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SubjectRe: [PATCH] perf: ARM: Record the user-mode PC in the call chain.
Will Deacon [will.deacon@arm.com] wrote:
| On Thu, Jun 20, 2013 at 04:17:06AM +0100, Jed Davis wrote:
| > On Tue, Jun 18, 2013 at 02:13:19PM +0100, Will Deacon wrote:
| > > On Fri, Jun 14, 2013 at 12:21:11AM +0100, Jed Davis wrote:
| > > > With this change, we no longer lose the innermost entry in the user-mode
| > > > part of the call chain. See also the x86 port, which includes the ip.
| > > >
| > > > It's possible to partially work around this problem by post-processing
| > > > the data to use the PERF_SAMPLE_IP value, but this works only if the CPU
| > > > wasn't in the kernel when the sample was taken.
| > >
| > > Thanks. I guess we need something similar for arm64 too. Could you cook a
| > > similar patch please?
| >
| > Done (and tested, on the ARM V8 Foundation Model).
|
| Wow, I didn't expect you to test it. Thanks!
|
| > It looked as if the powerpc and sparc ports might have similar issues,
| > but I haven't checked on them yet.
|
| I can extract some hardware from the pile in the corner of my flat if you
| want a hand with testing.

Interesting. Powerpc already saves the next_ip for the first entry:

arch/powerpc/perf/callchain.c:

perf_callchain_user_64():
...

next_ip = perf_instruction_pointer(regs);
lr = regs->link;
sp = regs->gpr[1];
perf_callchain_store(entry, next_ip);

for (;;) {


Do you have a test case/output that I can run on Power ?

We actually have the opposite problem where we see duplication in
callchains like this. We considered not saving the "link register"
but that seems to break for a "leaf-node" functions.

13.65% sprintft libc-2.12.so [.] __random
|
--- __random
|
|--62.82%-- __random
| |
| |--97.31%-- rand
| | do_my_sprintf
| | main
| | generic_start_main
| | __libc_start_main
| | 0x0
| |
| --2.69%-- do_my_sprintf
| main
| generic_start_main
| __libc_start_main
| 0x0
|
--37.18%-- rand
|
|--93.30%-- rand
| do_my_sprintf
| main
| generic_start_main
| __libc_start_main
| 0x0
|
--6.70%-- do_my_sprintf
main
generic_start_main
__libc_start_main
0x0



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