Re: [PATCH v4 0/5] mm: cma: add some debug information for CMA

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Mon, Mar 16, 2015 at 07:06:55PM +0300, Stefan Strogin wrote:
> Hi all.
> 
> Here is the fourth version of a patch set that adds some debugging facility for
> CMA.
> 
> This patch set is based on next-20150316.
> It is also available on git:
> git://github.com/stefanstrogin/linux -b cmainfo-v4
> 
> We want an interface to see a list of currently allocated CMA buffers and some
> useful information about them (like /proc/vmallocinfo but for physically
> contiguous buffers allocated with CMA).
> 
> For example. We want a big (megabytes) CMA buffer to be allocated in runtime
> in default CMA region. If someone already uses CMA then the big allocation
> could fail. If it happened then with such an interface we could find who used
> CMA at the moment of failure, who caused fragmentation and so on. Ftrace also
> would be helpful here, but with ftrace we can see the whole history of
> allocations and releases, whereas with this patch set we can see a snapshot of
> CMA region with actual information about its allocations.

Hello,

Hmm... I still don't think that this is really helpful to find root
cause of fragmentation. Think about following example.

Assume 1024 MB CMA region.

128 MB allocation * 4
1 MB allocation
128 MB allocation
128 MB release * 4 (first 4)
try 512 MB allocation

With above sequences, fragmentation happens and 512 MB allocation would
be failed. We can get information about 1 MB allocation and 128 MB one
from the buffer list as you suggested, but, fragmentation are related
to whole sequence of allocation/free history, not snapshot of allocation.

Thanks.

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