linux-kernel - Re: [PATCH v6 2/3] percpu: add PERCPU_ATOM

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Message-ID: <alpine.DEB.2.20.1511021210250.28799@east.gentwo.org>
Date:	Mon, 2 Nov 2015 12:11:33 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Catalin Marinas <catalin.marinas@....com>
cc:	mark.rutland@....com, Jungseok Lee <jungseoklee85@...il.com>,
	linux-mm@...ck.org, barami97@...il.com, will.deacon@....com,
	linux-kernel@...r.kernel.org, takahiro.akashi@...aro.org,
	james.morse@....com, tj@...nel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v6 2/3] percpu: add PERCPU_ATOM_SIZE for a generic percpu
 area setup

On Mon, 2 Nov 2015, Catalin Marinas wrote:

> On Mon, Nov 02, 2015 at 10:48:17AM -0600, Christoph Lameter wrote:
> > On Mon, 2 Nov 2015, Catalin Marinas wrote:
> > > I haven't looked at the patch 3/3 in detail but I'm pretty sure I'll NAK
> > > the approach (and the definition of PERCPU_ATOM_SIZE), therefore
> > > rendering this patch unnecessary. IIUC, this is used to enforce some
> > > alignment of the per-CPU IRQ stack to be able to check whether the
> > > current stack is process or IRQ on exception entry. But there are other,
> > > less intrusive ways to achieve the same (e.g. x86).
> >
> > The percpu allocator allows the specification of alignment requirements.
>
> Patch 3/3 does something like this:
>
> DEFINE_PER_CPU(char [IRQ_STACK_SIZE], irq_stacks) __aligned(IRQ_STACK_SIZE)
>
> where IRQ_STACK_SIZE > PAGE_SIZE. AFAICT, setup_per_cpu_areas() doesn't
> guarantee alignment greater than PAGE_SIZE.

And we cannot use percpu_alloc() instead? Aligning the whole of the percpu
area because one allocation requires it?


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