On 1/23/19 11:55 AM, Vineet Gupta wrote:
> Hi,
>
> I noticed a small anomaly in perf report -v output on ARC and x86 as well.
>
> A simple program which sits in tight loop, compiled for x86_64
>
> void main() { while(1) {} }
>
> $ gcc -g tight.c
> $ ./a.out &
> $ perf record -e cycles -p 26703
> $ perf report -n -v --stdio | egrep "(main|Symbol)"
>
> |# Overhead Samples Command Shared Object Symbol
>
> | 99.93% 55753 a.out /home/arc/test/a.out 0x4da B [.] main
>
> | ^^^^^
>
> The printer address for Symbols is *not* the actual address in elf, but rather VMA
> start offset.
>
> 0x4da = 0x4004da - 0x0000000000400000
>
> $ objdump -d ./a.out
>
> | 00000000004004d6 <main>:
> | 4004d6: 55 push %rbp
> | 4004d7: 48 89 e5 mov %rsp,%rbp
> | 4004da: eb fe jmp 4004da <main+0x4>
> | 4004dc: 0f 1f 40 00 nopl 0x0(%rax)
>
> $ readelf -a ./a.out
>
> | Program Headers:
> | Type Offset VirtAddr PhysAddr
> | FileSiz MemSiz Flags Align
> | Program Headers:
> | LOAD 0x0000000000000000 0x0000000000400000 0x0000000000400000
> 0x000000000000068c 0x000000000000068c R E 200000
>
>
> This is problematic in narrowing down the hotspot instruction, when the binary
> itself is. One needs to do the offset addition manually to find the actual hotspot
> location.
>
> | 99.79% 100064 a.out /home/arc/test/a.out 0x4da B [.] 0x00000000000004da
>
> ^^^^^^^^^^^^^^^^
>
> Is this considered an issue ? Would the fix to print the actual symbol address
> (and recorded in raw perf event data) break some existing tooling etc.
>
@Arnaldo any ideas ?
this is being done in tools/perf/util/symbol-elf.c
symsrc__init
if (dso->kernel == DSO_TYPE_USER)
ss->adjust_symbols = true;
dso__load_sym
dso->adjust_symbols = runtime_ss->adjust_symbols || ref_reloc(kmap);
} else if ((used_opd && runtime_ss->adjust_symbols) ||
(!used_opd && syms_ss->adjust_symbols)) {
*sym.st_value -= shdr.sh_addr - shdr.sh_offset;*
}
I investigated a bit more and this goes back to 2009:
Initially the adj was done for prelink binaries: commit f5812a7a336fb
("perf_counter tools: Adjust only prelinked symbol's addresses")
+ self->prelinked = elf_section_by_name(elf, &ehdr, &shdr,
+ ".gnu.prelink_undo",
+ NULL) != NULL;
+ if (self->prelinked) {
if (verbose >= 2)
printf("adjusting symbol: ...
sym.st_value -= shdr.sh_addr - shdr.sh_offset;
+ }
commit 30d7a77dd5a97 ("perf_counter tools: Adjust symbols in ET_EXEC files too")
started doing this for any executable
- self->prelinked = elf_section_by_name(elf, &ehdr, &shdr,
- ".gnu.prelink_undo",
- NULL) != NULL;
+ self->adjust_symbols = (ehdr.e_type == ET_EXEC ||
+ elf_section_by_name(elf, &ehdr, &shdr,
+ ".gnu.prelink_undo",
+
