linux-kernel - Re: [PATCH v3 4/6] sched/fair: Remove scale_load_down() for load

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Message-ID: <20160428102532.GY3430@twins.programming.kicks-ass.net>
Date:	Thu, 28 Apr 2016 12:25:32 +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 4/6] sched/fair: Remove scale_load_down() for load_avg

On Tue, Apr 05, 2016 at 12:12:29PM +0800, Yuyang Du wrote:
> Currently, load_avg = scale_load_down(load) * runnable%. The extra scaling
> down of load does not make much sense, because load_avg is primarily THE
> load and on top of that, we take runnable time into account.
> 
> We therefore remove scale_load_down() for load_avg. But we need to
> carefully consider the overflow risk if load has higher range
> (2*SCHED_FIXEDPOINT_SHIFT). The only case an overflow may occur due
> to us is on 64bit kernel with increased load range. In that case,
> the 64bit load_sum can afford 4251057 (=2^64/47742/88761/1024)
> entities with the highest load (=88761*1024) always runnable on one
> single cfs_rq, which may be an issue, but should be fine. Even if this
> occurs at the end of day, on the condition where it occurs, the
> load average will not be useful anyway.

I do feel we need a little more words on the actual ramification of
overflowing here.

Yes, having 4m tasks on a single runqueue will be somewhat unlikely, but
if it happens, then what will the user experience? How long (if ever)
does it take for numbers to correct themselves etc..

> Signed-off-by: Yuyang Du <yuyang.du@...el.com>
> [update calculate_imbalance]
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>

This SoB Chain suggests you wrote it and Vincent send it on, yet this
email is from you and Vincent isn't anywhere. Something's not right.