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

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Message-Id: <20111227135836.7102f41b.akpm@linux-foundation.org>
Date:	Tue, 27 Dec 2011 13:58:36 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	avi@...hat.com, nate@...nel.net, cl@...ux-foundation.org,
	oleg@...hat.com, axboe@...nel.dk, vgoyal@...hat.com,
	linux-kernel@...r.kernel.org, jaxboe@...ionio.com
Subject: Re: [PATCHSET] block, mempool, percpu: implement percpu mempool and
 fix blkcg percpu alloc deadlock

On Tue, 27 Dec 2011 13:44:21 -0800
Tejun Heo <tj@...nel.org> wrote:

> I'm not convinced trying to put this into GFP_KERNEL context would
> work.  Short of that, the next best thing would be making percpu
> allocator useable from memory reclaim path, right?

Well..  All allocations which are weaker than GFP_KERNEL are to be
discouraged.  That being said...

>  But that would
> involved a lot more churn and complexity without much added benefit,
> given that this type of use cases aren't expected to be common - and
> I'm fairly sure it isn't given track record of past few years.

I don't think it would be too hard to add an alloc_percpu_gfp().  Add
the gfp_t to a small number of functions (two or three?) then change
pcpu_mem_zalloc() to always use kzalloc() if (flags & GFP_KERNEL !=
GFP_KERNEL).  And that's it?

But the question is: is this a *good* thing to do?  It would be nice if
kernel developers understood that GFP_KERNEL is strongly preferred and
that they should put in effort to use it.  But there's a strong
tendency for people to get themselves into a sticky corner then take
the easy way out, resulting in less robust code.  Maybe calling the
function alloc_percpu_i_really_suck() would convey the hint.

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