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

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Message-ID: <20151124014527.GA32335@js1304-P5Q-DELUXE>
Date:	Tue, 24 Nov 2015 10:45:28 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Steven Rostedt <rostedt@...dmis.org>
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 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?

Thanks.
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