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Date: Thu, 27 Oct 2011 23:06:31 -0500 (CDT)
From: Christoph Lameter <cl@gentwo.org>
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To: Gilad Ben-Yossef <gilad@benyossef.com>
cc: linux-kernel@vger.kernel.org, Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Russell King <linux@arm.linux.org.uk>, linux-mm@kvack.org,
        Pekka Enberg <penberg@kernel.org>, Matt Mackall <mpm@selenic.com>,
        Sasha Levin <levinsasha928@gmail.com>
Subject: Re: [PATCH v2 6/6] slub: only preallocate cpus_with_slabs if
 offstack
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On Sun, 23 Oct 2011, Gilad Ben-Yossef wrote:

> We need a cpumask to track cpus with per cpu cache pages
> to know which cpu to whack during flush_all. For
> CONFIG_CPUMASK_OFFSTACK=n we allocate the mask on stack.
> For CONFIG_CPUMASK_OFFSTACK=y we don't want to call kmalloc
> on the flush_all path, so we preallocate per kmem_cache
> on cache creation and use it in flush_all.

I think the on stack alloc should be the default because we can then avoid
the field in kmem_cache and the associated logic with managing the field.
Can we do a GFP_ATOMIC allocation in flush_all()? If the alloc
fails then you can still fallback to send an IPI to all cpus.