Re: drm/radeon spamming alloc_contig_range: [xxx, yyy) PFNs busy busy

On Thu 01-12-16 17:03:52, Michal Nazarewicz wrote:

On Thu, Dec 01 2016, Michal Hocko wrote:

quoted

Let's also CC Marek

On Thu 01-12-16 08:43:40, Vlastimil Babka wrote:

quoted

On 12/01/2016 08:21 AM, Michal Hocko wrote:

quoted

Forgot to CC Joonsoo. The email thread starts more or less here
http://lkml.kernel.org/r/20161130092239.GD18437@dhcp22.suse.cz

On Thu 01-12-16 08:15:07, Michal Hocko wrote:

quoted

On Wed 30-11-16 20:19:03, Robin H. Johnson wrote:
[...]

quoted

alloc_contig_range: [83f2a3, 83f2a4) PFNs busy

Huh, do I get it right that the request was for a _single_ page? Why do
we need CMA for that?

Ugh, good point. I assumed that was just the PFNs that it failed to migrate
away, but it seems that's indeed the whole requested range. Yeah sounds some
part of the dma-cma chain could be smarter and attempt CMA only for e.g.
costly orders.

Is there any reason why the DMA api doesn't try the page allocator first
before falling back to the CMA? I simply have a hard time to see why the
CMA should be used (and fragment) for small requests size.

There actually may be reasons to always go with CMA even if small
regions are requested.  CMA areas may be defined to map to particular
physical addresses and given device may require allocations from those
addresses.  This may be more than just a matter of DMA address space.
I cannot give you specific examples though and I might be talking
nonsense.

I am not familiar with this code so I cannot really argue but a quick
look at rmem_cma_setup doesn't suggest any speicific placing or
anything...

-- 
Michal Hocko
SUSE Labs

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