Re: [PATCH v4 2/2] mm/page_ref: add tracepoint to track down page reference manipulation

[Joonsoo Kim <js1304@gmail.com>]

2016-03-03 1:58 GMT+09:00 Vlastimil Babka <vbabka@suse.cz>:
> 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.

Okay, better!
Andrew, do you want fixup patch from me or could you simply handle it?

Thanks.

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