Re: The same symbol is printed twice when use tracepoint to get stack

[Alan Maguire]

On 14/01/2026 15:09, Tao Chen wrote:
> Hi guys,
> 
> When using tracepoints to retrieve stack information, I observed that perf_trace_sched_migrate_task was printed twice. And the issue also occurs with tools using libbpf.
>

You may need the fix Jiri provided for x86_64 [1]. Eugene mentioned that
the issue persists for arm64 however [2].

Alan

[1] https://lore.kernel.org/bpf/20251104215405.168643-2-jolsa@xxxxxxxxxx/
[2] https://lore.kernel.org/all/a38fed68-67bc-98ce-8e12-743342121ae3@xxxxxxxxxx/
 
> sudo bpftrace -e '
> tracepoint:sched:sched_migrate_task {
> printf("Task %s migrated by:\n", args->comm);
> print(kstack);
> }'
> 
> Task kcompactd0 migrated by:
> 
>         perf_trace_sched_migrate_task+9
>         perf_trace_sched_migrate_task+9
>         set_task_cpu+353
>         detach_task+77
>         detach_tasks+281
>         sched_balance_rq+452
>         sched_balance_newidle+504
>         pick_next_task_fair+84
>         __pick_next_task+66
>         pick_next_task+43
>         __schedule+332
>         schedule+41
>         schedule_hrtimeout_range+239
>         do_poll.constprop.0+668
>         do_sys_poll+499
>         __x64_sys_ppoll+220
>         x64_sys_call+5722
>         do_syscall_64+126
>         entry_SYSCALL_64_after_hwframe+118
> 
> Task jbd2/sda2-8 migrated by:
> 
>         perf_trace_sched_migrate_task+9
>         perf_trace_sched_migrate_task+9
>         set_task_cpu+353
>         try_to_wake_up+365
>         default_wake_function+26
>         autoremove_wake_function+18
>         __wake_up_common+118
>         __wake_up+55
>         __jbd2_log_start_commit+195
> 
> env:
> bpftrace v0.21.2
> ubuntu24.04，6.14.0-36-generic
> 
> The issue is as follows:
> https://github.com/bpftrace/bpftrace/issues/4949
> 
> 
> It seems that there is no special handling in the kernel.
> Does anyone has thoughts on this issue. Thanks.
> 
> BPF_CALL_4(bpf_get_stack_raw_tp, struct bpf_raw_tracepoint_args *, args,
>            void *, buf, u32, size, u64, flags)
> {
>         struct pt_regs *regs = get_bpf_raw_tp_regs();
>         int ret;
> 
>         if (IS_ERR(regs))
>                 return PTR_ERR(regs);
> 
>         perf_fetch_caller_regs(regs);
>         ret = bpf_get_stack((unsigned long) regs, (unsigned long) buf,
>                             (unsigned long) size, flags, 0);
>         put_bpf_raw_tp_regs();
>         return ret;
> }
>