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

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The problem doesn't seems to be caused by the assembly code. I checked the execution in GDB, and put breakpoint at "x264_pixel_avg_w16_avx2+0x4" and ignored with 10, 100, 1000, 10000 and 100000 occurrences. But, in all cases, GDB, successfully, displayed the whole backtrace. One of them is as follows: #0 0x00007fffeab68840 in x264_pixel_avg_w16_avx2 () at /usr/lib/x86_64-linux-gnu/ #1 0x00007fffeaabdad9 in x264_mb_mc_01xywh (h=0x5555561dea00, x=0, y=0, width=4, height=<optimized out>) at common/macroblock.c:128 #2 0x00007fffeab0dd37 in x264_macroblock_analyse (h=h@entry=0x5555561dea00) at encoder/analyse.c:3456 #3 0x00007fffeab48c6e in x264_slice_write (h=0x5555561dea00) at encoder/encoder.c:2739 #4 0x00007fffeab85286 in x264_stack_align () at /usr/lib/x86_64-linux-gnu/ #5 0x00007fffeab45bdc in x264_slices_write (h=h@entry=0x5555561dea00) at encoder/encoder.c:3079 #6 0x00007fffeab52062 in x264_encoder_encode (h=<optimized out>, pp_nal=pp_nal@entry=0x7fffffffca28, pi_nal=pi_nal@entry=0x7fffffffca20, pic_in=pic_in@entry=0x555555828098, pic_out=pic_out@entry=0x7fffffffca30) at encoder/encoder.c:3738 #7 0x00007ffff5ff8cd1 in X264_frame (ctx=0x555555827800, pkt=0x55555610acc0, frame=0x55555633a500, got_packet=0x7fffffffcc64) at src/libavcodec/libx264.c:337 #8 0x00007ffff5dbc6c7 in avcodec_encode_video2 (avctx=avctx@entry=0x555555827800, avpkt=0x55555610acc0, frame=frame@entry=0x55555633a500, got_packet_ptr=got_packet_ptr@entry=0x7fffffffcc64) at src/libavcodec/encode.c:307 #9 0x00007ffff5dbcaad in do_encode (avctx=0x555555827800, frame=0x55555633a500, got_packet=0x7fffffffcc64) at src/libavcodec/encode.c:378 #10 0x00007ffff5dbcc5a in avcodec_send_frame (avctx=0x555555827800, frame=0x55555633a500) at src/libavcodec/encode.c:427 #11 0x000055555557e253 in do_video_out (of=0x555555825020, ost=0x5555558208a0, next_picture=0x55555633a500, sync_ipts=<optimized out>) at src/fftools/ffmpeg.c:1288 #12 0x000055555558220f in reap_filters (flush=0) at src/fftools/ffmpeg.c:1506
#13 0x00005555555611b2 in transcode_step () at src/fftools/ffmpeg.c:4588
#14 0x00005555555611b2 in transcode () at src/fftools/ffmpeg.c:4632
#15 0x00005555555611b2 in main (argc=<optimized out>, argv=<optimized out>) at src/fftools/ffmpeg.c:4838

It seems that, there should be something wrong with the kernel-side implementation. Could anybody point me to the kernel implementation? I think it is dumped, here:

But, I do not know where in the kernel, the user call-stack is generated.
Any guesses?


On 2020-06-16 15:08, ahmadkhorrami wrote:

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:

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.


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

On Mon, Jun 15, 2020 at 1:32 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
On Sun, 14 Jun 2020 18:13:21 +0430
ahmadkhorrami <ahmadkhorrami@xxxxxxxx> wrote:


I used the following command to sample backtraces for a simple "ffmpeg"
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

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

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

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
7fffeab463bc x264_fdec_filter_row+0x69c
7fffeab49523 x264_slice_write+0x1873
7fffeab85285 x264_stack_align+0x15
7fffeab45bdb x264_slices_write+0xfb
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

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
7fffeab49204 x264_slice_write+0x1554
7fffeab85285 x264_stack_align+0x15
7fffeab45bdb x264_slices_write+0xfb
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:

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:
For this to work with dwarf unwinding it needs to have call frame information:


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@xxxxxxxxxxxxxxx and not
linux-trace-users. As linux-trace-users are more for ftrace and not

-- Steve

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