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

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Message-ID: <20150317014306.GB19483@js1304-P5Q-DELUXE>
Date:	Tue, 17 Mar 2015 10:43:06 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Stefan Strogin <s.strogin@...tner.samsung.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	aneesh.kumar@...ux.vnet.ibm.com,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Dmitry Safonov <d.safonov@...tner.samsung.com>,
	Pintu Kumar <pintu.k@...sung.com>,
	Weijie Yang <weijie.yang@...sung.com>,
	Laura Abbott <lauraa@...eaurora.org>,
	SeongJae Park <sj38.park@...il.com>,
	Hui Zhu <zhuhui@...omi.com>, Minchan Kim <minchan@...nel.org>,
	Dyasly Sergey <s.dyasly@...sung.com>,
	Vyacheslav Tyrtov <v.tyrtov@...sung.com>,
	Aleksei Mateosian <a.mateosian@...sung.com>,
	gregory.0xf0@...il.com, sasha.levin@...cle.com, gioh.kim@....com,
	pavel@....cz, stefan.strogin@...il.com
Subject: Re: [PATCH v4 0/5] mm: cma: add some debug information for CMA

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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