Re: [PATCH 1/2] perf trace: Only call bpf_get_current_pid_tgid() when filtering tasks

[Arnaldo Carvalho de Melo]

On Thu, Oct 01, 2026 at 11:54:39PM -0700, Ian Rogers wrote:
> In task_traced(), bpf_get_current_pid_tgid() was called before checking
> has_pids_to_trace. Because BPF helper calls are not marked pure, the
> compiler cannot sink the call past the has_pids_to_trace check, so the
> helper was called on every syscall enter and exit even when tracing
> system-wide.
> 
> Check has_pids_to_trace before calling bpf_get_current_pid_tgid().
> 
> Suggested-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>

Thanks, appreciated, applied,

- Arnaldo

> Assisted-by: Antigravity:gemini-3.1-pro
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> ---
>  tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
> index d16e55706335..0b2ef9541f84 100644
> --- a/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
> +++ b/tools/perf/util/bpf_skel/augmented_raw_syscalls.bpf.c
> @@ -494,13 +494,15 @@ static bool pid_filter__has(struct pids_filtered *pids, pid_t pid)
>  
>  static bool task_traced(void)
>  {
> -	u64 pid_tgid = bpf_get_current_pid_tgid();
> -	pid_t pid = uses_tgid ? pid_tgid >> 32 : (pid_t)pid_tgid;
> +	u64 pid_tgid;
> +	pid_t pid;
>  	bool *traced;
>  
>  	if (!has_pids_to_trace)
>  		return true;
>  
> +	pid_tgid = bpf_get_current_pid_tgid();
> +	pid = uses_tgid ? pid_tgid >> 32 : (pid_t)pid_tgid;
>  	traced = bpf_map_lookup_elem(&pids_to_trace, &pid);
>  	return traced && *traced;
>  }
> 
> base-commit: 148cd3adf2df9e53fd9b3070ae1178d4876f8c67
> -- 
> 2.56.0.rc1.315.gc6ed9934b7-goog
>