Re: [PATCH v5 1/8] perf: support specify vdso path in cmdline
From: Adrian Hunter
Date: Thu Jul 18 2024 - 13:02:41 EST
On 2/07/24 07:18, Changbin Du wrote:
> The vdso dumped from process memory (in buildid-cache) lacks debugging
> info. To annotate vdso symbols with source lines we need specify a
> debugging version.
>
> For x86, we can find them from your local build as
> arch/x86/entry/vdso/vdso{32,64}.so.dbg. Or they may reside in
> /lib/modules/<version>/vdso/vdso{32,64}.so on Ubuntu. But notice that
> the buildid has to match.
>
> $ sudo perf record -a
> $ sudo perf report --objdump=llvm-objdump \
> --vdso arch/x86/entry/vdso/vdso64.so.dbg,arch/x86/entry/vdso/vdso32.so.dbg
>
> Samples: 17K of event 'cycles:P', 4000 Hz, Event count (approx.): 1760
> __vdso_clock_gettime /work/linux-host/arch/x86/entry/vdso/vdso64.so.d
> Percent│ movq -48(%rbp),%rsi
> │ testq %rax,%rax
> │ ; return vread_hvclock();
> │ movq %rax,%rdx
> │ ; if (unlikely(!vdso_cycles_ok(cycles)))
> │ ↑ js eb
> │ ↑ jmp 74
> │ ; ts->tv_sec = vdso_ts->sec;
> 0.02 │147: leaq 2(%rbx),%rax
> │ shlq $4, %rax
> │ addq %r10,%rax
> │ ; while ((seq = READ_ONCE(vd->seq)) & 1) {
> 9.38 │152: movl (%r10),%ecx
>
> When doing cross platform analysis, we also need specify the vdso path if
> we are interested in its symbols.
>
Just realized this is about objdump. Sorry for not getting that
earlier.
Like perf tools, objdump follows the paradigm of attempting to
locate and use debug info transparently. However, objdump looks
at the installed debug info in /usr/lib/debug/.build-id/
For example, if the debug file is copied (or linked) there, then
objdump or llvm-objdump will find it:
$ llvm-objdump-18 -dS ~/.debug/\[vdso\]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso | head -20
~/.debug/[vdso]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso: file format elf64-x86-64
Disassembly of section .text:
00000000000006d0 <.text>:
6d0: 48 8d 3d 29 d9 ff ff leaq -0x26d7(%rip), %rdi # 0xffffffffffffe000
6d7: eb 19 jmp 0x6f2 <.text+0x22>
6d9: 4c 8b 0d 28 d9 ff ff movq -0x26d8(%rip), %r9 # 0xffffffffffffe008
6e0: 4c 8b 05 29 d9 ff ff movq -0x26d7(%rip), %r8 # 0xffffffffffffe010
6e7: 0f 01 f9 rdtscp
6ea: 66 90 nop
6ec: 8b 0f movl (%rdi), %ecx
6ee: 39 ce cmpl %ecx, %esi
6f0: 74 0e je 0x700 <.text+0x30>
6f2: 8b 37 movl (%rdi), %esi
6f4: 85 f6 testl %esi, %esi
6f6: 75 e1 jne 0x6d9 <.text+0x9>
6f8: 48 c7 c0 ff ff ff ff movq $-0x1, %rax
6ff: c3 retq
$ sudo ln -s /lib/modules/6.9.2-local/build/arch/x86/entry/vdso/vdso64.so.dbg /usr/lib/debug/.build-id/cf/702469f4637840fd6ba1a8d8a628ff83253d04.debug
$ llvm-objdump-18 -dS ~/.debug/\[vdso\]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso | head -20
~/.debug/[vdso]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso: file format elf64-x86-64
Disassembly of section .text:
00000000000006d0 <vread_hvclock>:
; hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg,
6d0: 48 8d 3d 29 d9 ff ff leaq -0x26d7(%rip), %rdi # 0xffffffffffffe000 <hvclock_page>
6d7: eb 19 jmp 0x6f2 <vread_hvclock+0x22>
; scale = READ_ONCE(tsc_pg->tsc_scale);
6d9: 4c 8b 0d 28 d9 ff ff movq -0x26d8(%rip), %r9 # 0xffffffffffffe008 <hvclock_page+0x8>
; offset = READ_ONCE(tsc_pg->tsc_offset);
6e0: 4c 8b 05 29 d9 ff ff movq -0x26d7(%rip), %r8 # 0xffffffffffffe010 <hvclock_page+0x10>
; asm volatile(ALTERNATIVE_2("rdtsc",
6e7: 0f 31 rdtsc
6e9: 90 nop
6ea: 90 nop
6eb: 90 nop
; } while (READ_ONCE(tsc_pg->tsc_sequence) != sequence);
6ec: 8b 0f movl (%rdi), %ecx
$
Would that solve your problem?
Notably, later versions of llvm-objdump have an option
--debug-file-directory which makes it possible to
have the debug info in a directory owned by the user,
for example:
$ sudo rm /usr/lib/debug/.build-id/cf/702469f4637840fd6ba1a8d8a628ff83253d04.debug
$ llvm-objdump-18 -dS ~/.debug/\[vdso\]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso | head -20
~/.debug/[vdso]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso: file format elf64-x86-64
Disassembly of section .text:
00000000000006d0 <.text>:
6d0: 48 8d 3d 29 d9 ff ff leaq -0x26d7(%rip), %rdi # 0xffffffffffffe000
6d7: eb 19 jmp 0x6f2 <.text+0x22>
6d9: 4c 8b 0d 28 d9 ff ff movq -0x26d8(%rip), %r9 # 0xffffffffffffe008
6e0: 4c 8b 05 29 d9 ff ff movq -0x26d7(%rip), %r8 # 0xffffffffffffe010
6e7: 0f 01 f9 rdtscp
6ea: 66 90 nop
6ec: 8b 0f movl (%rdi), %ecx
6ee: 39 ce cmpl %ecx, %esi
6f0: 74 0e je 0x700 <.text+0x30>
6f2: 8b 37 movl (%rdi), %esi
6f4: 85 f6 testl %esi, %esi
6f6: 75 e1 jne 0x6d9 <.text+0x9>
6f8: 48 c7 c0 ff ff ff ff movq $-0x1, %rax
6ff: c3 retq
$ mkdir -p /tmp/debug/.build-id/cf/
$ ln -s /lib/modules/6.9.2-local/build/arch/x86/entry/vdso/vdso64.so.dbg /tmp/debug/.build-id/cf/702469f4637840fd6ba1a8d8a628ff83253d04.debug
$ llvm-objdump-18 --debug-file-directory /tmp/debug -dS ~/.debug/\[vdso\]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso | head -20
~/.debug/[vdso]/cf702469f4637840fd6ba1a8d8a628ff83253d04/vdso: file format elf64-x86-64
Disassembly of section .text:
00000000000006d0 <vread_hvclock>:
; hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg,
6d0: 48 8d 3d 29 d9 ff ff leaq -0x26d7(%rip), %rdi # 0xffffffffffffe000 <hvclock_page>
6d7: eb 19 jmp 0x6f2 <vread_hvclock+0x22>
; scale = READ_ONCE(tsc_pg->tsc_scale);
6d9: 4c 8b 0d 28 d9 ff ff movq -0x26d8(%rip), %r9 # 0xffffffffffffe008 <hvclock_page+0x8>
; offset = READ_ONCE(tsc_pg->tsc_offset);
6e0: 4c 8b 05 29 d9 ff ff movq -0x26d7(%rip), %r8 # 0xffffffffffffe010 <hvclock_page+0x10>
; asm volatile(ALTERNATIVE_2("rdtsc",
6e7: 0f 31 rdtsc
6e9: 90 nop
6ea: 90 nop
6eb: 90 nop
; } while (READ_ONCE(tsc_pg->tsc_sequence) != sequence);
6ec: 8b 0f movl (%rdi), %ecx
$