linux-kernel - Re: TTM page pool allocator

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Message-ID: <4A676438.6090008@shipmail.org>
Date:	Wed, 22 Jul 2009 21:10:48 +0200
From:	Thomas Hellström <thomas@...pmail.org>
To:	Jerome Glisse <glisse@...edesktop.org>
CC:	Dave Airlie <airlied@...il.com>, linux-kernel@...r.kernel.org,
	dri-devel@...ts.sf.net
Subject: Re: TTM page pool allocator

Jerome Glisse wrote:
> On Wed, 2009-07-22 at 10:27 +0200, Thomas Hellström wrote:
>   
>> Jerome Glisse wrote:
>>     
>>> On Thu, 2009-06-25 at 17:53 +0200, Thomas Hellström wrote:
>>>   
>>>       
>>>> Jerome Glisse skrev:
>>>>     
>>>>         
>>>>> Hi,
>>>>>
>>>>> Thomas i attach a reworked page pool allocator based on Dave works,
>>>>> this one should be ok with ttm cache status tracking. It definitely
>>>>> helps on AGP system, now the bottleneck is in mesa vertex's dma
>>>>> allocation.
>>>>>
>>>>> Cheers,
>>>>> Jerome
>>>>>   
>>>>>       
>>>>>           
>>>> Hi, Jerome!
>>>> In general it looks very good. Some things that need fixing:
>>>>
>>>> 1)  We must have a way to hand back pages. I still not quite understand 
>>>> how the shrink callbacks work and whether they are applicable. Another 
>>>> scheme would be to free, say 1MB when we have at least 2MB available.
>>>>
>>>> 2) We should avoid including AGP headers if AGP is not configured. 
>>>> Either reimplement unmap_page_from_agp or map_page_into_agp or move them 
>>>> out from the AGP headers. We've hade complaints before from people with 
>>>> AGP free systems that the code doesn't compile.
>>>>
>>>> 3) Since we're allocating (and freeing) in batches we should use the 
>>>> set_pages_array() interface to avoid a global tlb flush per page.
>>>>
>>>> 4) We could now skip the ttm_tt_populate() in ttm_tt_set_caching, since 
>>>> it will always allocate cached pages and then transition them.
>>>>
>>>>     
>>>>         
>>> Okay 4) is bad, what happens (my brain is a bit meltdown so i might be
>>> wrong) :
>>> 1 - bo get allocated tt->state = unpopulated
>>> 2 - bo is mapped few page are faulted tt->state = unpopulated
>>> 3 - bo is cache transitioned but tt->state == unpopulated but
>>>     they are page which have been touch by the cpu so we need
>>>     to clflush them and transition them, 
>>>       
>> Right.
>>
>>     
>>> this never happen if
>>>     we don't call ttm_tt_populate and proceed with the remaining
>>>     of the cache transitioning functions
>>>   
>>>       
>> Why can't we just skip ttm_tt_populate and proceed with the remaining of 
>> the cache transitioning functions? Then all pages currently allocated to 
>> the TTM will be transitioned.
>>
>>     
>
> Because not all tt->pages entry have valid page some are null and thus
> we have to go through the whole table and transition page per page. I
> did that and quick benchmark showed that it's faster to fully populate
> and fully transition.
>
>   
OK.
That's probably sufficient for now.

/Thomas


> Cheers,
> Jerome
>
>   



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