Chapter 1: Kernel Overview

1. Chapter 1: Kernel Overview

2. Objectives • Review development and history of UNIX and Linux. • Clarify the nature and extent of the kernel within the context of the operating system proper. • Identify operating system design goals and tradeoffs. • Review established architectural approaches for implementing an operating system. • Introduce the notion of the “core” Linux kernel. • Understand the Linux approach to hardware (architecture) abstraction and independence. • Explore the layout of the Linux source code tree.

3. History • UNIX: 1969 Thompson & Ritchie AT&T Bell Labs • BSD: 1978 Berkeley Software Distribution • Commercial Vendors: Sun, HP, IBM, SGI, DEC • GNU: 1984 Richard Stallman, FSF • POSIX: 1986 IEEE Portable Operating System unIX • Minix: 1987 Andy Tannenbaum • SVR4: 1989 AT&T and Sun • Linux: 1991 Linus Torvalds Intel 386 (i386) • Open Source: GPL, LGPL, Cathedral and the Bazaar

4. Linux Features • “UNIX-like” operating system • “aims at” standards compliance • “all the features you would expect in a modern UNIX” • preemptive multitasking • virtual memory (protected memory, paging) • shared libraries • demand loading, dynamic kernel modules • shared copy-on-write executables • TCP/IP networking • other features: • SMP support, large memory, large files • advanced networking, advanced filesystems • efficient, stable, highly portable, supports most device hardware • active development community, support, documentation, open source • GUIs, applications

5. What’s a Kernel? • aka: executive, system monitor, nucleus • controls and mediates access to hardware • implements and supports fundamental abstractions • processes, files, devices, users, net, etc. • schedules “fair” sharing of system resources • memory, cpu, disk, descriptors, etc. • enforces security and protection • responds to user requests for service (system calls) • performs routine maintenance, system checks, etc.

6. Kernel Design Goals • performance: efficiency, speed • utilize resources to capacity, low overhead, code size • stability: robustness, resilience • uptime, graceful degradation • capability: features, flexibility, compatibility • security, protection • protect users from each other, secure system from bad guys • portability • clarity • extensibility

7. Design Tradeoffs • Butler Lampson: “choose any three design goals” • efficiency vs. protection • more checks, more overhead • clarity vs. compatibility • ugly implementation of “broken” standards (e.g. signals) • flexibility vs. security • the more you can do, the more potential security holes! • not all are antagonistic • portability tends to enhance code clarity

8. Waterloo Diagrams mm ipc fs sched vfs sched ipc net mm net Conceptual Concrete

9. Stephen Tweedie’s Diagram User Processes Scheduler Syscalls VM Traps Process Manager Socket Manager VFS Memory Allocator Math Support Network Protocols File Systems Char Devices Block Devices Packet Requestor IO Requestor Net Devices

10. Vahalia’s Diagram from Unix Internals: The New Frontiers Uresh Vahalia / Prentice-Hall 1996 elf a.out coff file NFS exec switch device FFS virtual memory framework vnode/vfs interface s5fs anonymous core utilities scheduler framework block device switch time-sharing disk STREAMS real-time tape system network tty

11. “Core” Kernel Applications System Libraries (libc) System Call Interface I/O Related Process Related File Systems Scheduler Modules Networking Memory Management Device Drivers IPC Architecture-Dependent Code Hardware

12. Architectural Approaches • monolithic • layered • modularized • micro-kernel • virtual machine

13. Isolating Hardware Dependencies • architecture (cpu) • dependent (/arch) • independent (everything else) • abstract dependencies behind functions and macros • link appropriate version at compile-time • device-dependencies isolated in device drivers • provide general abstractions that map to reality • e.g. three-level page tables • tradeoff: exploiting special hardware features

14. Source Tree Layout /usr/src/linux scripts Documentation ipc kernel init net arch mm lib fs drivers include 802 appletalk atm ax25 bridge core decnet econet ethernet ipv4 ipv6 ipx irda khttpd lapb … acorn atm block cdrom char dio fc4 i2c i2o ide ieee1394 isdn macintosh misc net … adfs affs autofs autofs4 bfs code cramfs devfs devpts efs ext2 fat hfs hpfs … asm-alpha asm-arm asm-generic asm-i386 asm-ia64 asm-m68k asm-mips asm-mips64 … linux math-emu net pcmcia scsi video adfs affs autofs autofs4 bfs code cramfs devfs devpts efs ext2 fat hfs hpfs … alpha arm i386 ia64 m68k mips mips64 ppc s390 sh sparc sparc64

15. linux/Documentation • spotty but important collection of developer-generated documentation; you need to read what’s in here! • recent effort to produce javadoc-style documentation from source header comments using OpenDoc • an ambitious open-source kernel book effort has begun; see kernelbook.sourceforge.net for details • some especially interesting entries: • kernel-docs.txt (a bit out of date but good) • filesystems/ (very extensive) • networking/ (very extensive) • kmod.txt • oops-tracing.txt • spinlocks.txt (the official story from Linus)

16. linux/arch • subdirectories for each current port • each contains kernel, lib, mm, boot and other directories whose contents override code stubs in architecture independent code • lib contains highly-optimized common utility routines such as memcpy, checksums, etc. • arch as of 2.4: • alpha, arm, i386, ia64, m68k, mips, mips64 • ppc, s390, sh, sparc, sparc64

17. linux/drivers • largest amount of code in the kernel tree (~1.5M) • device, bus, platform and general directories • drivers/char – n_tty.c is the default line discipline • drivers/block – elevator.c, genhd.c, linear.c, ll_rw_blk.c, raidN.c • drivers/net –specific drivers and general routines Space.c and net_init.c • drivers/scsi – scsi_*.c files are generic; sd.c (disk), sr.c (CD-ROM), st.c (tape), sg.c (generic) • general: • cdrom, ide, isdn, parport, pcmcia, • pnp, sound, telephony, video • buses – fc4, i2c, nubus, pci, sbus, tc, usb • platforms – acorn, macintosh, s390, sgi

18. linux/fs • contains: • virtual filesystem (VFS) framework • subdirectories for actual filesystems • vfs-related files: • exec.c, binfmt_*.c - files for mapping new process images • devices.c, blk_dev.c – device registration, block device support • super.c, filesystems.c • inode.c, dcache.c, namei.c, buffer.c, file_table.c • open.c, read_write.c, select.c, pipe.c, fifo.c • fcntl.c, ioctl.c, locks.c, dquot.c, stat.c

19. linux/include • include/asm-* • architecture-dependent include subdirectories • include/linux • header info needed both by the kernel and user apps • usually linked to /usr/include/linux • kernel-only portions guarded by #ifdefs • #ifdef __KERNEL__ • /* kernel stuff */ • #endif • other directories: • math-emu • net • pcmcia • scsi • video

20. linux/init • just two files: version.c, main.c • version.c – contains the version banner that prints at boot • main.c – architecture-independent boot code • start_kernel is the primary entry point

21. linux/ipc • System V IPC facilities • if disabled at compile-time, util.c exports stubs that simply return –ENOSYS • one file for each facility: • sem.c – semaphores • shm.c – shared memory • msg.c – message queues

22. linux/kernel • the core kernel code • sched.c – “the main kernel file” • scheduler, wait queues, timers, alarms, task queues • process control • fork.c, exec.c, signal.c, exit.c • acct.c, capability.c, exec_domain.c • kernel module support • kmod.c, ksyms.c, module.c • other operations • time.c, resource.c, dma.c, softirq.c, itimer.c • printk.c, info.c, panic.c, sysctl.c, sys.c

23. linux/lib • kernel code cannot call standard C library routines • files: • brlock.c – “Big Reader” spinlocks • cmdline.c – kernel command line parsing routines • errno.c – global definition of errno • inflate.c – “gunzip” part of gzip.c used during boot • string.c – portable string code • usually replaced by optimized, architecture-dependent routines • vsprintf.c – libc replacement

24. linux/mm • paging and swapping • swap.c, swapfile.c (paging devices), swap_state.c (cache) • vmscan.c – paging policies, kwapd • page_io.c – low-level page transfer • allocation and deallocation • slab.c – slab allocator • page_alloc.c – page-based allocator zone allocator • vmalloc.c – kernel virtual-memory allocator • memory mapping • memory.c – paging, fault-handling, page table code • filemap.c – file mapping • mmap.c, mremap.c, mlock.c, mprotect.c

25. linux/net • changing too fast! • i haven’t figured it out yet 

26. linux/scripts • scripts for: • menu-based kernel configuration • kernel patching • generating kernel documentation

27. Sizes (linux-2.4.0-test2) size directory entries files loc 90M /usr/src/linux/ 19 7645 2.6M 4.5M Documentation 97 380 na 16.5M arch 12 1685 466K 54M drivers 31 2256 1.5M 5.6M fs 70 489 150K 14.2M include 19 2262 285K 28K init 2 2 1K 120K ipc 6 6 4.5K 332K kernel 25 25 12K 80K lib 8 8 2K 356K mm 19 19 12K 5.8M net 33 453 162K 400K scripts 26 42 12K

28. Summary • Linux is a modular, UNIX-like monolithic kernel • Kernel is the heart of the OS that executes with special hardware permission (kernel mode) • “Core kernel” provides framework, data structures, support for drivers, modules, subsystems • Kernel designers must consider many competing goals • Linux source tree mirrors kernel structure • Architecture dependent source subtrees live in /arch • “main” lives in /kernel/init.c • lxr.linux.no is a nice web-based source browser