We'll test the BTF encoder using perf's eBPF integration, but really we can plain
use clang directly, setting up all its options.

Using perf's integration will save some time here, to see all it does, use
'perf trace -vv' plus the options used below, then all the steps will be shown.

Build perf from the latest kernel sources, use it with clang/llvm like:

  [root@seventh ~]# clang --version
  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c)
  Target: x86_64-unknown-linux-gnu
  Thread model: posix
  InstalledDir: /usr/local/bin
  [root@seventh ~]# llc --version | head -17
  LLVM (http://llvm.org/):
    LLVM version 8.0.0svn
    DEBUG build with assertions.
    Default target: x86_64-unknown-linux-gnu
    Host CPU: skylake

    Registered Targets:
      aarch64    - AArch64 (little endian)
      aarch64_be - AArch64 (big endian)
      amdgcn     - AMD GCN GPUs
      arm        - ARM
      arm64      - ARM64 (little endian)
      armeb      - ARM (big endian)
      bpf        - BPF (host endian)
      bpfeb      - BPF (big endian)
      bpfel      - BPF (little endian)
      hexagon    - Hexagon
  [root@seventh ~]# 

Then enable saving the object file build as part of perf's handling of foo.c type
events, i.e. eBPF programs that will be compiled with clang and then loaded with
sys_bpf() to possibly insert events in perf's ring buffer via bpf_perf_event_output(),
or interact with the system via bpf_trace_printk() or just work as filters, etc:

  # cat ~/.perfconfig
  [llvm]
	dump-obj = true

Then run a simple example, found in the kernel sources:

  # perf trace -e tools/perf/examples/bpf/hello.c cat /etc/passwd > /dev/null
  LLVM: dumping tools/perf/examples/bpf/hello.o
     0.000 __bpf_stdout__:Hello, world
     0.028 __bpf_stdout__:Hello, world
     0.291 __bpf_stdout__:Hello, world
  #

Notice that "LLVM: dumping..." line, look at the ELF sections in that file:

  [root@seventh perf]# readelf -SW tools/perf/examples/bpf/hello.o
  There are 11 section headers, starting at offset 0x220:

  Section Headers:
    [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
    [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
    [ 1] .strtab           STRTAB          0000000000000000 00018c 00008d 00      0   0  1
    [ 2] .text             PROGBITS        0000000000000000 000040 000000 00  AX  0   0  4
    [ 3] syscalls:sys_enter_openat PROGBITS        0000000000000000 000040 000088 00  AX  0   0  8
    [ 4] .relsyscalls:sys_enter_openat REL             0000000000000000 000178 000010 10     10   3  8
    [ 5] maps              PROGBITS        0000000000000000 0000c8 00001c 00  WA  0   0  4
    [ 6] .rodata.str1.1    PROGBITS        0000000000000000 0000e4 00000e 01 AMS  0   0  1
    [ 7] license           PROGBITS        0000000000000000 0000f2 000004 00  WA  0   0  1
    [ 8] version           PROGBITS        0000000000000000 0000f8 000004 00  WA  0   0  4
    [ 9] .llvm_addrsig     LOOS+0xfff4c03  0000000000000000 000188 000004 00   E 10   0  1
    [10] .symtab           SYMTAB          0000000000000000 000100 000078 18      1   1  8
  Key to Flags:
    W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
    L (link order), O (extra OS processing required), G (group), T (TLS),
    C (compressed), x (unknown), o (OS specific), E (exclude),
    p (processor specific)
  [root@seventh perf]#

No DWARF debugging info, so we need to further customize ~/.perfconfig LLVM section:

  [root@seventh perf]# cat ~/.perfconfig
  [llvm]
	  dump-obj = true
	  clang-opt = -g
  [root@seventh perf]# perf trace -e tools/perf/examples/bpf/hello.c cat /etc/passwd > /dev/null
  LLVM: dumping tools/perf/examples/bpf/hello.o
       0.000 __bpf_stdout__:Hello, world
       0.015 __bpf_stdout__:Hello, world
       0.184 __bpf_stdout__:Hello, world
  [root@seventh perf]# 
  [root@seventh perf]# readelf -SW tools/perf/examples/bpf/hello.o
  There are 26 section headers, starting at offset 0xe20:

  Section Headers:
    [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
    [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
    [ 1] .strtab           STRTAB          0000000000000000 000cf4 000127 00      0   0  1
    [ 2] .text             PROGBITS        0000000000000000 000040 000000 00  AX  0   0  4
    [ 3] syscalls:sys_enter_openat PROGBITS        0000000000000000 000040 000088 00  AX  0   0  8
    [ 4] .relsyscalls:sys_enter_openat REL             0000000000000000 000a80 000010 10     25   3  8
    [ 5] maps              PROGBITS        0000000000000000 0000c8 00001c 00  WA  0   0  4
    [ 6] .rodata.str1.1    PROGBITS        0000000000000000 0000e4 00000e 01 AMS  0   0  1
    [ 7] license           PROGBITS        0000000000000000 0000f2 000004 00  WA  0   0  1
    [ 8] version           PROGBITS        0000000000000000 0000f8 000004 00  WA  0   0  4
    [ 9] .debug_str        PROGBITS        0000000000000000 0000fc 0001d2 01  MS  0   0  1
    [10] .debug_loc        PROGBITS        0000000000000000 0002ce 000023 00      0   0  1
    [11] .debug_abbrev     PROGBITS        0000000000000000 0002f1 0000e3 00      0   0  1
    [12] .debug_info       PROGBITS        0000000000000000 0003d4 000182 00      0   0  1
    [13] .rel.debug_info   REL             0000000000000000 000a90 000210 10     25  12  8
    [14] .debug_ranges     PROGBITS        0000000000000000 000556 000030 00      0   0  1
    [15] .debug_macinfo    PROGBITS        0000000000000000 000586 000001 00      0   0  1
    [16] .debug_pubnames   PROGBITS        0000000000000000 000587 00006e 00      0   0  1
    [17] .rel.debug_pubnames REL             0000000000000000 000ca0 000010 10     25  16  8
    [18] .debug_pubtypes   PROGBITS        0000000000000000 0005f5 000056 00      0   0  1
    [19] .rel.debug_pubtypes REL             0000000000000000 000cb0 000010 10     25  18  8
    [20] .debug_frame      PROGBITS        0000000000000000 000650 000028 00      0   0  8
    [21] .rel.debug_frame  REL             0000000000000000 000cc0 000020 10     25  20  8
    [22] .debug_line       PROGBITS        0000000000000000 000678 0000a7 00      0   0  1
    [23] .rel.debug_line   REL             0000000000000000 000ce0 000010 10     25  22  8
    [24] .llvm_addrsig     LOOS+0xfff4c03  0000000000000000 000cf0 000004 00   E 25   0  1
    [25] .symtab           SYMTAB          0000000000000000 000720 000360 18      1  32  8
  Key to Flags:
    W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
    L (link order), O (extra OS processing required), G (group), T (TLS),
    C (compressed), x (unknown), o (OS specific), E (exclude),
    p (processor specific)
  [root@seventh perf]#

Now lets use 'pahole --btf_encode' (or 'pahole -J') to add an ELF section to that object
file with the conversion from the DWARF sections to a new one, for BTF:

  [root@seventh perf]# pahole --btf_encode tools/perf/examples/bpf/hello.o

  [root@seventh perf]# readelf -SW tools/perf/examples/bpf/hello.o
  There are 27 section headers, starting at offset 0x1080:

  Section Headers:
    [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
    [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
    [ 1] .text             PROGBITS        0000000000000000 000040 000000 00  AX  0   0  4
    [ 2] syscalls:sys_enter_openat PROGBITS        0000000000000000 000040 000088 00  AX  0   0  8
    [ 3] maps              PROGBITS        0000000000000000 0000c8 00001c 00  WA  0   0  4
    [ 4] .rodata.str1.1    PROGBITS        0000000000000000 0000e4 00000e 01 AMS  0   0  1
    [ 5] license           PROGBITS        0000000000000000 0000f2 000004 00  WA  0   0  1
    [ 6] version           PROGBITS        0000000000000000 0000f8 000004 00  WA  0   0  4
    [ 7] .debug_str        PROGBITS        0000000000000000 0000fc 0001d2 01  MS  0   0  1
    [ 8] .debug_loc        PROGBITS        0000000000000000 0002ce 000023 00      0   0  1
    [ 9] .debug_abbrev     PROGBITS        0000000000000000 0002f1 0000e3 00      0   0  1
    [10] .debug_info       PROGBITS        0000000000000000 0003d4 000182 00      0   0  1
    [11] .debug_ranges     PROGBITS        0000000000000000 000556 000030 00      0   0  1
    [12] .debug_macinfo    PROGBITS        0000000000000000 000586 000001 00      0   0  1
    [13] .debug_pubnames   PROGBITS        0000000000000000 000587 00006e 00      0   0  1
    [14] .debug_pubtypes   PROGBITS        0000000000000000 0005f5 000056 00      0   0  1
    [15] .debug_frame      PROGBITS        0000000000000000 000650 000028 00      0   0  8
    [16] .debug_line       PROGBITS        0000000000000000 000678 0000a7 00      0   0  1
    [17] .symtab           SYMTAB          0000000000000000 000720 000360 18     25  32  8
    [18] .relsyscalls:sys_enter_openat REL             0000000000000000 000a80 000010 10     17   2  8
    [19] .rel.debug_info   REL             0000000000000000 000a90 000210 10     17  10  8
    [20] .rel.debug_pubnames REL             0000000000000000 000ca0 000010 10     17  13  8
    [21] .rel.debug_pubtypes REL             0000000000000000 000cb0 000010 10     17  14  8
    [22] .rel.debug_frame  REL             0000000000000000 000cc0 000020 10     17  15  8
    [23] .rel.debug_line   REL             0000000000000000 000ce0 000010 10     17  16  8
    [24] .llvm_addrsig     LOOS+0xfff4c03  0000000000000000 000cf0 000004 00   E  0   0  1
    [25] .strtab           STRTAB          0000000000000000 000cf4 00019c 00      0   0  1
    [26] .BTF              PROGBITS        0000000000000000 000e90 0001ea 00      0   0  1
  Key to Flags:
    W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
    L (link order), O (extra OS processing required), G (group), T (TLS),
    C (compressed), x (unknown), o (OS specific), E (exclude),
    p (processor specific)
  readelf: tools/perf/examples/bpf/hello.o: Warning: possibly corrupt ELF header - it has a non-zero program header offset, but no program headers
  [root@seventh perf]# 

That new ".BTF" section should then be parseable by the kernel, that has a BTF
decoder, something not available for pahole at this time, but that will come in
a later version.

When pahole tries to read the DWARF info in that BPF ELF file, hello.o, we can se
a problem that will require us to add another option to the .perfconfig llvm section:

  # pahole tools/perf/examples/bpf/hello.o
  struct clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c) {
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566ec377 clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*     0     4 */
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566ec377 clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*     4     4 */
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566ec377 clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*     8     4 */
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566ec377 clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*    12     4 */
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566ec377 clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*    16     4 */
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566ec377 clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*    20     4 */
	  clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566ec377 clang version 8.0.0 (http://llvm.org/git/clang.git 8587270a739ee30c926a76d5657e65e85b560f6e) (http://llvm.org/git/llvm.git 0566eefef9c3777bd780ec4cbb9efa764633b76c); /*    24     4 */

	  /* size: 28, cachelines: 1, members: 7 */
	  /* last cacheline: 28 bytes */
  };
#

We need to pass some options to llvm, via the llvm.opts variable in ~/.perfconfig:

  [root@seventh perf]# cat ~/.perfconfig 
  [llvm]
	  dump-obj = true
	  clang-opt = -g
	  opts = -mattr=dwarfris
  [root@seventh perf]# perf trace -e tools/perf/examples/bpf/hello.c cat /etc/passwd > /dev/null
  LLVM: dumping tools/perf/examples/bpf/hello.o
       0.000 __bpf_stdout__:Hello, world
       0.018 __bpf_stdout__:Hello, world
       0.209 __bpf_stdout__:Hello, world
  [root@seventh perf]# pahole tools/perf/examples/bpf/hello.o
  struct bpf_map {
	  unsigned int               type;                 /*     0     4 */
	  unsigned int               key_size;             /*     4     4 */
	  unsigned int               value_size;           /*     8     4 */
	  unsigned int               max_entries;          /*    12     4 */
	  unsigned int               map_flags;            /*    16     4 */
	  unsigned int               inner_map_idx;        /*    20     4 */
	  unsigned int               numa_node;            /*    24     4 */

	  /* size: 28, cachelines: 1, members: 7 */
	  /* last cacheline: 28 bytes */
  };
  [root@seventh perf]#

This is not needed when using elfutils >= 0.173, pahole will just work as above.

Now we need to go test the kernel, and to load that file with a BTF section we
can also use perf, passing the .o file instead of the .c one, skipping the
compilation phase and using the modified .o file, we will also run in system
wide mode, so taht we can keep that BPF object loaded and attached to the
tracepoint, so that we can use the kernel facilities to inspect the BTF file as
read and processed by the kernel:

  # perf trace -e tools/perf/examples/bpf/hello.c 2> /dev/null

Now to look if the kernel has the bpf filesystem:

  [acme@jouet perf]$ grep bpf /proc/filesystems 
  nodev	bpf
  [acme@jouet perf]$
  [root@jouet ~]# mount -t bpf nodev /sys/fs/bpf
  [root@jouet ~]# mount | grep bpf
  nodev on /sys/fs/bpf type bpf (rw,relatime)
  [root@jouet ~]# cd /sys/fs/bpf
  [root@jouet bpf]# ls -la
  total 0
  drwxrwxrwt.  2 root root 0 Aug 15 17:42 .
  drwxr-xr-x. 10 root root 0 Aug 13 15:04 ..
  [root@jouet bpf]#

Work is planned to allow using BTF info to pretty print from the bpf fs, see:

  https://www.spinics.net/lists/netdev/msg518606.html
  Date: Sat, 11 Aug 2018

<quote>
For bpftool, BTF pretty print support is missing
for per-cpu maps. bpffs print for per-cpu hash/array maps
need to be added as well. Will add them later.

Acked-by: Yonghong Song <yhs@xxxxxx>
</quote>

To see what libbpf and its users, like perf, does when a ".BTF" ELF section is
found in a BPF object being loaded via sys_bpf(), we can use 'perf ftrace' to
show the sequence of events inside the kernel to load, validade and initialize
data structures related to the request:

  # perf ftrace -G *btf* perf trace -e tools/perf/examples/bpf/hello.o cat /etc/passwd
  3)               |  bpf_btf_load() {
  3)               |    capable() {
  3)               |      ns_capable_common() {
  3)               |        security_capable() {
  3)   0.048 us    |          cap_capable();
  3)               |          selinux_capable() {
  3)   0.092 us    |            cred_has_capability();
  3)   0.444 us    |          }
  3)   1.387 us    |        }
  3)   1.764 us    |      }
  3)   2.168 us    |    }
  3)               |    btf_new_fd() {
  3)               |      kmem_cache_alloc_trace() {
  3)               |        _cond_resched() {
  3)   0.041 us    |          rcu_all_qs();
  3)   0.407 us    |        }
  3)   0.040 us    |        should_failslab();
  3)   0.161 us    |        prefetch_freepointer();
  3)   0.097 us    |        memcg_kmem_put_cache();
  3)   2.719 us    |      }
  3)               |      kmem_cache_alloc_trace() {
  3)               |        _cond_resched() {
  3)   0.040 us    |          rcu_all_qs();
  3)   0.409 us    |        }
  3)   0.040 us    |        should_failslab();
  3)   0.110 us    |        prefetch_freepointer();
  3)   0.099 us    |        memcg_kmem_put_cache();
  3)   2.296 us    |      }
  3)   0.054 us    |      bpf_check_uarg_tail_zero();
  3)               |      __check_object_size() {
  3)   0.152 us    |        __virt_addr_valid();
  3)   0.047 us    |        __check_heap_object();
  3)   0.040 us    |        check_stack_object();
  3)   1.465 us    |      }
  3)   0.041 us    |      btf_sec_info_cmp();
  3)               |      kvmalloc_node() {
  3)               |        __kmalloc_node() {
  3)   0.051 us    |          kmalloc_slab();
  3)               |          _cond_resched() {
  3)   0.042 us    |            rcu_all_qs();
  3)   0.401 us    |          }
  3)   0.038 us    |          should_failslab();
  3)   0.040 us    |          memcg_kmem_put_cache();
  3)   2.168 us    |        }
  3)   2.591 us    |      }
  3)               |      __check_object_size() {
  3)   0.108 us    |        __virt_addr_valid();
  3)   0.050 us    |        __check_heap_object();
  3)   0.039 us    |        check_stack_object();
  3)   1.469 us    |      }
  3)               |      btf_struct_check_meta() {
  3)   0.057 us    |        __btf_verifier_log_type();
  3)   0.057 us    |        btf_verifier_log_member();
  3)   0.043 us    |        btf_verifier_log_member();
  3)   0.042 us    |        btf_verifier_log_member();
  3)   0.043 us    |        btf_verifier_log_member();
  3)   0.043 us    |        btf_verifier_log_member();
  3)               |        btf_verifier_log_member() {
  3)   ==========> |
  3)               |        smp_irq_work_interrupt() {
  3)               |          irq_enter() {
  3)               |            rcu_irq_enter() {
  3)   0.038 us    |              rcu_nmi_enter();
  3)   0.412 us    |            }
  3)   0.054 us    |            irqtime_account_irq();
  3)   1.409 us    |          }
  3)               |          __wake_up() {
  3)               |            __wake_up_common_lock() {
  3)   0.040 us    |              _raw_spin_lock_irqsave();
  3)   0.051 us    |              __wake_up_common();
  3)   0.044 us    |              _raw_spin_unlock_irqrestore();
  3)   1.155 us    |            }
  3)   1.508 us    |          }
  3)               |          irq_exit() {
  3)   0.062 us    |            irqtime_account_irq();
  3)   0.038 us    |            idle_cpu();
  3)               |            rcu_irq_exit() {
  3)   0.038 us    |              rcu_nmi_exit();
  3)   0.419 us    |            }
  3)   1.601 us    |          }
  3)   6.230 us    |        }
  3)   <========== |
  3)   0.088 us    |        } /* btf_verifier_log_member */
  3)   0.041 us    |        btf_verifier_log_member();
  3) + 10.759 us   |      }
  3)               |      kvmalloc_node() {
  3)               |        __kmalloc_node() {
  3)   0.043 us    |          kmalloc_slab();
  3)               |          _cond_resched() {
  3)   0.037 us    |            rcu_all_qs();
  3)   0.455 us    |          }
  3)   0.040 us    |          should_failslab();
  3)   0.037 us    |          memcg_kmem_put_cache();
  3)   2.227 us    |        }
  3)   2.624 us    |      } /* kvmalloc_node */
  3)               |      kvfree() {
  3)   0.048 us    |        kfree();
  3)   0.662 us    |      }
  3)               |      btf_int_check_meta() {
  3)   0.043 us    |        __btf_verifier_log_type();
  3)   0.457 us    |      }
  3)               |      btf_array_check_meta() {
  3)   0.041 us    |        __btf_verifier_log_type();
  3)   0.393 us    |      }
  3)               |      btf_int_check_meta() {
  3)   0.094 us    |        __btf_verifier_log_type();
  3)   0.447 us    |      }
  3)               |      btf_int_check_meta() {
  3)   0.043 us    |        __btf_verifier_log_type();
  3)   0.573 us    |      }
  3)               |      btf_int_check_meta() {
  3)   0.085 us    |        __btf_verifier_log_type();
  3)   0.446 us    |      }
  3)               |      btf_ref_type_check_meta() {
  3)   0.042 us    |        __btf_verifier_log_type();
  3)   0.451 us    |      }
  3)               |      btf_ref_type_check_meta() {
  3)   0.042 us    |        __btf_verifier_log_type();
  3)   0.427 us    |      }
  3)               |      btf_ref_type_check_meta() {
  3)   0.042 us    |        __btf_verifier_log_type();
  3)   0.397 us    |      }
  3)               |      btf_ref_type_check_meta() {
  3)   0.041 us    |        __btf_verifier_log_type();
  3)   0.399 us    |      }
  3)               |      btf_int_check_meta() {
  3)   0.043 us    |        __btf_verifier_log_type();
  3)   0.602 us    |      }
  3)               |      btf_ref_type_check_meta() {
  3)   0.040 us    |        __btf_verifier_log_type();
  3)   0.733 us    |      }
  3)               |      btf_array_check_meta() {
  3)   0.094 us    |        __btf_verifier_log_type();
  3)   0.452 us    |      }
  3)               |      kvmalloc_node() {
  3)               |        __kmalloc_node() {
  3)   0.039 us    |          kmalloc_slab();
  3)               |          _cond_resched() {
  3)   0.041 us    |            rcu_all_qs();
  3)   0.579 us    |          }
  3)   0.039 us    |          should_failslab();
  3)   0.042 us    |          memcg_kmem_put_cache();
  3)   2.538 us    |        }
  3)   2.886 us    |      }
  3)               |      kvmalloc_node() {
  3)               |        __kmalloc_node() {
  3)   0.041 us    |          kmalloc_slab();
  3)               |          _cond_resched() {
  3)   0.038 us    |            rcu_all_qs();
  3)   0.708 us    |          }
  3)   0.038 us    |          should_failslab();
  3)   0.040 us    |          memcg_kmem_put_cache();
  3)   2.483 us    |        }
  3)   2.829 us    |      }
  3)               |      kvmalloc_node() {
  3)               |        __kmalloc_node() {
  3)   0.057 us    |          kmalloc_slab();
  3)               |          _cond_resched() {
  3)   0.040 us    |            rcu_all_qs();
  3)   0.533 us    |          }
  3)   0.039 us    |          should_failslab();
  3)   0.038 us    |          memcg_kmem_put_cache();
  3)   2.680 us    |        }
  3)   3.171 us    |      }
  3)   0.054 us    |      env_stack_push();
  3)               |      btf_struct_resolve() {
  3)   0.051 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.039 us    |        btf_int_check_member();
  3)   0.039 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.039 us    |        btf_int_check_member();
  3)   0.040 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.040 us    |        btf_int_check_member();
  3)   0.039 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.099 us    |        btf_int_check_member();
  3)   0.040 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.042 us    |        btf_int_check_member();
  3)   0.040 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.038 us    |        btf_int_check_member();
  3)   0.038 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.039 us    |        btf_int_check_member();
  3)   6.545 us    |      }
  3)   0.053 us    |      env_stack_push();
  3)               |      btf_array_resolve() {
  3)   0.039 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.090 us    |        btf_type_id_size();
  3)   0.060 us    |        btf_type_int_is_regular();
  3)   0.058 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.051 us    |        btf_type_id_size();
  3)   0.055 us    |        btf_type_int_is_regular();
  3)   3.414 us    |      }
  3)   0.041 us    |      btf_type_id_size();
  3)   0.057 us    |      env_stack_push();
  3)               |      btf_ptr_resolve() {
  3)   0.056 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.054 us    |        env_stack_push();
  3)   1.056 us    |      }
  3)   0.063 us    |      btf_ptr_resolve();
  3)               |      btf_ptr_resolve() {
  3)   0.049 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.086 us    |        btf_type_id_size();
  3)   1.052 us    |      }
  3)   0.045 us    |      env_stack_push();
  3)   0.060 us    |      btf_ptr_resolve();
  3)   0.045 us    |      env_stack_push();
  3)               |      btf_ptr_resolve() {
  3)   0.039 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.062 us    |        btf_type_id_size();
  3)   1.325 us    |      }
  3)   0.054 us    |      env_stack_push();
  3)               |      btf_modifier_resolve() {
  3)   0.061 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.043 us    |        btf_type_id_size();
  3)   0.877 us    |      }
  3)   0.052 us    |      env_stack_push();
  3)               |      btf_array_resolve() {
  3)   0.060 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.051 us    |        btf_type_id_size();
  3)   0.042 us    |        btf_type_int_is_regular();
  3)   0.040 us    |        env_type_is_resolve_sink.isra.19();
  3)   0.042 us    |        btf_type_id_size();
  3)   0.041 us    |        btf_type_int_is_regular();
  3)   2.822 us    |      }
  3)   0.048 us    |      btf_type_id_size();
  3)               |      kvfree() {
  3)   0.148 us    |        kfree();
  3)   0.685 us    |      }
  3)   0.287 us    |      kfree();
  3)   0.042 us    |      _raw_spin_lock_bh();
  3)               |      kmem_cache_alloc() {
  3)   0.040 us    |        should_failslab();
  3)   0.111 us    |        prefetch_freepointer();
  3)   0.094 us    |        memcg_kmem_put_cache();
  3)   2.139 us    |      }
  3)               |      _raw_spin_unlock_bh() {
  3)   0.079 us    |        __local_bh_enable_ip();
  3)   0.460 us    |      }
  3)               |      anon_inode_getfd() {
  3)               |        get_unused_fd_flags() {
  3)               |          __alloc_fd() {
  3)   0.040 us    |            _raw_spin_lock();
  3)   0.041 us    |            expand_files();
  3)   1.374 us    |          }
  3)   1.759 us    |        }
  3)               |        anon_inode_getfile() {
  3)               |          d_alloc_pseudo() {
  3)               |            __d_alloc() {
  3)               |              kmem_cache_alloc() {
  3)               |                _cond_resched() {
  3)   0.035 us    |                  rcu_all_qs();
  3)   0.507 us    |                }
  3)   0.040 us    |                should_failslab();
  3)               |                memcg_kmem_get_cache() {
  3)   0.091 us    |                  get_mem_cgroup_from_mm();
  3)   0.633 us    |                }
  3)   0.111 us    |                prefetch_freepointer();
  3)   0.082 us    |                memcg_kmem_put_cache();
  3)   4.178 us    |              }
  3)   0.162 us    |              d_set_d_op();
  3)   5.545 us    |            }
  3)   6.270 us    |          }
  3)   0.112 us    |          mntget();
  3)   0.125 us    |          ihold();
  3)               |          d_instantiate() {
  3)   0.120 us    |            security_d_instantiate();
  3)   0.106 us    |            _raw_spin_lock();
  3)               |            __d_instantiate() {
  3)   0.069 us    |              d_flags_for_inode();
  3)   0.090 us    |              _raw_spin_lock();
  3)   1.483 us    |            }
  3)   2.767 us    |          }
  3)               |          alloc_file() {
  3)               |            get_empty_filp() {
  3)               |              kmem_cache_alloc() {
  3)               |                _cond_resched() {
  3)   0.039 us    |                  rcu_all_qs();
  3)root:x:0:0:root:/root:/bin/bash
  bin:x:1:1:bin:/bin:/sbin/nologin
  daemon:x:2:2:daemon:/sbin:/sbin/nologin
  adm:x:3:4:adm:/var/adm:/sbin/nologin
  <SNIP rest of /proc/passwd contents>
    0.382 us    |                }
  3)   0.040 us    |                should_failslab();
  3)               |                memcg_kmem_get_cache() {
  3)   0.039 us    |                  get_mem_cgroup_from_mm();
  3)   0.626 us    |                }
  3)   0.050 us    |                prefetch_freepointer();
  3)   0.059 us    |                memcg_kmem_put_cache();
  3)   3.280 us    |              }
  3)               |              security_file_alloc() {
  3)               |                selinux_file_alloc_security() {
  3)               |                  kmem_cache_alloc() {
  3)               |                    _cond_resched() {
  3)   0.038 us    |                      rcu_all_qs();
  3)   0.422 us    |                    }
  3)   0.040 us    |                    should_failslab();
  3)   0.051 us    |                    prefetch_freepointer();
  3)   0.054 us    |                    memcg_kmem_put_cache();
  3)   2.660 us    |                  }
  3)   3.062 us    |                }
  3)   3.548 us    |              }
  3)   0.039 us    |              __mutex_init();
  3)   8.091 us    |            }
  3)   8.617 us    |          }
  3) + 20.810 us   |        }
  3)               |        fd_install() {
  3)   0.054 us    |          __fd_install();
  3)   0.723 us    |        }
  3) + 24.438 us   |      }
  3) ! 109.639 us  |    }
  3) ! 112.925 us  |  }
  3)               |  btf_release() {
  3)               |    btf_put() {
  3)   0.145 us    |      _raw_spin_lock_irqsave();
  3)               |      call_rcu_sched() {
  3)               |        __call_rcu() {
  3)   0.082 us    |          rcu_segcblist_enqueue();
  3)   1.323 us    |        }
  3)   1.782 us    |      }
  3)   0.069 us    |      _raw_spin_unlock_irqrestore();
  3)               |      call_rcu_sched() {
  3)               |        __call_rcu() {
  3)   0.069 us    |          rcu_segcblist_enqueue();
  3)   0.541 us    |        }
  3)   0.984 us    |      }
  3)   5.210 us    |    }
  3)   5.954 us    |  }

This should be enough for us to validate pahole's BTF encoder, and now one can
use 'pahole -F btf' to obtain mostly the same results as with the default use
of '-F dwarf', modulo things like explicit alignments that are not present in
BTF and need some work to be inferred from existing non-natural alignment holes.

- Arnaldo