Re: [PATCH] perf callchain: Handle multiple address spaces

[Thomas Richter]

On 4/30/26 04:57, Namhyung Kim wrote:
> On Wed, Apr 29, 2026 at 09:14:38AM +0200, Thomas Richter wrote:
>> On 4/21/26 18:30, Namhyung Kim wrote:
>>> Hello,
>>>
>>> On Tue, Apr 14, 2026 at 02:42:41PM +0200, Thomas Richter wrote:
>>>> perf test 'perf inject to convert DWARF callchains to regular ones'
>>>> fails on s390. It was introduced with
>>>> commit 92ea788d2af4 ("perf inject: Add --convert-callchain option")
>>>>
>>>> The failure comes the difference in output. Without the inject script to
>>>> convert DWARF the callchains is:
>>>>  # ./perf record -F 999 --call-graph dwarf -- perf test -w noploop
>>>>  # ./perf report -i perf.data --stdio --no-children -q \
>>>> 					 --percent-limit=1 > /tmp/111
>>>>  # cat /tmp/111
>>>>     99.30%  perf-noploop  perf               [.] noploop
>>>>             |
>>>>             ---noploop
>>>>                run_workload (inlined)
>>>>                cmd_test
>>>>                run_builtin (inlined)
>>>>                handle_internal_command
>>>>                run_argv (inlined)
>>>>                main
>>>>                __libc_start_call_main
>>>>                __libc_start_main_impl (inlined)
>>>>                _start
>>>>  #
>>>>
>>>> With the inject script step the output is:
>>>>  # ./perf inject -i perf.data --convert-callchain -o /tmp/perf-inject-1.out
>>>>  # ./perf report -i /tmp/perf-inject-1.out --stdio --no-children -q \
>>>> 		--percent-limit=1 > /tmp/222
>>>>  # cat /tmp/222
>>>>     99.40%  perf-noploop  perf               [.] noploop
>>>>             |
>>>>             ---noploop
>>>>                run_workload (inlined)
>>>>                cmd_test
>>>>                run_builtin (inlined)
>>>>                handle_internal_command
>>>>                run_argv (inlined)
>>>>                main
>>>>                _start
>>>>  # diff /tmp/111 /tmp/222
>>>>  1c1
>>>>  <     99.30%  perf-noploop  perf               [.] noploop
>>>>  ---
>>>>  >     99.40%  perf-noploop  perf               [.] noploop
>>>>  10,11d9
>>>>  <                __libc_start_call_main
>>>>  <                __libc_start_main_impl (inlined)
>>>>  #
>>>>
>>>> The difference are the symbols __libc_start_call_main and
>>>> __libc_start_main_impl.
>>>>
>>>> On x86_64, kernel and user space share a single virtual address space,
>>>> with the kernel mapped to the upper end of memory. The instruction
>>>> pointer value alone is sufficient to distinguish between user space
>>>> and kernel space addresses. This is not true for s390, which uses
>>>> separate address spaces for user and kernel. The same virtual address
>>>> can be valid in both address spaces, so the instruction pointer value
>>>> alone cannot determine whether an address belongs to the kernel or
>>>> user space. Instead, perf must rely on the cpumode metadata derived
>>>> from the processor status word (PSW) at sample time.
>>>>
>>>> In function perf_event__convert_sample_callchain() the first part
>>>> copies a kernel callchain and context entries, if any.
>>>> It then appends additional entries ignoring the address space
>>>> architecture. Taking that into account, the symbols at addresses
>>>>
>>>>    0x3ff970348cb __libc_start_call_main
>>>>    0x3ff970349c5 __libc_start_main_impl
>>>>
>>>> (located after the kernel address space on s390) are now included.
>>>>
>>>> Output before:
>>>>  # ./perf test 83
>>>>  83: perf inject to convert DWARF callchains to regular ones : FAILED!
>>>>
>>>> Output after:
>>>>  # ./perf test 83
>>>>  83: perf inject to convert DWARF callchains to regular ones : Ok
>>>>
>>>> Question to Namhyung:
>>>> In function perf_event__convert_sample_callchain() just before the
>>>> for() loop this patch modifies, the kernel callchain is copied,
>>>> see this comment and the next 5 lines:
>>>>    /* copy kernel callchain and context entries */ 
>>>> Then why is machine__kernel_ip() needed in the for() loop, when
>>>> the kernel entries have been copied just before the loop?
>>>
>>> IIRC I wanted to make sure to have PERF_CONTEXT_* part in the raw
>>> callchains.
>>>
>>>>
>>>> Note: This patch was tested on x86_64 virtual machine and succeeded.
>>>>
>>>> Fixes: 92ea788d2af4 ("perf inject: Add --convert-callchain option")
>>>> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
>>>>
>>>> Signed-off-by: Thomas Richter <tmricht@xxxxxxxxxxxxx>
>>>
>>> Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx>
>>>
>>> Thanks,
>>> Namhyung
>>>
>>
>> Hi Namhyung,
>>
>> is there something which needs to be done on my side to get this patch picked?
>> It still fails on s390.
>>
>> Please adviseon how to proceed.
> 
> Arnaldo will take care of 7.2 development cycle.
> 

Today we are at v7.1-rc2. Does this mean the fix won't make it into v7.1 at all
and we have to wait roughly 2 month?

Thanks.

-- 
Thomas Richter, Dept 3303, IBM s390 Linux Development, Boeblingen, Germany
--
IBM Deutschland Research & Development GmbH

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