linux-kernel - Re: [PATCH 23/32] Generic dynamic per cpu refcounting

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Message-ID: <20130128175951.GA22465@mtj.dyndns.org>
Date:	Mon, 28 Jan 2013 09:59:51 -0800
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	linux-fsdevel@...r.kernel.org, zab@...hat.com, bcrl@...ck.org,
	jmoyer@...hat.com, axboe@...nel.dk, viro@...iv.linux.org.uk,
	tytso@....edu, Oleg Nesterov <oleg@...hat.com>,
	srivatsa.bhat@...ux.vnet.ibm.com, Christoph Lameter <cl@...ux.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 23/32] Generic dynamic per cpu refcounting

Hello, Kent.

On Mon, Jan 28, 2013 at 09:53:04AM -0800, Kent Overstreet wrote:
> > Like Tejun, I'd prefer to see it always alloc up-front, because it
> > avoids the _noalloc variant (which is backwards: please hand gfp_t, so
> > you don't hide the alloc) and heuristics.
> 
> Problem with gfp_t is alloc_percpu() doesn't take it. I don't know why,
> but this all goes away with Tejun's idea for allocating from a pool
> refilled by workqueue.

It goes down to how we allocate page tables.  percpu depends on
vmalloc space allocation which in turn depends on page table
allocation which unfortunately assumes GFP_KERNEL and is spread all
across different architectures.  Adding @gfp to it came up a couple
times but the cases weren't strong enough to push it all the way
through.  There are some aspects that I like about forcing GFP_KERNEL
on all percpu allocations but if there are strong enough cases and
someone is willing enough to push it through, maybe.

Thanks.

-- 
tejun
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