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

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Message-Id: <20111228091402.c45a08f6.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 28 Dec 2011 09:14:02 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
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

On Tue, 27 Dec 2011 09:52:49 -0800
Tejun Heo <tj@...nel.org> wrote:

> 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.
> 

Hm, but this never causes -ENOMEM error, at all.

Thanks,
-Kame

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