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

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Date:	Thu, 21 Aug 2008 07:46:49 +1000
From:	"Dave Airlie" <airlied@...il.com>
To:	"Rene Herman" <rene.herman@...access.nl>
Cc:	"Venki Pallipadi" <venkatesh.pallipadi@...el.com>,
	"Ingo Molnar" <mingo@...e.hu>,
	"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 Thu, Aug 21, 2008 at 7:40 AM, Rene Herman <rene.herman@...access.nl> wrote:
> On 20-08-08 21:41, Venki Pallipadi wrote:
>
>> 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.
>
> Okay, thanks for the confirmation.
>
> Now, how to fix...
>
> Firstly, it seems we can conclude that any expectancy of a short PAT list is
> simply destroyed by AGP. I believe the best thing migh be to look into
> "fixing" AGP rather than PAT for now?
>
> In a sense the entire purpose of the AGP GART is collecting non contiguous
> pages but given that in practice it's generally still just one or at most a
> few regions, going to multi-page allocs sounds most appetising to me.
>
> All in tree AGP drivers except sgi-agp use agp_generic_alloc_page(), ali via
> m1541_alloc_page and i460 via i460_alloc_page.

In the future we will be getting more smaller AGP allocs, so the other
problem needs a fix as well.

http://git.kernel.org/?p=linux/kernel/git/airlied/agp-2.6.git;a=shortlog;h=agp-pageattr2

contains some code I started on before that moves the interfaces
around,  Shaohua has been looking at
it as it needs the changes to the set_pages interface as well, which
is where I ran out of time/steam last time.

However with alloc/free pages we could change to a higher order
allocation function as long as it fell back to lower
orders internally.

Dave.

>
> Rene.
>
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