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

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Date:	Tue, 17 Mar 2015 11:04:02 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Stefan Strogin <s.strogin@...tner.samsung.com>, 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, 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 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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