Part 9 · 1 chapters · ~8 min

Reading the Source

Navigating the kernel tree with Elixir cross-referencer and cscope, finding a syscall's implementation, reading git history and blame, LKML and lore.kernel.org, the patch workflow, maintainers and MAINTAINERS, building and booting your own kernel in QEMU, and a reading plan.

A reading plan

code

# find any syscall's implementation
git grep -n 'SYSCALL_DEFINE3(read,'                 # fs/read_write.c
# browse with cross-references: https://elixir.bootlin.com/linux/latest/source
# why is this line here?
git log -L '/^ssize_t vfs_read/,+20:fs/read_write.c' --oneline | head
git log --grep='EEVDF' --oneline kernel/sched/ | head
# the discussion behind a patch: search its subject on lore.kernel.org
# who to ask about a file
scripts/get_maintainer.pl -f mm/vmscan.c

# build and boot your own kernel in QEMU (minutes, safe)
make defconfig && make -j$(nproc)
qemu-system-x86_64 -kernel arch/x86/boot/bzImage -initrd initramfs.cpio.gz -nographic -append console=ttyS0

week	read	with
1	fs/read_write.c, fs/open.c	strace a program and match each syscall
2	kernel/fork.c (copy_process), kernel/exit.c	trace fork with bpftrace
3	mm/memory.c (handle_mm_fault), mm/mmap.c	count faults with perf
4	kernel/sched/fair.c (EEVDF pick)	/proc/<pid>/sched
5	net/ipv4/tcp_input.c (receive path)	ss -tmi, nstat
6	kernel/cgroup/, kernel/nsproxy.c	unshare and /sys/fs/cgroup

Books: Linux Kernel Development (Love) for concepts, Understanding the Linux Kernel (Bovet and Cesati) for depth, LWN.net articles for current changes, and Brendan Gregg's Systems Performance for the observable side.