linux-kernel - Re: AGP and PAT (induced?) problem (on AMD family 6)

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Date:	Wed, 20 Aug 2008 12:41:28 -0700
From:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
To:	Rene Herman <rene.herman@...access.nl>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>,
	Dave Airlie <airlied@...il.com>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Arjan van de Ven <arjan@...radead.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Dave Jones <davej@...emonkey.org.uk>
Subject: Re: AGP and PAT (induced?) problem (on AMD family 6)

On Wed, Aug 20, 2008 at 07:27:22AM -0700, Rene Herman wrote:
> On 20-08-08 12:50, Rene Herman wrote:
> 
> > On 20-08-08 12:04, Ingo Molnar wrote:
> >
> >> * Venki Pallipadi <venkatesh.pallipadi@...el.com> wrote:
> >>
> >>>> I'd really like a reply from the AGP or PAT side right about now.
> >>> Hmm. Looks like there are more than 16000 entries in the PAT list!
> >>
> >> hm, btw., why is that?
> >
> > Because 64M of AGP memory divided by 4K pages is 16K. That is, the
> > underlying problem seems to be AGP drivers using order 0 allocations.
> > I'm looking.
> >
> > Do note also that this means that Venki's change would not constitite a
> > correct/final fix. Sure, caching the last entry speeds up traversing a
> > 16K entry list but the issue is that there shouldn't be a 16K entry
> > list. Through AGP, or maybe even by coalescing entries in the PAT list
> > if that's at all possible (I guess it's not really).
> >
> > Even if such a more fundamental fix isn't (easily) available, the PAT
> > code already comments that the list, which is sorted by ->start value,
> > is expected to be short, and should be turned into an rbtree if it isn't
> > which might be slightly less of a bandaid.
> >
> > Dave Airlie (as the MAINTAINERS entry) can't be arsed to answer email it
> > seems so I've added Dave Jones for a possible comment from the AGP side.
> > If I'm reading this right upto now, still many AGP driver (among which
> > my amd-k7-agp) are affected.
> 
> This was based on a wrong reading; I was looking at the GATT allocation.
> 
> I'm giving up looking until someone can tell me whether or not those 16K
> entries are expected though. I have just one AGP card in a PAT capable
> machine.
> 

OK. I have reproduced this list size issue locally and this order 1
allocation and set_memory_uc on that allocation is actually coming from
agp_allocate_memory() -> agp_generic_alloc_page() -> map_page_into_agp()
agp_allocate_memory breaks higher order page requests into order 1 allocs.

On my system I see multiple agp_allocate_memory requests for nrpages
8841, 1020, 16, 2160, 2160, 8192. Together they end up resulting in
more than 22K entries in PAT pages.

Thanks,
Venki

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