Uhuh, apologies for my perf report formatting! I'll try to figure it out for next time, meanwhile you can find it better formatted here https://paste.debian.net/1257405/ On Mon, Oct 17, 2022 at 8:49 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > On Mon, 17 Oct 2022 19:55:06 +0200 > Florent Revest <revest@xxxxxxxxxxxx> wrote: > > > Note that I can't really make sense of the perf report with indirect > > calls. it always reports it spent 12% of the time in > > rethook_trampoline_handler but I verified with both a WARN in that > > function and a breakpoint with a debugger, this function does *not* > > get called when running this "bench trig-fentry" benchmark. Also it > > wouldn't make sense for fprobe_handler to call it so I'm quite > > confused why perf would report this call and such a long time spent > > there. Anyone know what I could be missing here ? > > The trace shows __bpf_prog_exit, which I'm guessing is tracing the end of > the function. Right? Actually no, this function is called to end the context of a BPF program execution. Here it is called at the end of the fentry program (so still before the traced function). I hope the pastebin helps clarify this! > In which case I believe it must call rethook_trampoline_handler: > > -> fprobe_handler() /* Which could use some "unlikely()" to move disabled > paths out of the hot path */ > > /* And also calls rethook_try_get () which does a cmpxchg! */ > > -> ret_hook() > -> arch_rethook_prepare() > Sets regs->lr = arch_rethook_trampoline > > On return of the function, it jumps to arch_rethook_trampoline() > > -> arch_rethook_trampoline() > -> arch_rethook_trampoline_callback() > -> rethook_trampoline_handler() This is indeed what happens when an fexit program is also attached. But when running "bench trig-fentry", only an fentry program is attached so bpf_fprobe_entry returns a non-zero value and fprobe doesn't call rethook_hook. Also, in this situation arch_rethook_trampoline is called by the traced function's return but in the perf report, iiuc, it shows up as being called from fprobe_handler and that should never happen. I wonder if this is some sort of stack unwinding artifact during the perf record? > So I do not know how it wouldn't trigger the WARNING or breakpoint if you > added it there. By the way, the WARNING does trigger if I also attach an fexit program (then rethook_hook is called). But I made sure we skip the whole rethook logic if no fexit program is attached so bench trig-fentry should not go through rethook_trampoline_handler.
