[feature] Inherit resource limits from cgroups (#1336)

When GTS is running in a container runtime which has configured CPU or
memory limits or under an init system that uses cgroups to impose CPU
and memory limits the values the Go runtime sees for GOMAXPROCS and
GOMEMLIMIT are still based on the host resources, not the cgroup.

At least for the throttling middlewares which use GOMAXPROCS to
configure their queue size, this can result in GTS running with values
too big compared to the resources that will actuall be available to it.

This introduces 2 dependencies which can pick up resource contraints
from the current cgroup and tune the Go runtime accordingly. This should
result in the different queues being appropriately sized and in general
more predictable performance. These dependencies are a no-op on
non-Linux systems or if running in a cgroup that doesn't set a limit on
CPU or memory.

The automatic tuning of GOMEMLIMIT can be disabled by either explicitly
setting GOMEMLIMIT yourself or by setting AUTOMEMLIMIT=off. The
automatic tuning of GOMAXPROCS can similarly be counteracted by setting
GOMAXPROCS yourself.

vendor/github.com/cilium/ebpf/Makefile

@@ -0,0 +1,67 @@
+# The development version of clang is distributed as the 'clang' binary,
+# while stable/released versions have a version number attached.
+# Pin the default clang to a stable version.
+CLANG ?= clang-11
+CFLAGS := -target bpf -O2 -g -Wall -Werror $(CFLAGS)
+
+# Obtain an absolute path to the directory of the Makefile.
+# Assume the Makefile is in the root of the repository.
+REPODIR := $(shell dirname $(realpath $(firstword $(MAKEFILE_LIST))))
+UIDGID := $(shell stat -c '%u:%g' ${REPODIR})
+
+IMAGE := $(shell cat ${REPODIR}/testdata/docker/IMAGE)
+VERSION := $(shell cat ${REPODIR}/testdata/docker/VERSION)
+
+# clang <8 doesn't tag relocs properly (STT_NOTYPE)
+# clang 9 is the first version emitting BTF
+TARGETS := \
+	testdata/loader-clang-7 \
+	testdata/loader-clang-9 \
+	testdata/loader-clang-11 \
+	testdata/invalid_map \
+	testdata/raw_tracepoint \
+	testdata/invalid_map_static \
+	testdata/initialized_btf_map \
+	testdata/strings \
+	internal/btf/testdata/relocs
+
+.PHONY: all clean docker-all docker-shell
+
+.DEFAULT_TARGET = docker-all
+
+# Build all ELF binaries using a Dockerized LLVM toolchain.
+docker-all:
+	docker run --rm --user "${UIDGID}" \
+		-v "${REPODIR}":/ebpf -w /ebpf --env MAKEFLAGS \
+		"${IMAGE}:${VERSION}" \
+		make all
+
+# (debug) Drop the user into a shell inside the Docker container as root.
+docker-shell:
+	docker run --rm -ti \
+		-v "${REPODIR}":/ebpf -w /ebpf \
+		"${IMAGE}:${VERSION}"
+
+clean:
+	-$(RM) testdata/*.elf
+	-$(RM) internal/btf/testdata/*.elf
+
+all: $(addsuffix -el.elf,$(TARGETS)) $(addsuffix -eb.elf,$(TARGETS))
+
+testdata/loader-%-el.elf: testdata/loader.c
+	$* $(CFLAGS) -mlittle-endian -c $< -o $@
+
+testdata/loader-%-eb.elf: testdata/loader.c
+	$* $(CFLAGS) -mbig-endian -c $< -o $@
+
+%-el.elf: %.c
+	$(CLANG) $(CFLAGS) -mlittle-endian -c $< -o $@
+
+%-eb.elf : %.c
+	$(CLANG) $(CFLAGS) -mbig-endian -c $< -o $@
+
+# Usage: make VMLINUX=/path/to/vmlinux vmlinux-btf
+.PHONY: vmlinux-btf
+vmlinux-btf: internal/btf/testdata/vmlinux-btf.gz
+internal/btf/testdata/vmlinux-btf.gz: $(VMLINUX)
+	objcopy --dump-section .BTF=/dev/stdout "$<" /dev/null | gzip > "$@"