Re: [PATCH] perf record: skip synthesize event when open evsel failed

[Shuai Xue <xueshuai@linux.alibaba.com>]

在 2025/10/31 01:32, Ian Rogers 写道:
> On Wed, Oct 29, 2025 at 5:55 AM Shuai Xue <xueshuai@linux.alibaba.com> wrote:
>>
>>
>>
>> 在 2025/10/24 10:45, Shuai Xue 写道:
>>>
>>>
>>> 在 2025/10/24 00:08, Ian Rogers 写道:
>>>> On Wed, Oct 22, 2025 at 6:50 PM Shuai Xue <xueshuai@linux.alibaba.com> wrote:
>>>>>
>>>>> When using perf record with the `--overwrite` option, a segmentation fault
>>>>> occurs if an event fails to open. For example:
>>>>>
>>>>>     perf record -e cycles-ct -F 1000 -a --overwrite
>>>>>     Error:
>>>>>     cycles-ct:H: PMU Hardware doesn't support sampling/overflow-interrupts. Try 'perf stat'
>>>>>     perf: Segmentation fault
>>>>>         #0 0x6466b6 in dump_stack debug.c:366
>>>>>         #1 0x646729 in sighandler_dump_stack debug.c:378
>>>>>         #2 0x453fd1 in sigsegv_handler builtin-record.c:722
>>>>>         #3 0x7f8454e65090 in __restore_rt libc-2.32.so[54090]
>>>>>         #4 0x6c5671 in __perf_event__synthesize_id_index synthetic-events.c:1862
>>>>>         #5 0x6c5ac0 in perf_event__synthesize_id_index synthetic-events.c:1943
>>>>>         #6 0x458090 in record__synthesize builtin-record.c:2075
>>>>>         #7 0x45a85a in __cmd_record builtin-record.c:2888
>>>>>         #8 0x45deb6 in cmd_record builtin-record.c:4374
>>>>>         #9 0x4e5e33 in run_builtin perf.c:349
>>>>>         #10 0x4e60bf in handle_internal_command perf.c:401
>>>>>         #11 0x4e6215 in run_argv perf.c:448
>>>>>         #12 0x4e653a in main perf.c:555
>>>>>         #13 0x7f8454e4fa72 in __libc_start_main libc-2.32.so[3ea72]
>>>>>         #14 0x43a3ee in _start ??:0
>>>>>
>>>>> The --overwrite option implies --tail-synthesize, which collects non-sample
>>>>> events reflecting the system status when recording finishes. However, when
>>>>> evsel opening fails (e.g., unsupported event 'cycles-ct'), session->evlist
>>>>> is not initialized and remains NULL. The code unconditionally calls
>>>>> record__synthesize() in the error path, which iterates through the NULL
>>>>> evlist pointer and causes a segfault.
>>>>>
>>>>> To fix it, move the record__synthesize() call inside the error check block, so
>>>>> it's only called when there was no error during recording, ensuring that evlist
>>>>> is properly initialized.
>>>>>
>>>>> Fixes: 4ea648aec019 ("perf record: Add --tail-synthesize option")
>>>>> Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>
>>>>
>>>> This looks great! I wonder if we can add a test, perhaps here:
>>>> https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/tests/shell/record.sh?h=perf-tools-next#n435
>>>> something like:
>>>> ```
>>>> $ perf record -e foobar -F 1000 -a --overwrite -o /dev/null -- sleep 0.1
>>>> ```
>>>> in a new test subsection for test_overwrite? foobar would be an event
>>>> that we could assume isn't present. Could you help with a test
>>>> covering the problems you've uncovered and perhaps related flags?
>>>>
>>>
>>> Hi, Ian,
>>>
>>> Good suggestion, I'd like to add a test. But foobar may not a good case.
>>>
>>> Regarding your example:
>>>
>>>     perf record -e foobar -a --overwrite -o /dev/null -- sleep 0.1
>>>     event syntax error: 'foobar'
>>>                          \___ Bad event name
>>>
>>>     Unable to find event on a PMU of 'foobar'
>>>     Run 'perf list' for a list of valid events
>>>
>>>      Usage: perf record [<options>] [<command>]
>>>         or: perf record [<options>] -- <command> [<options>]
>>>
>>>         -e, --event <event>   event selector. use 'perf list' to list available events
>>>
>>>
>>> The issue with using foobar is that it's an invalid event name, and the
>>> perf parser will reject it much earlier. This means the test would exit
>>> before reaching the part of the code path we want to verify (where
>>> record__synthesize() could be called).
>>>
>>> A potential alternative could be testing an error case such as EACCES:
>>>
>>>     perf record -e cycles -C 0 --overwrite -o /dev/null -- sleep 0.1
>>>
>>> This could reproduce the scenario of a failure when attempting to access
>>> a valid event, such as due to permission restrictions. However, the
>>> limitation here is that users may override
>>> /proc/sys/kernel/perf_event_paranoid, which affects whether or not this
>>> test would succeed in triggering an EACCES error.
>>>
>>>
>>> If you have any other suggestions or ideas for a better way to simulate
>>> this situation, I'd love to hear them.
>>>
>>> Thanks.
>>> Shuai
>>
>> Hi, Ian,
>>
>> Gentle ping.
> 
> Sorry, for the delay. I was trying to think of a better way given the
> problems you mention and then got distracted. I wonder if a legacy
> event that core PMUs never implement would be a good candidate to
> test. For example, the event "node-prefetch-misses" is for "Local
> memory prefetch misses" but the memory controller tends to be a
> separate PMU and this event is never implemented to my knowledge.
> Running this locally I see:
> 
> ```
> $ perf record -e node-prefetch-misses -a --overwrite -o /dev/null -- sleep 0.1
> Lowering default frequency rate from 4000 to 1750.
> Please consider tweaking /proc/sys/kernel/perf_event_max_sample_rate.
> Error:
> Failure to open event 'cpu_atom/node-prefetch-misses/' on PMU
> 'cpu_atom' which will be removed.
> No fallback found for 'cpu_atom/node-prefetch-misses/' for error 2
> Error:
> Failure to open event 'cpu_core/node-prefetch-misses/' on PMU
> 'cpu_core' which will be removed.
> No fallback found for 'cpu_core/node-prefetch-misses/' for error 2
> Error:
> Failure to open any events for recording.
> perf: Segmentation fault
>     #0 0x55a487ad8b87 in dump_stack debug.c:366
>     #1 0x55a487ad8bfd in sighandler_dump_stack debug.c:378
>     #2 0x55a4878c6f94 in sigsegv_handler builtin-record.c:722
>     #3 0x7f72aae49df0 in __restore_rt libc_sigaction.c:0
>     #4 0x55a487b57ef8 in __perf_event__synthesize_id_index
> synthetic-events.c:1862
>     #5 0x55a487b58346 in perf_event__synthesize_id_index synthetic-events.c:1943
>     #6 0x55a4878cb2a3 in record__synthesize builtin-record.c:2150
>     #7 0x55a4878cdada in __cmd_record builtin-record.c:2963
>     #8 0x55a4878d11ca in cmd_record builtin-record.c:4453
>     #9 0x55a48795b3cc in run_builtin perf.c:349
>     #10 0x55a48795b664 in handle_internal_command perf.c:401
>     #11 0x55a48795b7bd in run_argv perf.c:448
>     #12 0x55a48795bb06 in main perf.c:555
>     #13 0x7f72aae33ca8 in __libc_start_call_main libc_start_call_main.h:74
>     #14 0x7f72aae33d65 in __libc_start_main_alias_2 libc-start.c:128
>     #15 0x55a4878acf41 in _start perf[52f41]
> Segmentation fault
> ```


Hi, Ian，

Is node-prefetch-misses a platform specific event? Running it on ARM Yitian 710
and Intel SPR platform, I see:

$sudo perf record -e node-prefetch-misses
Error:
The node-prefetch-misses event is not supported.

Thanks.
Shuai