Linux Kernel Architecture

Mauerer app04.tex V1 - 09/04/2008 6:12pm Page 1231

Appendix D: System Startup

ticks = jiffies;

while (ticks == jiffies)

/* nothing */;

/* Go .. */

ticks = jiffies;

__delay(loops_per_jiffy);

ticks = jiffies - ticks;

if (ticks)

break;

}

/*

* Do a binary approximation to get loops_per_jiffy set to

* equal one clock (up to lps_precision bits)

*/

loops_per_jiffy >>= 1;

loopbit = loops_per_jiffy;

while ( lps_precision-- && (loopbit >>= 1) ) {

loops_per_jiffy |= loopbit;

ticks = jiffies;

while (ticks == jiffies)

/* nothing */;

ticks = jiffies;

__delay(loops_per_jiffy);

if (jiffies != ticks) /* longer than 1 tick */

loops_per_jiffy &= ~loopbit;

}

/* Round the value and print it */

printk("%lu.%02lu BogoMIPS (lpj=%lu)\n",

loops_per_jiffy/(500000/HZ),

(loops_per_jiffy/(5000/HZ)) % 100,

loops_per_jiffy);

}

}

The following construction is particularly interesting (although in C, it would not normally make

sense or would produce an endless loop):

init/main.c

ticks = jiffies;

while (ticks == jiffies)

/* nothing */;

However, the loop does terminate at some point because the value ofjiffiesis incremented

by 1 in the interrupt handler routine at each tick of the system clock (which sleeps with the

frequencyHZ). As a result, the condition in thewhileloops produces an incorrect value after a

certain time, thus causing the loop to terminate.

❑ pidmap_initallocates the array in which the free positions of the PID allocator are saved. It also
reserves the (unused) PID 0 for all PID types.

❑ fork_initallocates thetask_structslab cache (providing there is no architecture-specific
mechanism to generate and cachetask_structinstances) and calculates the maximum number
of threads that can be generated.