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

[Thomas Richter <tmricht@linux.ibm.com>]

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@kernel.org>
>>
>> Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
> 
> Acked-by: Namhyung Kim <namhyung@kernel.org>
> 
> 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.

While at looking at my backlog, what about this patch
[PATCH] perf test: probe_vfs_getname add do_getname() probing fallback 

Thanks a lot for your help

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

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