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

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Date:	Fri, 28 Feb 2014 12:04:19 +0000
From:	Matthew Leach <matthew.leach@....com>
To:	Paul Turner <pjt@...gle.com>
Cc:	linux-kernel@...r.kernel.org,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	Peter Ziljstra <peterz@...radead.org>
Subject: A question regarding TG load tracking functions in fair.c

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?

Thanks,
-- 
Matt

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