bpf: Use nproc --all for __NR_CPUS__

This uses `-D__NR_CPUS__=$(nproc --all)` (or `GetNumPossibleCPUs` when
invoked from Go) to compile the datapath.

This fixes an issue where cilium monitor fails to report any events
on AKS, due to the `perf_event_array` map duplicates being created
with different max_entries sizes, presumably causing the datapath
to write to the first one, while the agent is reading from the second
one.

This bug occurs for example on AKS due to the present/possible cpuset on
the VMs. The default Standard_D2s_v3 node size has 2 present CPUs, but
128 possible CPUs in /sys/devices/system/cpu.

Fixes: #12070

Signed-off-by: Sebastian Wicki <sebastian@isovalent.com>

bpf/Makefile.bpf

@@ -1,7 +1,7 @@
 # Copyright 2017-2020 Authors of Cilium
 # SPDX-License-Identifier: Apache-2.0
 
-FLAGS := -I$(ROOT_DIR)/bpf/include -I$(ROOT_DIR)/bpf -D__NR_CPUS__=$(shell nproc) -O2 -g
+FLAGS := -I$(ROOT_DIR)/bpf/include -I$(ROOT_DIR)/bpf -D__NR_CPUS__=$(shell nproc --all) -O2 -g
 
 CLANG_FLAGS := ${FLAGS} -target bpf -std=gnu89 -nostdinc -emit-llvm
 # eBPF verifier enforces unaligned access checks where necessary, so don't

bpf/init.sh

@@ -278,7 +278,7 @@ function bpf_compile()
 	      -Wno-gnu-variable-sized-type-not-at-end			\
 	      -Wdeclaration-after-statement				\
 	      -I. -I$DIR -I$LIB -I$LIB/include				\
-	      -D__NR_CPUS__=$(nproc)					\
+	      -D__NR_CPUS__=$(nproc --all)					\
 	      -DENABLE_ARP_RESPONDER=1					\
 	      -DHANDLE_NS=1						\
 	      $EXTRA_OPTS						\

pkg/datapath/loader/compile.go

@@ -23,10 +23,10 @@ import (
 	"io/ioutil"
 	"os"
 	"path"
-	"runtime"
 	"sync"
 
 	"github.com/cilium/cilium/pkg/command/exec"
+	"github.com/cilium/cilium/pkg/common"
 	"github.com/cilium/cilium/pkg/datapath/linux/probes"
 	"github.com/cilium/cilium/pkg/logging/logfields"
 	"github.com/cilium/cilium/pkg/option"
@@ -90,7 +90,7 @@ type directoryInfo struct {
 
 var (
 	standardCFlags = []string{"-O2", "-target", "bpf", "-std=gnu89",
-		"-nostdinc", fmt.Sprintf("-D__NR_CPUS__=%d", runtime.NumCPU()),
+		"-nostdinc", fmt.Sprintf("-D__NR_CPUS__=%d", common.GetNumPossibleCPUs(log)),
 		"-Wall", "-Wextra", "-Werror", "-Wshadow",
 		"-Wno-address-of-packed-member",
 		"-Wno-unknown-warning-option",

test/bpf/Makefile

@@ -3,7 +3,7 @@
 
 include ../../Makefile.defs
 
-FLAGS := -I../../bpf/ -I../../bpf/include -I. -D__NR_CPUS__=$(shell nproc) -O2
+FLAGS := -I../../bpf/ -I../../bpf/include -I. -D__NR_CPUS__=$(shell nproc --all) -O2
 FLAGS_CLANG := -Wall -Wextra -Werror -Wshadow -Wno-unused-parameter
 FLAGS_CLANG += -Wno-address-of-packed-member
 FLAGS_CLANG += -Wno-unknown-warning-option