linux-kernel - Re: [RESEND RFC v4 1/3] mm: add tracepoint for scanning pages

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Date:	Fri, 04 Sep 2015 12:44:59 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Vlastimil Babka <vbabka@...e.cz>,
	Ebru Akagunduz <ebru.akagunduz@...il.com>, linux-mm@...ck.org
CC:	akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
	n-horiguchi@...jp.nec.com, aarcange@...hat.com,
	iamjoonsoo.kim@....com, xiexiuqi@...wei.com, gorcunov@...nvz.org,
	linux-kernel@...r.kernel.org, mgorman@...e.de, rientjes@...gle.com,
	aneesh.kumar@...ux.vnet.ibm.com, hughd@...gle.com,
	hannes@...xchg.org, mhocko@...e.cz, boaz@...xistor.com,
	raindel@...lanox.com
Subject: Re: [RESEND RFC v4 1/3] mm: add tracepoint for scanning pages

On 09/04/2015 12:07 PM, Vlastimil Babka wrote:
> On 09/03/2015 10:51 PM, Ebru Akagunduz wrote:
>> Using static tracepoints, data of functions is recorded.
>> It is good to automatize debugging without doing a lot
>> of changes in the source code.
>>
>> This patch adds tracepoint for khugepaged_scan_pmd,
>> collapse_huge_page and __collapse_huge_page_isolate.
>>
>> Signed-off-by: Ebru Akagunduz <ebru.akagunduz@...il.com>
>> Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
>> Acked-by: Rik van Riel <riel@...hat.com>
>> ---
>> Changes in v2:
>>  - Nothing changed
>>
>> Changes in v3:
>>  - Print page address instead of vm_start (Vlastimil Babka)
>>  - Define constants to specify exact tracepoint result (Vlastimil Babka)
>>
>> Changes in v4:
>>  - Change the constant prefix with SCAN_ instead of MM_ (Vlastimil Babka)
>>  - Move the constants into the enum (Vlastimil Babka)
>>  - Move the constants from mm.h to huge_memory.c
>>    (because only will be used in huge_memory.c) (Vlastimil Babka)
>>  - Print pfn in tracepoints (Vlastimil Babka)
>>  - Print scan result as string in tracepoint (Vlastimil Babka)
>>    (I tried to make same things to print string like mm/compaction.c.
>>     My patch does not print string, I skip something but could not see why)
> 
> How do you print the trace? Do you cat /sys/kernel/debug/tracing/trace_pipe
> or use some tool such as trace-cmd? I have just recently realized that tools
> don't print strings in the compaction tracepoints, which lead to a patch [1].
> You could convert this patch in the same way and then it should work with
> tracing tools. Sorry for previously suggesting a wrong example to follow.
> 
> [1] https://lkml.org/lkml/2015/8/27/373

Well that explains why doing the same thing as compaction.c
resulted in the strings not being printed!  Ebru and I got
confused over that for quite a while :)

Thanks for pointing us to the fix.

Ebru, can you use tracepoint macros like in Vlastimil's patch
above, so your tracepoints work?

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