linux-kernel - Re: [PATCH 00/10] Stitch LBR call stack

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Date:   Mon, 7 Oct 2019 16:06:24 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     peterz@...radead.org, acme@...nel.org,
        linux-kernel@...r.kernel.org, jolsa@...nel.org,
        namhyung@...nel.org, ak@...ux.intel.com,
        vitaly.slobodskoy@...el.com, pavel.gerasimov@...el.com
Subject: Re: [PATCH 00/10] Stitch LBR call stack



On 10/7/2019 2:24 PM, Ingo Molnar wrote:
> 
> * kan.liang@...ux.intel.com <kan.liang@...ux.intel.com> wrote:
> 
>> Performance impact:
>> The processing time may increase with the LBR stitching approach
>> enabled. The impact depends on the number of samples with stitched LBRs.
>>
>> For sqlite's tcltest,
>> perf record --call-graph lbr -- make tcltest
>> perf report --stitch-lbr
>>
>> There are 4.11% samples has stitched LBRs.
>> Total number of samples:                        2833728
>> The number of samples with stitched LBRs        116478
>>
>> The processing time of perf report increases 6.8%
>> Without --stitch-lbr:                           55906106 usec
>> With --stitch-lbr:                              59728701 usec
>>
>> For a simple test case tchain_edit with 43 depth of call stacks.
>> perf record --call-graph lbr -- ./tchain_edit
>> perf report --stitch-lbr
>>
>> There are 99.9% samples has stitched LBRs.
>> Total number of samples:                        10915
>> The number of samples with stitched LBRs        10905
>>
>> The processing time of perf report increases 67.4%
>> Without --stitch-lbr:                           11970508 usec
>> With --stitch-lbr:                              20036055 usec
> 
> That cost seems pretty high, while the feature sounds useful - is there
> any way to speed this up?
> 

For each LBR entry, perf tool will calculate and generate an appended 
node for callchain_cursor.
The stitched LBR entries are from previous sample. It looks like we 
don't need to do the calculation again for them. That should speed up 
the whole process. I will do more test for it.

Thanks,
Kan