Re: Perf Script Erroneous User Stack Trace

[Steven Rostedt <rostedt@goodmis.org>]

On Sun, 14 Jun 2020 18:13:21 +0430
ahmadkhorrami <ahmadkhorrami@ut.ac.ir> wrote:

> 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.

No problem, but please note that perf questions are more likely to be
answered via: linux-perf-users@vger.kernel.org and not
linux-trace-users. As linux-trace-users are more for ftrace and not
perf.

-- Steve