Subject: Re: [PATCH v4 2/2] mm/page_ref: add tracepoint to track down page
 reference manipulation
To: js1304@gmail.com, Andrew Morton <akpm@linux-foundation.org>
References: <1456448282-897-1-git-send-email-iamjoonsoo.kim@lge.com>
 <1456448282-897-2-git-send-email-iamjoonsoo.kim@lge.com>
Cc: Michal Nazarewicz <mina86@mina86.com>, Minchan Kim <minchan@kernel.org>,
        Mel Gorman <mgorman@techsingularity.net>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <56D71BB2.5060503@suse.cz>
Date: Wed, 2 Mar 2016 17:58:26 +0100
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On 02/26/2016 01:58 AM, js1304@gmail.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>
> CMA allocation should be guaranteed to succeed by definition, but,
> unfortunately, it would be failed sometimes. It is hard to track down
> the problem, because it is related to page reference manipulation and
> we don't have any facility to analyze it.
>
> This patch adds tracepoints to track down page reference manipulation.
> With it, we can find exact reason of failure and can fix the problem.
> Following is an example of tracepoint output. (note: this example is
> stale version that printing flags as the number. Recent version will
> print it as human readable string.)
>
> Enabling this feature bloat kernel text 30 KB in my configuration.
>
>     text    data     bss     dec     hex filename
> 12127327        2243616 1507328 15878271         f2487f vmlinux_disabled
> 12157208        2258880 1507328 15923416         f2f8d8 vmlinux_enabled
>

That's not bad, and it's even configurable. Thanks for taking the extra 
care about overhead since v1.

> Note that, due to header file dependency problem between mm.h and
> tracepoint.h, this feature has to open code the static key functions
> for tracepoints. Proposed by Steven Rostedt in following link.
>
> https://lkml.org/lkml/2015/12/9/699
>
> v3:
> o Add commit description and code comment why this patch open code
> the static key functions for tracepoints.
> o Notify that example is stale version.
> o Add "depends on TRACEPOINTS".
>
> v2:
> o Use static key of each tracepoints to avoid function call overhead
> when tracepoints are disabled.
> o Print human-readable page flag thanks to newly introduced %pgp option.
> o Add more description to Kconfig.debug.
>
> Acked-by: Michal Nazarewicz <mina86@mina86.com>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> +config DEBUG_PAGE_REF
> +	bool "Enable tracepoint to track down page reference manipulation"
> +	depends on DEBUG_KERNEL
> +	depends on TRACEPOINTS
> +	---help---
> +	  This is the feature to add tracepoint for tracking down page reference
> +	  manipulation. This tracking is useful to diagnosis functional failure
> +	  due to migration failure caused by page reference mismatch. Be

OK.

> +	  careful to turn on this feature because it could bloat some kernel
> +	  text. In my configuration, it bloats 30 KB. Although kernel text will
> +	  be bloated, there would be no runtime performance overhead if
> +	  tracepoint isn't enabled thanks to jump label.

I would just write something like:

Enabling this feature adds about 30 KB to the kernel code, but runtime 
performance overhead is virtually none until the tracepoints are 
actually enabled.