linux-kernel - Re: [PATCH] radeon: use vmalloc instead of kmalloc

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Date:	Tue, 23 Jun 2009 17:14:23 +0200
From:	Thomas Hellström <thomas@...pmail.org>
To:	Dave Airlie <airlied@...il.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Jerome Glisse <jglisse@...hat.com>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.sf.net
Subject: Re: [PATCH] radeon: use vmalloc instead of kmalloc

Dave Airlie skrev:
> On Tue, Jun 23, 2009 at 9:35 PM, Peter Zijlstra<peterz@...radead.org> wrote:
>   
>> On Mon, 2009-06-22 at 19:26 +0200, Jerome Glisse wrote:
>>     
>>> We don't need to allocated contiguous pages in cs codepath
>>> so use vmalloc instead.
>>>       
>> Best would be to not require >PAGE_SIZE allocations at all of course.
>>     
>
> It gets messy when you have copy from user and spinlocks, it would be nice
> to just parse the userspace PAGE_SIZE at a time, but it would mean dropping
> a lock we probably need to hold.
>
>   
>> But barring that, it would be great to have something like:
>>
>>  ptr = kmalloc(size, GFP_KERNEL | __GFP_NOWARN);
>>  if (!ptr)
>>    ptr = vmalloc(size);
>>     
>
> we have a drm_calloc_large workaround already for the Intel driver which also
> need this.
>   
One problem with multiple vmallocs per command submission is 
performance. Judging from previous work, drivers that are doing this 
tend to get very cpu-hungry. Since Radeon is only allowing a single 
process into the command submission path at once, what about 
pre-allocating a single larger vmalloced buffer at first command 
submission and take care to flush user-space before the submitted 
command stream gets too big.

>> Also, how long do these allocations live? vmalloc space can be quite
>> limited (i386) and it can fragment too.
>>     
>
> Only an ioctl lifetime so they aren't that bad. We however do have some vmaps
> that might be quite large, mainly the fbcon framebuffer (up to 8MB in
> some cases)
>   
That one would be ioremapped, not vmapped right? Not that it matters 
because it's using vmalloc space anyway, but it wouldn't be worse than a 
traditionally ioremapped framebuffer.

Thomas

> Dave.
>
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