linux-kernel - Re: [RFC PATCH 4/4] sched: Upload nohz full CPU load on task enqueue/dequeue

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Message-ID: <20160120182503.GP6357@twins.programming.kicks-ass.net>
Date:	Wed, 20 Jan 2016 19:25:03 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Byungchul Park <byungchul.park@....com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>
Subject: Re: [RFC PATCH 4/4] sched: Upload nohz full CPU load on task
 enqueue/dequeue

On Wed, Jan 20, 2016 at 06:21:07PM +0100, Frederic Weisbecker wrote:

> You mean a periodic call to the above from the housekeepers?
> 
> I didn't think about doing that because you nacked that approach with
> scheduler_tick(). This isn't much different.

This does _one_ thing, namely load accounting. scheduler_tick() does a
whole bunch of things, some which really do not make much sense to do
remotely.

> It means the housekeeper is entirely dedicated to full dynticks CPUs.

Depends on how many there are, and how many housekeepers. I would
suggest one housekeeper cpu per node or so to make sure this keeps
working.

Then again, these load values aren't super important, esp. not for the
nohz_full case where you typically don't care about load balancing.