linux-kernel - Re: swiotlb detection should be memory hotplug aware ?

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Date:	Fri, 23 Jul 2010 10:59:45 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Alok Kataria <akataria@...are.com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	"lenb@...nel.org" <lenb@...nel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Petr Vandrovec <petr@...are.com>
Subject: Re: swiotlb detection should be memory hotplug aware ?

On Fri, Jul 23, 2010 at 04:33:32PM +0200, Andi Kleen wrote:
> 
> >I thought SRAT has NUMA affinity information - so for example my AMD
> >desktop box has that, but it does not support hotplug capability.
> >
> >I think first your 'hotplug_possible' code needs to be more specific -
> >not just check if SRAT exists, but also if there are swaths of memory
> >that are non-populated. It would also help if there was some indication
> >of whether the box truly does a hardware hotplug - is there a way to do
> >this?
> 
> The SRAT declares hotplug memory ranges in advance. And Linux
> already uses this
> information in the SRAT parser (just the code for doing this is a
> bit dumb, I have a rewrite
> somewhere)
> 
> The only drawback is that some older systems claimed to have large
> hotplug memory ranges
> when they didn't actually support it. So it's better to not do
> anything with a lot
> of overhead.
> 
> So yes it would be reasonable to let swiotlb (and possibly other
> code sizing itself
> based on memory) call into the SRAT parser and check the hotplug ranges too.
> 
> BTW longer term swiotlb should be really more dynamic anyways and grow
> and shrink on demand. I attempted this some time ago with my DMA

I was thinking about this at some point. I think the first step is to 
make SWIOTLB use the debugfs to actually print out how much of its
buffers are used - and see if the 64MB is a good fit.

The shrinking part scares me - I think it might be more prudent to first
explore on how to grow it. The big problem looks to allocate a physical
contiguity set of pages. And I guess SWIOTLB would need to change from
using one big region to something of a pool system?

> allocator patchkit,
> unfortunately that didn't go forward.

I wasn't present at that time so I don't know what the issues were - you
wouldn't have a link to LKML for this?
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