Linux Kernel Architecture

Mauerer app04.tex V1 - 09/04/2008 6:12pm Page 1232

Appendix D: System Startup

❑ proc_caches_initinitiates slab caches for the remaining data structures involved in the process

description. The following structures are considered:sighand,signal,files,fs,fs_struct,

andmm_struct.

❑ buffer_initgenerates a cache forbuffer_heads and calculates the value of the

max_buffer_headsvariable so that the buffer heads never use more than 10 percent of

the memory inZONE_NORMAL.

❑ vfs_caches_initcreates caches for various data structures needed by the virtual filesystem

(VFS) layer.

❑ radix_tree_initcreates a slab cache forradix_tree_nodeinstances needed by memory man-

agement.

❑ page_writeback_initinitializes the flushing mechanism and, more specifically, defines the

limit value for dirty pages after which the mechanism comes into effect.

❑ proc_root_initinitializes the inode cache of theprocfilesystem, registers the process

filesystem (procfs) in the kernel, and generates the central filesystem entries — for example,

/proc/meminfo,/proc/uptime,/proc/version,andsoon.

SearchingforKnown SystemErrors

Software is not the only thing that has bugs — mishaps also occur when implementing processors and,

as a result, chips do not function as they should. Fortunately, most error situations can be remedied with

workarounds. However, before workarounds can beput in place, the kernel needs to know whether a

particular processor does, in fact, have any bugs. This can be established using the architecture-specific

check_bugsfunction.

For instance, the following code is available for IA-32 systems:

arch/x86/kernel/cpu/bugs.c
static void __init check_bugs(void)
{
identify_boot_cpu();

check_config();
check_fpu();
check_hlt();
check_popad();
init_utsname()->machine[1] = ’0’ + (boot_cpu_data.x86 >6?6:boot_cpu_data.x86);
alternative_instructions();
}

The last statement (alternative_instructions) also invokes a function that replaces certain assem-

bler instructions — depending on processor type — with faster, more modern alternatives. This enables

distributors to create kernel images that are capable of running on a wide variety of machines without

having to forgo more recent features.

For comparison of the CPU quality, here is thecheck_bugsroutine of S390, Alpha, Extensa, H8300, v850,

FRV, Blackfin, Cris, PA-RISC, and PPC64:

static void check_bugs(void)