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

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Message-ID: <20151123092604.7ec1397d@gandalf.local.home>
Date:	Mon, 23 Nov 2015 09:26:04 -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 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.

-- Steve
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