Nysal reported that userspace backtraces are missing in offcputime bcc
tool. As an example:
$ sudo ./bcc/tools/offcputime.py -uU
Tracing off-CPU time (us) of user threads by user stack... Hit Ctrl-C to
end.
^C
write
- python (9107)
8
write
- sudo (9105)
9
mmap
- python (9107)
16
clock_nanosleep
- multipathd (697)
3001604
The offcputime bcc tool attaches a bpf program to a kprobe on
finish_task_switch(), which is usually hit on a syscall from userspace.
With the switch to system call vectored, we zero out LR value in user
pt_regs on syscall entry. BPF uses perf callchain infrastructure for
capturing stack traces, and this stores LR as the second entry in the
stack trace. Since this is NULL, userspace unwinders assume that there
are no further entries resulting in a truncated userspace stack trace.
Rather than fixing all userspace unwinders to ignore/skip past the
second entry, store NIP as LR so that there continues to be a valid,
though duplicate entry.
With this change:
$ sudo ./bcc/tools/offcputime.py -uU
Tracing off-CPU time (us) of user threads by user stack... Hit Ctrl-C to
end.
^C
write
write
[unknown]
[unknown]
[unknown]
[unknown]
[unknown]
PyObject_VectorcallMethod
[unknown]
[unknown]
PyObject_CallOneArg
PyFile_WriteObject
PyFile_WriteString
[unknown]
[unknown]
PyObject_Vectorcall
_PyEval_EvalFrameDefault
PyEval_EvalCode
[unknown]
[unknown]
[unknown]
_PyRun_SimpleFileObject
_PyRun_AnyFileObject
Py_RunMain
[unknown]
Py_BytesMain
[unknown]
__libc_start_main
- python (1293)
7
write
write
[unknown]
sudo_ev_loop_v1
sudo_ev_dispatch_v1
[unknown]
[unknown]
[unknown]
[unknown]
__libc_start_main
- sudo (1291)
7
syscall
syscall
bpf_open_perf_buffer_opts
[unknown]
[unknown]
[unknown]
[unknown]
_PyObject_MakeTpCall
PyObject_Vectorcall
_PyEval_EvalFrameDefault
PyEval_EvalCode
[unknown]
[unknown]
[unknown]
_PyRun_SimpleFileObject
_PyRun_AnyFileObject
Py_RunMain
[unknown]
Py_BytesMain
[unknown]
__libc_start_main
- python (1293)
11
clock_nanosleep
clock_nanosleep
nanosleep
sleep
[unknown]
[unknown]
__clone
- multipathd (698)
3001661
Reported-by: Nysal Jan K.A <ny...@linux.ibm.com>
Signed-off-by: Naveen N Rao <nav...@kernel.org>
---
arch/powerpc/kernel/interrupt_64.S | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/powerpc/kernel/interrupt_64.S
b/arch/powerpc/kernel/interrupt_64.S
index bd863702d812..5cf3758a19d3 100644
--- a/arch/powerpc/kernel/interrupt_64.S
+++ b/arch/powerpc/kernel/interrupt_64.S
@@ -53,6 +53,7 @@ _ASM_NOKPROBE_SYMBOL(system_call_vectored_\name)
ld r1,PACAKSAVE(r13)
std r10,0(r1)
std r11,_NIP(r1)
+ std r11,_LINK(r1)
std r12,_MSR(r1)
std r0,GPR0(r1)
std r10,GPR1(r1)
@@ -70,7 +71,6 @@ _ASM_NOKPROBE_SYMBOL(system_call_vectored_\name)
std r9,GPR13(r1)
SAVE_NVGPRS(r1)
std r11,_XER(r1)
- std r11,_LINK(r1)
std r11,_CTR(r1)
li r11,\trapnr
base-commit: 414e92af226ede4935509b0b5e041810c92e003f
--
2.43.0
