On 11/12/24 8:56 AM, Alan Maguire wrote:
On 12/11/2024 01:51, Yonghong Song wrote:
On 11/11/24 7:39 AM, Alan Maguire wrote:
On 08/11/2024 18:05, Yonghong Song wrote:
Song Liu reported that a kernel func (perf_event_read()) cannot be
traced
in certain situations since the func is not in vmlinux bTF. This happens
in kernels 6.4, 6.9 and 6.11 and the kernel is built with pahole 1.27.
The perf_event_read() signature in kernel (kernel/events/core.c):
static int perf_event_read(struct perf_event *event, bool group)
Adding '-V' to pahole command line, and the following error msg can
be found:
skipping addition of 'perf_event_read'(perf_event_read) due to
unexpected register used for parameter
Eventually the error message is attributed to the setting
(parm->unexpected_reg = 1) in parameter__new() function.
The following is the dwarf representation for perf_event_read():
0x0334c034: DW_TAG_subprogram
DW_AT_low_pc (0xffffffff812c6110)
DW_AT_high_pc (0xffffffff812c640a)
DW_AT_frame_base (DW_OP_reg7 RSP)
DW_AT_GNU_all_call_sites (true)
DW_AT_name ("perf_event_read")
DW_AT_decl_file ("/rw/compile/kernel/events/core.c")
DW_AT_decl_line (4641)
DW_AT_prototyped (true)
DW_AT_type (0x03324f6a "int")
0x0334c04e: DW_TAG_formal_parameter
DW_AT_location (0x007de9fd:
[0xffffffff812c6115, 0xffffffff812c6141):
DW_OP_reg5 RDI
[0xffffffff812c6141, 0xffffffff812c6323):
DW_OP_reg14 R14
[0xffffffff812c6323, 0xffffffff812c63fe):
DW_OP_GNU_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
[0xffffffff812c63fe, 0xffffffff812c6405):
DW_OP_reg14 R14
[0xffffffff812c6405, 0xffffffff812c640a):
DW_OP_GNU_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value)
DW_AT_name ("event")
DW_AT_decl_file ("/rw/compile/kernel/events/
core.c")
DW_AT_decl_line (4641)
DW_AT_type (0x0333aac2 "perf_event *")
0x0334c05e: DW_TAG_formal_parameter
DW_AT_location (0x007dea82:
[0xffffffff812c6137, 0xffffffff812c63f2):
DW_OP_reg12 R12
[0xffffffff812c63f2, 0xffffffff812c63fe):
DW_OP_GNU_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value
[0xffffffff812c63fe, 0xffffffff812c640a):
DW_OP_reg12 R12)
DW_AT_name ("group")
DW_AT_decl_file ("/rw/compile/kernel/events/
core.c")
DW_AT_decl_line (4641)
DW_AT_type (0x03327059 "bool")
By inspecting the binary, the second argument ("bool group") is used
in the function. The following are the disasm code:
ffffffff812c6110 <perf_event_read>:
ffffffff812c6110: 0f 1f 44 00 00 nopl (%rax,%rax)
ffffffff812c6115: 55 pushq %rbp
ffffffff812c6116: 41 57 pushq %r15
ffffffff812c6118: 41 56 pushq %r14
ffffffff812c611a: 41 55 pushq %r13
ffffffff812c611c: 41 54 pushq %r12
ffffffff812c611e: 53 pushq %rbx
ffffffff812c611f: 48 83 ec 18 subq $24, %rsp
ffffffff812c6123: 41 89 f4 movl %esi, %r12d
<=========== NOTE that here '%esi' is used and moved to '%r12d'.
ffffffff812c6126: 49 89 fe movq %rdi, %r14
ffffffff812c6129: 65 48 8b 04 25 28 00 00 00 movq %gs:40,
%rax
ffffffff812c6132: 48 89 44 24 10 movq %rax, 16(%rsp)
ffffffff812c6137: 8b af a8 00 00 00 movl 168(%rdi), %ebp
ffffffff812c613d: 85 ed testl %ebp, %ebp
ffffffff812c613f: 75 3f jne
0xffffffff812c6180 <perf_event_read+0x70>
ffffffff812c6141: 66 2e 0f 1f 84 00 00 00 00 00 nopw %cs:
(%rax,%rax)
ffffffff812c614b: 0f 1f 44 00 00 nopl (%rax,%rax)
ffffffff812c6150: 49 8b 9e 28 02 00 00 movq 552(%r14), %rbx
ffffffff812c6157: 48 89 df movq %rbx, %rdi
ffffffff812c615a: e8 c1 a0 d7 00 callq
0xffffffff82040220 <_raw_spin_lock_irqsave>
ffffffff812c615f: 49 89 c7 movq %rax, %r15
ffffffff812c6162: 41 8b ae a8 00 00 00 movl 168(%r14), %ebp
ffffffff812c6169: 85 ed testl %ebp, %ebp
ffffffff812c616b: 0f 84 9a 00 00 00 je
0xffffffff812c620b <perf_event_read+0xfb>
ffffffff812c6171: 48 89 df movq %rbx, %rdi
ffffffff812c6174: 4c 89 fe movq %r15, %rsi
<=========== NOTE: %rsi is overwritten
......
ffffffff812c63f0: 41 5c popq %r12
<============ POP r12
ffffffff812c63f2: 41 5d popq %r13
ffffffff812c63f4: 41 5e popq %r14
ffffffff812c63f6: 41 5f popq %r15
ffffffff812c63f8: 5d popq %rbp
ffffffff812c63f9: e9 e2 a8 d7 00 jmp
0xffffffff82040ce0 <__x86_return_thunk>
ffffffff812c63fe: 31 c0 xorl %eax, %eax
ffffffff812c6400: e9 be fe ff ff jmp
0xffffffff812c62c3 <perf_event_read+0x1b3>
It is not clear why dwarf didn't encode %rsi in locations. But
DW_OP_GNU_entry_value(DW_OP_reg4 RSI) tells us that RSI is live at
the entry of perf_event_read(). So this patch tries to search
DW_OP_GNU_entry_value/DW_OP_entry_value location/expression so if
the expected parameter register matchs the register in
DW_OP_GNU_entry_value/DW_OP_entry_value, then the original parameter
is not optimized.
For one of internal 6.11 kernel, there are 62498 functions in BTF and
perf_event_read() is not there. With this patch, there are 61552
functions
in BTF and perf_event_read() is included.
hi Yonghong,
I'm confused by these numbers. I would have thought your changes would
have led to a net increase of functions encoded in vmlinux BTF since we
are now likely catching more cases where registers are expected. When I
ran your patches against an LLVM-built kernel, that's what I saw; 70
additional functions were recognized as having expected parameters, and
thus were encoded in BTF. In your case it looks like we lost nearly 1000
functions. Any idea what's going on there? If you can share your config,
LLVM version I can dig into this from my side too. Thanks!
Attached is my config (based on one of meta internal configs). I tried
with master branch with head:
7b6e5bfa2541380b478ea1532880210ea3e39e11 (HEAD -> master, origin/master,
origin/HEAD) Merge branch 'refactor-lock-management'
ae6e3a273f590a2b64f14a9fab3546c3a8f44ed4 bpf: Drop special callback
reference handling
f6b9a69a9e56b2083aca8a925fc1a28eb698e3ed bpf: Refactor active lock
management
I am using pahole v1.27.
I am using an llvm built from upstream. The following is llvm-project head:
beb12f92c71981670e07e47275efc6b5647011c1 (HEAD -> main) [RISCV] Add
+optimized-nfN-segment-load-store (#114414)
6bad4514c938b3b48c0c719b8dd98b3906f2c290 [AArch64] Extend vector mull
test coverage. NFC
915b910d800d7fab6a692294ff1d7075d8cba824 [libc] Fix typos in proxy type
headers (#114717)
98ea1a81a28a6dd36941456c8ab4ce46f665f57a [IPO] Remove unused includes
(NFC) (#114716)
With the above setup, when to do
pahole -JV --
btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs vmlinux >& log.pahole
You will find the below info in the log:
skipping addition of 'perf_event_read'(perf_event_read) due to
unexpected register used for paramet
In the dwarf:
0x02122746: DW_TAG_subprogram
DW_AT_low_pc (0xffffffff81299740)
DW_AT_high_pc (0xffffffff812999f7)
DW_AT_frame_base (DW_OP_reg7 RSP)
DW_AT_GNU_all_call_sites (true)
DW_AT_name ("perf_event_read")
DW_AT_decl_file ("/home/yhs/work/bpf-next/kernel/events/
core.c")
DW_AT_decl_line (4746)
DW_AT_prototyped (true)
DW_AT_type (0x020f95f5 "int")
0x02122760: DW_TAG_formal_parameter
DW_AT_location (0x00769b72:
[0xffffffff81299745, 0xffffffff81299764):
DW_OP_reg5 RDI
[0xffffffff81299764, 0xffffffff81299937):
DW_OP_reg3 RBX
[0xffffffff81299937, 0xffffffff812999f0):
DW_OP_GNU_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
[0xffffffff812999f0, 0xffffffff812999f7):
DW_OP_reg3 RBX)
DW_AT_name ("event")
DW_AT_decl_file ("/home/yhs/work/bpf-next/
kernel/events/core.c")
DW_AT_decl_line (4746)
DW_AT_type (0x0210f654 "perf_event *")
0x02122770: DW_TAG_formal_parameter
DW_AT_location (0x00769c61:
[0xffffffff81299758, 0xffffffff81299926):
DW_OP_reg6 RBP
[0xffffffff81299926, 0xffffffff812999f0):
DW_OP_GNU_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value
[0xffffffff812999f0, 0xffffffff812999f7):
DW_OP_reg6 RBP)
DW_AT_name ("group")
DW_AT_decl_file ("/home/yhs/work/bpf-next/
kernel/events/core.c")
The above is slightly different from our production kernel where Song
reported. But essence is the same.
The second parameter needs to check DW_OP_GNU_entry_value(DW_OP_reg4
RSI) to ensure the second
argument is available.
My patch is supposed to only make improvement. I am curiously why you
get less functions encoded in BTF.
Thanks for the config etc! When I build bpf-next using master branch
llvm and this config, I see
with baseline (master branch pahole): 62371 functions, no perf_event_read
your series on top of master branch pahole: 62433 functions,
perf_event_read present
So that's consistent with what I've seen with other configs; more
functions are present in vmlinux BTF since we are now seeing more cases
where parameters are in fact consistent. The part that confuses me
though is the numbers you initially reported above
"for one of internal 6.11 kernel, there are 62498 functions in BTF and
perf_event_read() is not there. With this patch, there are 61552
functions in BTF and perf_event_read() is included."
These numbers suggest you lost nearly 1000 functions when building
vmlinux BTF with pahole using this series. That's the part I don't
understand - we should just see a gain in numbers of functions in
vmlinux BTF, right? Did you mean 62552 functions rather than 61552 perhaps?
Sorry, really embarrassing. it is typo. Indeed it should be 62552 functions
in BTF instead.
Alan
Alan
Reported-by: Song Liu <song@xxxxxxxxxx>
Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
dwarf_loader.c | 81 +++++++++++++++++++++++++++++++++++---------------
1 file changed, 57 insertions(+), 24 deletions(-)
[...]
