linux-kernel - Re: [patch 10/15] sched/migration: Move calc_load_migrate() into CPU

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Message-ID: <20160712143745.430733e7@kryten>
Date:	Tue, 12 Jul 2016 14:37:45 +1000
From:	Anton Blanchard <anton@...ba.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, rt@...utronix.de,
	Michael Ellerman <mpe@...erman.id.au>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	shreyas@...ux.vnet.ibm.com
Subject: Re: [patch 10/15] sched/migration: Move calc_load_migrate() into
 CPU_DYING

Hi Thomas,

> It really does not matter when we fold the load for the outgoing cpu.
> It's almost dead anyway, so there is no harm if we fail to fold the
> few microseconds which are required for going fully away.

We are seeing the load average shoot up when hot unplugging CPUs (+1
for every CPU we offline) on ppc64. This reproduces on bare metal as
well as inside a KVM guest. A bisect points at this commit.

As an example, a completely idle box with 128 CPUS and 112 hot
unplugged:

# uptime
 04:35:30 up  1:23,  2 users,  load average: 112.43, 122.94, 125.54

Anton