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

[Sasha Levin]

On 03/16/2015 09:43 PM, Joonsoo Kim wrote:
> 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.

This is solvable by dumping task->comm in the tracepoint patch (1/5), right?


Thanks,
Sasha
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