linux-kernel - Re: [PATCH 1/2] perf tools powerpc: Fix callchain ip filtering

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Date:   Tue, 17 Apr 2018 12:29:25 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
To:     Sandipan Das <sandipan@...ux.vnet.ibm.com>
Cc:     acme@...nel.org, jolsa@...hat.com, linux-kernel@...r.kernel.org,
        naveen.n.rao@...ux.vnet.ibm.com, ravi.bangoria@...ux.vnet.ibm.com,
        sukadev@...ux.vnet.ibm.com, maynard@...ibm.com
Subject: Re: [PATCH 1/2] perf tools powerpc: Fix callchain ip filtering



On 04/12/2018 10:41 PM, Sandipan Das wrote:
> For powerpc64, if a probe is added for a function without specifying
> a line number, the corresponding trap instruction is placed at offset
> 0 (for big endian) or 8 (for little endian) from the start address of
> the function. This address is in the function prologue and the trap
> instruction preceeds the instructions to set up the stack frame.
>
> Therefore, at this point during execution, the return address for the
> function is yet to be written to its caller's stack frame. So, the LR
> value at index 2 of the callchain ips provided by the kernel is still
> valid and must not be skipped.
>
> This can be observed on a powerpc64le system running Fedora 27 as
> shown below.
>
>  # perf probe -x /usr/lib64/libc-2.26.so -a inet_pton
>  # perf record -e probe_libc:inet_pton/max-stack=3/ ping -6 -c 1 ::1
>  # perf script
>
> Without this patch, the output is:
>
>  ping 27909 [007] 532219.943481: probe_libc:inet_pton: (7fff99b0af28)
>                    15af28 __GI___inet_pton (/usr/lib64/libc-2.26.so)
>                    1105b4 getaddrinfo (/usr/lib64/libc-2.26.so)
>
> With this patch applied, the output is:
>
>  ping 27909 [007] 532219.943481: probe_libc:inet_pton: (7fff99b0af28)
>                    15af28 __GI___inet_pton (/usr/lib64/libc-2.26.so)
>                    10fa54 gaih_inet.constprop.7 (/usr/lib64/libc-2.26.so)
>                    1105b4 getaddrinfo (/usr/lib64/libc-2.26.so)
>
> Fixes: a60335ba3298 ("perf tools powerpc: Adjust callchain based on DWARF debug info")
> Signed-off-by: Sandipan Das <sandipan@...ux.vnet.ibm.com>
> ---

This change looks good to me but seems it fixed the issue
partially.Ex,

    # readelf --debug-dump=frames-interp /lib64/libc-2.26.so | less
    ...
    00005778 0000000000000024 0000577c FDE cie=00000000 pc=0000000000048b30..0000000000048c64
       LOC           CFA      r31   ra   
    0000000000048b30 r1+0     u     u    
    0000000000048b40 r1+0     c-8   r0   
    0000000000048b58 r1+64    c-8   c+16 
    0000000000048bd8 r1+0     c-8   c+16 
    0000000000048be4 r1+0     u    
    0000000000048bf0 r1+64    c-8   c+16 

0000000000048b30..0000000000048c64 is arandom() function from libc:

    0000000000048b30 <random>:
       48b30:       1c 00 4c 3c     addis   r2,r12,28
       48b34:       d0 e5 42 38     addi    r2,r2,-6704
       48b38:       a6 02 08 7c     mflr    r0
       48b3c:       f8 ff e1 fb     std     r31,-8(r1)
       48b40:       00 00 00 60     nop
       48b44:       00 00 20 39     li      r9,0
       48b48:       80 b5 e2 3b     addi    r31,r2,-19072
       48b4c:       01 00 00 39     li      r8,1
       48b50:       10 00 01 f8     std     r0,16(r1)
       48b54:       c1 ff 21 f8     stdu    r1,-64(r1)
       48b58:       f0 8f 4d e9     ld      r10,-28688(r13)
       ...

Your change fixed the issue for 48b30..48b40. But not for
48b40..48b58.

I probed at 0x48b40.

    # ./perf record -g -e probe_libc:abs_48b40 ~/rand

perf report without Suka's and your change:

    # Children      Self  Trace output 
    # ........  ........  ..............
    #
       100.00%   100.00%  (7fffb7d28b40)
                |
                ---0
                   __libc_start_main
                   generic_start_main.isra.0
                   main
                   rand
                   __random

perf report with only Suka's change:

    # Children      Self  Trace output 
    # ........  ........  ..............
    #
       100.00%   100.00%  (7fffb7d28b40)
                |
                ---0
                   __libc_start_main
                   generic_start_main.isra.0
                   main
                   __random

perf report with Suka's and your change:

    # Children      Self  Trace output 
    # ........  ........  ..............
    #
       100.00%   100.00%  (7fffb7d28b40)
                |
                ---0
                   __libc_start_main
                   generic_start_main.isra.0
                   main
                   __random

I think rand() is a valid entry which is missing in last two cases.