Perf Script Erroneous User Stack Trace

ahmadkhorrami
2020-06-14T13:43:26Z

Hi,

I used the following command to sample backtraces for a simple "ffmpeg"
benchmark:
sudo perf record -d --call-graph dwarf,65528 -c 1000000 -e
mem_load_uops_retired.l3_miss:u ffmpeg -i
/media/ahmad/DATA/Videos/video.mp4 -threads 1 -vf spp out.mp4

As can be seen PEBS is not used, the stack size is set to the maximum
and the sampling period is quite large. I also limited the thread count,
but this is the first portion of "perf script --no-demangle" output:
ffmpeg 11750 6670.061261: 1000000 mem_load_uops_retired.l3_miss:u:
0 5080021 N/A|SNP N/A|TLB N/A|LCK N/A
7fffeab68844 x264_pixel_avg_w16_avx2+0x4
(/usr/lib/x86_64-linux-gnu/libx264.so.152)

ffmpeg 11750 6670.274835: 1000000 mem_load_uops_retired.l3_miss:u:
0 5080021 N/A|SNP N/A|TLB N/A|LCK N/A
7fffeab68844 x264_pixel_avg_w16_avx2+0x4
(/usr/lib/x86_64-linux-gnu/libx264.so.152)

ffmpeg 11750 6670.496159: 1000000 mem_load_uops_retired.l3_miss:u:
0 5080021 N/A|SNP N/A|TLB N/A|LCK N/A
7fffeab8ef89 x264_pixel_sad_x4_16x16_avx2+0x49
(/usr/lib/x86_64-linux-gnu/libx264.so.152)

ffmpeg 11750 6670.852598: 1000000 mem_load_uops_retired.l3_miss:u:
0 5080021 N/A|SNP N/A|TLB N/A|LCK N/A
7fffeaac97b3 pixel_memset+0x293 (inlined)
7fffeaac97b3 plane_expand_border+0x293 (inlined)
7fffeaac97b3 x264_frame_expand_border_filtered+0x293
(/usr/lib/x86_64-linux-gnu/libx264.so.152)
7fffeab463bc x264_fdec_filter_row+0x69c
(/usr/lib/x86_64-linux-gnu/libx264.so.152)
7fffeab49523 x264_slice_write+0x1873
(/usr/lib/x86_64-linux-gnu/libx264.so.152)
7fffeab85285 x264_stack_align+0x15
(/usr/lib/x86_64-linux-gnu/libx264.so.152)
7fffeab45bdb x264_slices_write+0xfb
(/usr/lib/x86_64-linux-gnu/libx264.so.152)
5555561e3d87 [unknown] ([heap])

ffmpeg 11750 6671.110007: 1000000 mem_load_uops_retired.l3_miss:u:
0 5080021 N/A|SNP N/A|TLB N/A|LCK N/A
7fffeab6cdde x264_frame_init_lowres_core_avx2+0x8e
(/usr/lib/x86_64-linux-gnu/libx264.so.152)

ffmpeg 11750 6671.463562: 1000000 mem_load_uops_retired.l3_miss:u:
0 5080021 N/A|SNP N/A|TLB N/A|LCK N/A
7fffeaabf806 x264_macroblock_load_pic_pointers+0x886 (inlined)
7fffeaabf806 x264_macroblock_cache_load+0x886 (inlined)
7fffeaabf806 x264_macroblock_cache_load_progressive+0x886
(/usr/lib/x86_64-linux-gnu/libx264.so.152)
7fffeab49204 x264_slice_write+0x1554
(/usr/lib/x86_64-linux-gnu/libx264.so.152)
7fffeab85285 x264_stack_align+0x15
(/usr/lib/x86_64-linux-gnu/libx264.so.152)
7fffeab45bdb x264_slices_write+0xfb
(/usr/lib/x86_64-linux-gnu/libx264.so.152)
1c [unknown] ([unknown])

None of the backtraces are correct. Because none of them begin with
"__start" or "__GI___clone". I also used "LBR", instead. But it has more
size constraints and, therefore, not suitable. The important thing to
note is that the problem occurs only with user space events (and for all
events that I checked). I do not think that the problem is with
DebugInfo. Because I manually used "perf_event_open()" system call
(without using "Perf") and the problem was still there (with raw
callstack IPs).

Therefore, I assumed that the problem is inside the kernel. Precisely,
it should be where the userspace callchain is extracted or dumped. I
looked for the latter (i.e., the callchain dump implementation) and it
seemed to be here:
https://github.com/torvalds/linux/blob/master/kernel/events/core.c#L6786

But I could not (or, equivalently, did not know how to) view the user
callchain instruction pointers.
Am I on the right track? Does anybody know the kernel mechanism for
extracting userspace callchains?

Please accept my apology for my frequent questions. I tried to get
around the problem, myself, but it has taken more than three complete
days and I'm stuck!
I really appreciate any suggestions.

Regards.