linux-kernel - Re: [PATCH 2/2] mm/page_ref: add tracepoint to track down page reference manipulation

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Date:	Wed, 9 Dec 2015 15:01:54 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Nazarewicz <mina86@...a86.com>,
	Minchan Kim <minchan@...nel.org>, Mel Gorman <mgorman@...e.de>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org
Subject: Re: [PATCH 2/2] mm/page_ref: add tracepoint to track down page
 reference manipulation

On Thu, 3 Dec 2015 13:16:58 +0900
Joonsoo Kim <iamjoonsoo.kim@....com> wrote:

> On Tue, Nov 24, 2015 at 10:45:28AM +0900, Joonsoo Kim wrote:
> > On Mon, Nov 23, 2015 at 09:26:04AM -0500, Steven Rostedt wrote:  
> > > On Mon, 23 Nov 2015 17:28:05 +0900
> > > Joonsoo Kim <iamjoonsoo.kim@....com> wrote:
> > >   
> > > > On Fri, Nov 20, 2015 at 11:42:25AM -0500, Steven Rostedt wrote:  
> > > > > On Fri, 20 Nov 2015 15:33:25 +0900
> > > > > Joonsoo Kim <iamjoonsoo.kim@....com> wrote:
> > > > > 
> > > > >     
> > > > > > Steven, is it possible to add tracepoint to inlined fucntion such as
> > > > > > get_page() in include/linux/mm.h?    
> > > > > 
> > > > > I highly recommend against it. The tracepoint code adds a bit of bloat,
> > > > > and if you inline it, you add that bloat to every use case. Also, it    
> > > > 
> > > > Is it worse than adding function call to my own stub function into
> > > > inlined function such as get_page(). I implemented it as following.
> > > > 
> > > > get_page()
> > > > {
> > > >         atomic_inc()
> > > >         stub_get_page()
> > > > }
> > > > 
> > > > stub_get_page() in foo.c
> > > > {
> > > >         trace_page_ref_get_page()
> > > > }  
> > > 
> > > Now you just slowed down the fast path. But what you could do is:
> > > 
> > > get_page()
> > > {
> > > 	atomic_inc();
> > > 	if (trace_page_ref_get_page_enabled())
> > > 		stub_get_page();
> > > }
> > > 
> > > Now that "trace_page_ref_get_page_enabled()" will turn into:
> > > 
> > > 	if (static_key_false(&__tracepoint_page_ref_get_page.key)) {
> > > 
> > > which is a jump label (nop when disabled, a jmp when enabled). That's
> > > less bloat but doesn't solve the include problem. You still need to add
> > > the include of that will cause havoc with other tracepoints.  
> > 
> > Yes, It also has a include dependency problem so I can't use
> > trace_page_ref_get_page_enabled() in mm.h. BTW, I tested following
> > implementation and it works fine.
> > 
> > extern struct tracepoint __tracepoint_page_ref_get_page;
> > 
> > get_page()
> > {
> >         atomic_inc()
> >         if (static_key_false(&__tracepoint_page_ref_get_page.key))
> >                 stub_get_page()
> > }
> > 
> > This would not slow down fast path although it can't prevent bloat.
> > I know that it isn't good code practice, but, this page reference
> > handling functions have complex include dependency so I'm not sure
> > I can solve it completely. For this special case, can I use
> > this raw data structure?
> >   
> 
> Steven, any comment?

Sorry for the later reply, I was going to reply but then got called off
to do something else, and then forgot about this message :-/

I wanted you to look at what Andi has done here:

http://lkml.kernel.org/r/1449018060-1742-2-git-send-email-andi@firstfloor.org

 and here

http://lkml.kernel.org/r/1449018060-1742-3-git-send-email-andi@firstfloor.org

-- Steve
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