Re: [PATCH bpf-next 5/5] selftests/bpf: add benchmark for uprobe vs. user_prog

On Sun, Aug 2, 2020 at 9:47 PM Song Liu [off-list ref] wrote:

quoted

On Aug 2, 2020, at 6:51 PM, Andrii Nakryiko [off-list ref] wrote:

On Sat, Aug 1, 2020 at 1:50 AM Song Liu [off-list ref] wrote:

quoted

Add a benchmark to compare performance of
 1) uprobe;
 2) user program w/o args;
 3) user program w/ args;
 4) user program w/ args on random cpu.

Can you please add it to the existing benchmark runner instead, e.g.,
along the other bench_trigger benchmarks? No need to re-implement
benchmark setup. And also that would also allow to compare existing
ways of cheaply triggering a program vs this new _USER program?

Will try.

quoted

If the performance is not significantly better than other ways, do you
think it still makes sense to add a new BPF program type? I think
triggering KPROBE/TRACEPOINT from bpf_prog_test_run() would be very
nice, maybe it's possible to add that instead of a new program type?
Either way, let's see comparison with other program triggering
mechanisms first.

Triggering KPROBE and TRACEPOINT from bpf_prog_test_run() will be useful.
But I don't think they can be used instead of user program, for a couple
reasons. First, KPROBE/TRACEPOINT may be triggered by other programs
running in the system, so user will have to filter those noise out in
each program. Second, it is not easy to specify CPU for KPROBE/TRACEPOINT,
while this feature could be useful in many cases, e.g. get stack trace
on a given CPU.

Right, it's not as convenient with KPROBE/TRACEPOINT as with the USER
program you've added specifically with that feature in mind. But if
you pin user-space thread on the needed CPU and trigger kprobe/tp,
then you'll get what you want. As for the "noise", see how
bench_trigger() deals with that: it records thread ID and filters
everything not matching. You can do the same with CPU ID. It's not as
automatic as with a special BPF program type, but still pretty simple,
which is why I'm still deciding (for myself) whether USER program type
is necessary :)

Thanks,
Song