linux-kernel - Re: [PATCH v2 6/7] mm/page_owner: keep track of page owners

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Date:	Mon, 24 Nov 2014 12:09:52 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Mel Gorman <mgorman@...e.de>, Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan@...nel.org>, Dave Hansen <dave@...1.net>,
	Michal Nazarewicz <mina86@...a86.com>,
	Jungsoo Son <jungsoo.son@....com>,
	Ingo Molnar <mingo@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/7] mm/page_owner: keep track of page owners

On Fri, Nov 21, 2014 at 03:38:32PM -0800, Andrew Morton wrote:
> On Fri, 21 Nov 2014 17:14:05 +0900 Joonsoo Kim <iamjoonsoo.kim@....com> wrote:
> 
> > This is the page owner tracking code which is introduced
> > so far ago. It is resident on Andrew's tree, though, nobody
> > tried to upstream so it remain as is. Our company uses this feature
> > actively to debug memory leak or to find a memory hogger so
> > I decide to upstream this feature.
> > 
> > This functionality help us to know who allocates the page.
> > When allocating a page, we store some information about
> > allocation in extra memory. Later, if we need to know
> > status of all pages, we can get and analyze it from this stored
> > information.
> > 
> > In previous version of this feature, extra memory is statically defined
> > in struct page, but, in this version, extra memory is allocated outside
> > of struct page. It enables us to turn on/off this feature at boottime
> > without considerable memory waste.
> > 
> > Although we already have tracepoint for tracing page allocation/free,
> > using it to analyze page owner is rather complex. We need to enlarge
> > the trace buffer for preventing overlapping until userspace program
> > launched. And, launched program continually dump out the trace buffer
> > for later analysis and it would change system behaviour with more
> > possibility rather than just keeping it in memory, so bad for debug.
> > 
> > Moreover, we can use page_owner feature further for various purposes.
> > For example, we can use it for fragmentation statistics implemented in
> > this patch. And, I also plan to implement some CMA failure debugging
> > feature using this interface.
> > 
> > I'd like to give the credit for all developers contributed this feature,
> > but, it's not easy because I don't know exact history. Sorry about that.
> > Below is people who has "Signed-off-by" in the patches in Andrew's tree.
> > 
> > ...
> >
> > --- a/Documentation/kernel-parameters.txt
> > +++ b/Documentation/kernel-parameters.txt
> > @@ -884,6 +884,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> >  			MTRR settings.  This parameter disables that behavior,
> >  			possibly causing your machine to run very slowly.
> >  
> > +	disable_page_owner
> > +			[KNL] Disable to store the information who requests
> > +			the page.
> 
> How about "Disable storage of the information about who allocated each
> page".
> 
> It seems odd that we have a disable flag.  Wouldn't it be less
> surprising to disable it by default and only enable if the boot option
> is provided?

Okay. Will do.

> 
> What is the overhead of page_owner if it is runtime-disabled, btw? 
> Will it be feasible for lots of people to just leave it enabled in
> config and to only turn it on when they want to use it?  That would be
> nice.  Please add a paragraph on this point to the changelog and the
> yet-to-be-written documentation.

- Without page owner
   text    data     bss     dec     hex filename
  40662    1493     644   42799    a72f mm/page_alloc.o

- With page owner
   text    data     bss     dec     hex filename
  40892    1493     644   43029    a815 mm/page_alloc.o
   1427      24       8    1459     5b3 mm/page_ext.o
   2722      50       0    2772     ad4 mm/page_owner.o

Roughly, 4 KB code is added in total. No more runtime memory is needed if
runtime-disabled. Size of page_alloc.o is 200 bytes bigger than disabled one.
Page owner addes two 'if' statements in allocator hotpath and two 'if'
statements in coldpath. If runtime-disabled, allocation performance would not
be affected by these few unlikely branches.

Will write this to yet-to-be-written documentation.

Thanks.
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