On Wed, Aug 5, 2020 at 10:16 AM Alexei Starovoitov <alexei.starovoi...@gmail.com> wrote: > > On Wed, Aug 05, 2020 at 04:47:30AM +0000, Song Liu wrote: > > > > Being able to trigger BPF program on a different CPU could enable many > > use cases and optimizations. The use case I am looking at is to access > > perf_event and percpu maps on the target CPU. For example: > > 0. trigger the program > > 1. read perf_event on cpu x; > > 2. (optional) check which process is running on cpu x; > > 3. add perf_event value to percpu map(s) on cpu x. > > If the whole thing is about doing the above then I don't understand why new > prog type is needed. Can prog_test_run support existing BPF_PROG_TYPE_KPROBE? > "enable many use cases" sounds vague. I don't think folks reading > the patches can guess those "use cases". > "Testing existing kprobe bpf progs" would sound more convincing to me.
Was just about to propose the same :) I wonder if generic test_run() capability to trigger test programs of whatever supported type on a specified CPU through IPI can be added. That way you can even use the XDP program to do what Song seems to need. TRACEPOINTs might also be a good fit here, given it seems simpler to let users specify custom tracepoint data for test_run(). Having the ability to unit-test KPROBE and TRACEPOINT, however rudimentary, is already a big win. > If the test_run framework can be extended to trigger kprobe with correct > pt_regs. > As part of it test_run would trigger on a given cpu with $ip pointing > to some test fuction in test_run.c. For local test_run the stack trace > would include bpf syscall chain. For IPI the stack trace would include > the corresponding kernel pieces where top is our special test function. > Sort of like pseudo kprobe where there is no actual kprobe logic, > since kprobe prog doesn't care about mechanism. It needs correct > pt_regs only as input context. > The kprobe prog output (return value) has special meaning though, > so may be kprobe prog type is not a good fit. It does? I don't remember returning 1 from KPROBE changing anything. I thought it's only the special bpf_override_return() that can influence the kernel function return result. > Something like fentry/fexit may be better, since verifier check_return_code() > enforces 'return 0'. So their return value is effectively "void". > Then prog_test_run would need to gain an ability to trigger > fentry/fexit prog on a given cpu.
