28 Feb 2014 13:29:22 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Matthew Leach <matthew.leach@....com>
Cc:	Paul Turner <pjt@...gle.com>, linux-kernel@...r.kernel.org,
	Dietmar Eggemann <Dietmar.Eggemann@....com>
Subject: Re: A question regarding TG load tracking functions in fair.c

On Fri, Feb 28, 2014 at 12:04:19PM +0000, Matthew Leach wrote:
> Hi Paul,
> 
> I have a question regarding two lines of code in the functions
> __update_cfs_rq_tg_load_contrib and __update_tg_runnable_avg. AFAICS
> these functions update a load-tracking signal for the local RQ and an
> aggregated signal in the TG for all RQs that belong to this TG. The
> update is surrounded by conditions:
> 
>   if (abs(contrib) > cfs_rq->tg_runnable_contrib / 64) {
> 
> for __update_tg_runnable_avg and
> 
>   if (force_update || abs(tg_contrib) > cfs_rq->tg_load_contrib / 8) {
> 
> for __update_cfs_rq_tg_load_contrib. Is this an optimisation to prevent
> the signals being updated for small variations? Also, could you please
> shed some light on the specific numbers used?

Yes, its an optimization; updating global state is expensive. I'll let
Paul explain the specific numbers.
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