Documentation/filesystems/proc.txt
And the implementation is in fs/proc/.
For processes - related information, it is mainly concentrated in task_mmu.c.
For interrupts enumeration (cat /proc/interrupts) the processing is done here:
arch/i386/kernel/irq.c
245: * /proc/interrupts printing:
246: */
247:
248: int show_interrupts(struct seq_file *p, void *v)
249: {
250: int i = *(loff_t *) v, j;
251: struct irqaction * action;
252: unsigned long flags;
253:
254: if (i == 0) {
255: seq_printf(p, " ");
256: for_each_online_cpu(j)
257: seq_printf(p, "CPU%-8d",j);
258: seq_putc(p, '\n');
259: }
260:
261: if (i < NR_IRQS) {
262: spin_lock_irqsave(&irq_desc[i].lock, flags);
263: action = irq_desc[i].action;
264: if (!action)
265: goto skip;
266: seq_printf(p, "%3d: ",i);
267: #ifndef CONFIG_SMP
268: seq_printf(p, "%10u ", kstat_irqs(i));
269: #else
270: for_each_online_cpu(j)
271: seq_printf(p, "%10u ", kstat_cpu(j).irqs[i]);
272: #endif
273: seq_printf(p, " %8s", irq_desc[i].chip->name);
274: seq_printf(p, "-%-8s", irq_desc[i].name);
275: seq_printf(p, " %s", action->name);
276:
277: for (action=action->next; action; action = action->next)
278: seq_printf(p, ", %s", action->name);
279:
280: seq_putc(p, '\n');
281: skip:
282: spin_unlock_irqrestore(&irq_desc[i].lock, flags);
283: } else if (i == NR_IRQS) {
284: seq_printf(p, "NMI: ");
285: for_each_online_cpu(j)
286: seq_printf(p, "%10u ", nmi_count(j));
287: seq_putc(p, '\n');
288: #ifdef CONFIG_X86_LOCAL_APIC
289: seq_printf(p, "LOC: ");
290: for_each_online_cpu(j)
291: seq_printf(p, "%10u ",
292: per_cpu(irq_stat,j).apic_timer_irqs);
293: seq_putc(p, '\n');
294: #endif
295: seq_printf(p, "ERR: %10u\n", atomic_read(&irq_err_count));
296: #if defined(CONFIG_X86_IO_APIC)
297: seq_printf(p, "MIS: %10u\n", atomic_read(&irq_mis_count));
298: #endif
299: }
300: return 0;
301: } 