Re: Sp! Re: Perf Script Erroneous User Stack Trace

ahmadkhorrami
2020-06-16T10:38:35Z

Hi Ian,
That's a good point. Thanks!
Now, I need to verify that.
1) I will focus on single element backtraces, such as this one:
x264_pixel_avg_w16_avx2+0x4

I will set a breakpoint at this address in GDB and sample its
occurrences (e.g., by ignoring every 1000 occurrences) while checking if
GDB can generate backtraces.

2) But to fully verify the cause of the problem, I need to know the
kernel-level mechanism/code location for capturing user-level
callchains.

Regards.

On 2020-06-16 01:23, Ian Rogers wrote:

> On Mon, Jun 15, 2020 at 1:32 PM Steven Rostedt <rostedt@goodmis.org>
> 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?

Hi Ahmad,

a lot of ffmpeg is hand written assembly such as:
https://github.com/FFmpeg/FFmpeg/blob/master/libavresample/x86/audio_convert.asm
For this to work with dwarf unwinding it needs to have call frame
information:
https://sourceware.org/binutils/docs/as/CFI-directives.html

Thanks,
Ian

>> 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