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

[Joonsoo Kim]

On Mon, Mar 16, 2015 at 09:54:18PM -0400, Sasha Levin wrote:
> 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?

Yes, it can be solved by 1/5.
I mean that I'm not sure patch 4/5 is really needed or not.

Thanks.
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