linux-kernel - Re: [PATCHSET] block, mempool, percpu: implement percpu mempool and fix blkcg percpu alloc deadlock

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Message-ID: <20111227175249.GE17712@google.com>
Date:	Tue, 27 Dec 2011 09:52:49 -0800
From:	Tejun Heo <tj@...nel.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, avi@...hat.com,
	nate@...nel.net, cl@...ux-foundation.org, oleg@...hat.com,
	axboe@...nel.dk, linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET] block, mempool, percpu: implement percpu mempool
 and fix blkcg percpu alloc deadlock

Hello, KAMEZAWA.

On Mon, Dec 26, 2011 at 03:05:31PM +0900, KAMEZAWA Hiroyuki wrote:
> From 433b56fd5644d4b1e695bc16bbf8dd78842999fd Mon Sep 17 00:00:00 2001
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Date: Mon, 26 Dec 2011 15:06:21 +0900
> Subject: [PATCH] percpu_counter: add lazy init
> 
> percpu_counter calls alloc_percpu(). This means percpu_counter_init()
> assumes GFP_KERNEL context. This may call vmalloc() in percpu_counter
> allocation and will have locks.
> 
> If a caller doesn't want to assume GFP_KERNEL, we need some tricks.
> This patch adds percpu_counter_init_lazy().
> 
> At lazy allocation, the function leaves fbc->counters as NULL and
> init fbc->counters by workqueue. This work item handling is done
> by fbc->list, so, the struct size of 'fbc' will not increase if
> a user configs CONFIG_HOTPLUG_CPU.

This is essentially more specialized form of the mempool approach.  It
doesn't seem any simpler to me while being less generic.  I don't see
what the upside would be.

Thanks.

-- 
tejun
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