Re: [bisected] "sched: Allow per-cpu kernel threads to run on online && !active" causes warning

From: Peter Zijlstra <peterz@infradead.org>
To: Tejun Heo <tj@kernel.org>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>,
	Ming Lei <tom.leiming@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Lai Jiangshan <laijs@cn.fujitsu.com>,
	Michael Holzheu <holzheu@linux.vnet.ibm.com>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [bisected] "sched: Allow per-cpu kernel threads to run on online && !active" causes warning
Date: Tue, 16 Aug 2016 17:29:49 +0200	[thread overview]
Message-ID: <20160816152949.GL30192@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20160816152027.GD9516@htj.duckdns.org>

On Tue, Aug 16, 2016 at 11:20:27AM -0400, Tejun Heo wrote:
> As long as the mapping doesn't change after the first onlining of the
> CPU, the workqueue side shouldn't be too difficult to fix up.  I'll
> look into it.  For memory allocations, as long as the cpu <-> node
> mapping is established before any memory allocation for the cpu takes
> place, it should be fine too, I think.

Don't we allocate per-cpu memory for 'cpu_possible_map' on boot? There's
a whole bunch of per-cpu memory users that does things like:

	for_each_possible_cpu(cpu) {
		struct foo *foo = per_cpu_ptr(&per_cpu_var, cpu);

		/* muck with foo */
	}

Which requires a cpu->node map for all possible cpus at boot time.