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

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

Hi,
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/libx264.so.152
#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/libx264.so.152
#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:
https://github.com/torvalds/linux/blob/master/kernel/events/core.c#L6786

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

Regards.

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:
> 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