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

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Date:	Fri, 19 Feb 2016 09:34:21 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	js1304@...il.com, Andrew Morton <akpm@...ux-foundation.org>,
	Michal Nazarewicz <mina86@...a86.com>,
	Minchan Kim <minchan@...nel.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Vlastimil Babka <vbabka@...e.cz>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-api@...r.kernel.org, Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH 2/2] mm/page_ref: add tracepoint to track down page
 reference manipulation

On (02/18/16 09:29), Steven Rostedt wrote:
> > diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
> > index 534249c..fd6d9a5 100644
> > --- a/include/linux/page_ref.h
> > +++ b/include/linux/page_ref.h
> > @@ -1,6 +1,54 @@
> >  #include <linux/atomic.h>
> >  #include <linux/mm_types.h>
> >  #include <linux/page-flags.h>
> > +#include <linux/tracepoint-defs.h>
> > +
> > +extern struct tracepoint __tracepoint_page_ref_set;
> > +extern struct tracepoint __tracepoint_page_ref_mod;
> > +extern struct tracepoint __tracepoint_page_ref_mod_and_test;
> > +extern struct tracepoint __tracepoint_page_ref_mod_and_return;
> > +extern struct tracepoint __tracepoint_page_ref_mod_unless;
> > +extern struct tracepoint __tracepoint_page_ref_freeze;
> > +extern struct tracepoint __tracepoint_page_ref_unfreeze;
> > +
> > +#ifdef CONFIG_DEBUG_PAGE_REF
> 
> Please add a comment here. Something to the effect of:
> 
> /*
>  * Ideally we would want to use the trace_<tracepoint>_enabled() helper
>  * functions. But due to include header file issues, that is not
>  * feasible. Instead we have to open code the static key functions.
>  *
>  * See trace_##name##_enabled(void) in include/linux/tracepoint.h
>  */
> 

not sure if it's worth mentioning in the comment, but the other
concern here is the performance impact of an extra function call,
I believe. otherwise, Joonsoo would just do:

in include/linux/page_ref.h

static inline void set_page_count(struct page *page, int v)
{
	atomic_set(&page->_count, v);
	__page_ref_set(page, v);
}
...



and in mm/debug_page_ref.c

void __page_ref_set(struct page *page, int v)
{
	if (trace_page_ref_set_enabled())
		trace_page_ref_set(page, v);
}
EXPORT_SYMBOL(__page_ref_set);
EXPORT_TRACEPOINT_SYMBOL(page_ref_set);
...

	-ss