selftests/bpf: add batched, mostly in-kernel BPF triggering benchmarks

Existing kprobe/fentry triggering benchmarks have 1-to-1 mapping between
one syscall execution and BPF program run. While we use a fast
get_pgid() syscall, syscall overhead can still be non-trivial.

This patch adds kprobe/fentry set of benchmarks significantly amortizing
the cost of syscall vs actual BPF triggering overhead. We do this by
employing BPF_PROG_TEST_RUN command to trigger "driver" raw_tp program
which does a tight parameterized loop calling cheap BPF helper
(bpf_get_numa_node_id()), to which kprobe/fentry programs are
attached for benchmarking.

This way 1 bpf() syscall causes N executions of BPF program being
benchmarked. N defaults to 100, but can be adjusted with
--trig-batch-iters CLI argument.

For comparison we also implement a new baseline program that instead of
triggering another BPF program just does N atomic per-CPU counter
increments, establishing the limit for all other types of program within
this batched benchmarking setup.

Taking the final set of benchmarks added in this patch set (including
tp/raw_tp/fmodret, added in later patch), and keeping for now "legacy"
syscall-driven benchmarks, we can capture all triggering benchmarks in
one place for comparison, before we remove the legacy ones (and rename
xxx-batched into just xxx).

$ benchs/run_bench_trigger.sh
usermode-count       :   79.500 ± 0.024M/s
kernel-count         :   49.949 ± 0.081M/s
syscall-count        :    9.009 ± 0.007M/s

fentry-batch         :   31.002 ± 0.015M/s
fexit-batch          :   20.372 ± 0.028M/s
fmodret-batch        :   21.651 ± 0.659M/s
rawtp-batch          :   36.775 ± 0.264M/s
tp-batch             :   19.411 ± 0.248M/s
kprobe-batch         :   12.949 ± 0.220M/s
kprobe-multi-batch   :   15.400 ± 0.007M/s
kretprobe-batch      :    5.559 ± 0.011M/s
kretprobe-multi-batch:    5.861 ± 0.003M/s

fentry-legacy        :    8.329 ± 0.004M/s
fexit-legacy         :    6.239 ± 0.003M/s
fmodret-legacy       :    6.595 ± 0.001M/s
rawtp-legacy         :    8.305 ± 0.004M/s
tp-legacy            :    6.382 ± 0.001M/s
kprobe-legacy        :    5.528 ± 0.003M/s
kprobe-multi-legacy  :    5.864 ± 0.022M/s
kretprobe-legacy     :    3.081 ± 0.001M/s
kretprobe-multi-legacy:   3.193 ± 0.001M/s

Note how xxx-batch variants are measured with significantly higher
throughput, even though it's exactly the same in-kernel overhead. As
such, results can be compared only between benchmarks of the same kind
(syscall vs batched):

fentry-legacy        :    8.329 ± 0.004M/s
fentry-batch         :   31.002 ± 0.015M/s

kprobe-multi-legacy  :    5.864 ± 0.022M/s
kprobe-multi-batch   :   15.400 ± 0.007M/s

Note also that syscall-count is setting a theoretical limit for
syscall-triggered benchmarks, while kernel-count is setting similar
limits for batch variants. usermode-count is a happy and unachievable
case of user space counting without doing any syscalls, and is mostly
the measure of CPU speed for such a trivial benchmark.

As was mentioned, tp/raw_tp/fmodret require kernel-side kfunc to produce
similar benchmark, which we address in a separate patch.

Note that run_bench_trigger.sh allows to override a list of benchmarks
to run, which is very useful for performance work.

Cc: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240326162151.3981687-3-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>