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?
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()
So I do not know how it wouldn't trigger the WARNING or breakpoint if you
added it there.
-- Steve
