Re: Perf Script Erroneous User Stack Trace

[ahmadkhorrami <ahmadkhorrami@ut.ac.ir>]

Hi,
Thanks, Steven!
It was full of patches. Therefore, I thought posting questions might not 
be OK. I will post my further questions, there.
Regards.

On 2020-06-16 01:01, Steven Rostedt wrote:

> 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