The Linux timer interrupt handler doesn’t do all that much directly. For x86, you’ll find the default PIT/HPET timer interrupt handler in arch/x86/kernel/time.c:

static irqreturn_t timer_interrupt(int irq, void *dev_id)
{
    global_clock_event->event_handler(global_clock_event);
    return IRQ_HANDLED;
}

This calls the event handler for global clock events, tick_handler_periodic by default, which updates the jiffies counter, calculates the global load, and updates a few other places where time is tracked.

As a side-effect of an interrupt occurring, __schedule might end up being called, so a timer interrupt can also lead to a task switch (like any other interrupt).

Changing CONFIG_HZ changes the timer interrupt’s periodicity. Increasing HZ means that it fires more often, so there’s more timer-related overhead, but less opportunity for task scheduling to wait for a while (so interactivity is improved); decreasing HZ means that it fires less often, so there’s less timer-related overhead, but a higher risk that tasks will wait to be scheduled (so throughput is improved at the expense of interactive responsiveness). As always, the best compromise depends on your specific workload. Nowadays CONFIG_HZ is less relevant for scheduling aspects anyway; see How to change the length of time-slices used by the Linux CPU scheduler?

See also How is an Interrupt handled in Linux?

The local timer interrupt is a timer implemented on the APIC that interrupts only a particular CPU instead of raising an interrupt that can be handled by any CPU. It's discussed in Bovet & Cesati's "Understanding the Linux Kernel". A snippet:

The local APIC present in recent 80x86 microprocessors (see the section “Interrupts and Exceptions” in Chapter 4) provides yet another time-measuring device: the CPU local timer.

The CPU local timer is a device similar to the Programmable Interval Timer (PIT) just described that can issue one-shot or periodic interrupts. There are, however, a few differences:

• The APIC’s timer counter is 32 bits long, while the PIT’s timer counter is 16 bits long; therefore, the local timer can be programmed to issue interrupts at very low frequencies (the counter stores the number of ticks that must elapse before the interrupt is issued).
• The local APIC timer sends an interrupt only to its processor, while the PIT raises a global interrupt, which may be handled by any CPU in the system.
• The APIC’s timer is based on the bus clock signal (or the APIC bus signal, in older machines). It can be programmed in such a way to decrease the timer counter every 1, 2, 4, 8, 16, 32, 64, or 128 bus clock signals. Conversely, the PIT, which makes use of its own clock signals, can be programmed in a more flexible way.

You don't generate timer interrupts. The hardware does (perhaps hundreds interrupts per second). And the kernel configures the hardware to do that.

Read also time(7), which is the way to deal with time related stuff from user-land Linux application programs (which probably should matter to you more).

If you cared about handling interrupts, you would write your own OS kernel and scheduler (and then the linux tag would be off-topic). See Operating systems: Three Easy Pieces (freely downloadable). For practical hints on OS development, see OSDEV wiki.

The operating system is here to hide you hardware details like interrupts and to manage hardware and provide you abstractions above it. /proc/interrupts is simply a way to query the OS kernel state. Its format varies with kernel versions and hardware (on my system the output is quite different from yours), and it mostly is useful for sysadmins (e.g. as hint for hardware malfunction, when some weird piece of hardware sends a lot of interrupts). See proc(5).

How can I manually generate an interrupt

Within a Linux system, you cannot.

BTW, Linux also has signal(7)-s (see also signal-safety(7)...) but these signals are not interrupts (even if some interrupts might indirectly trigger signals). And signals can indeed "interrupt" (in a very figurative way, not the same as hardware interrupts) your process.

^{(it could be possible that on your particular hardware and kernel, most "timer" interrupts go into the ioc0 line)}

Do you want to use signals or threads?

First, set up the signal handler or prepare a suitable thread function; see man 7 sigevent for details.

Next, create a suitable timer, using timer_create(). See man 2 timer_create for details.

Depending on what you do when the timer fires, you may wish to set the timer to either one-shot, or to repeat at a short interval afterwards. You use timer_settime() to both arm, and to disarm, the timer; see man 2 timer_settime for details.

In practical applications you usually need to multiplex the timer. Even though a process can create multiple timers, they are a limited resource. Especially timeout timers -- which are trivial, either setting a flag and/or sending a signal to a specific thread -- should use a single timer, which fires at the next timeout, sets the related timeout flag, and optionally send a signal (with an empty-body handler) to the desired thread to make sure it is interrupted. (For a single-thread process, the original signal delivery will interrupt blocking I/O calls.) Consider a server, responding to some request: the request itself might have a timeout on the order of a minute or so, while processing the request might need connection timeouts, I/O timeouts, and so on.

Now, the original question is interesting, because timers are powerful when used effectively. However, the example program is basically nonsense. Why don't you create say a program that sets one or more timers, each for example outputting something to standard output? Remember to use write() et al from unistd.h as they are async-signal safe, whereas printf() et cetera from stdio.h are not. (If your signal handlers use non-async-signal safe functions, the results are undefined. It usually works, but it's not guaranteed at all; it may just as well crash as work. Testing will not tell, as it is undefined.)

Edited to add: Here is a bare-bones example of multiplexed timeouts.

(To the extent possible under law, I dedicate all copyright and related and neighboring rights to the code snippets shown below to the public domain worldwide; see CC0 Public Domain Dedication. In other words, feel free to use the code below in any way you wish, just don't blame me for any problems with it.)

I used old-style GCC atomic built-ins, so it should be thread-safe. With a few additions, it should work for multithreaded code too. (You cannot use for example mutexes, because pthread_mutex_lock() is not async-signal safe. Atomically manipulating the timeout states should work, although there might be some races left if you disable a timeout just when it fires.)

#define _POSIX_C_SOURCE 200809L
#include <unistd.h>
#include <signal.h>
#include <time.h>
#include <errno.h>

#define   TIMEOUTS       16
#define   TIMEOUT_SIGNAL (SIGRTMIN+0)

#define   TIMEOUT_USED   1
#define   TIMEOUT_ARMED  2
#define   TIMEOUT_PASSED 4

static timer_t               timeout_timer;
static volatile sig_atomic_t timeout_state[TIMEOUTS] = { 0 };
static struct timespec       timeout_time[TIMEOUTS];


/* Return the number of seconds between before and after, (after - before).
 * This must be async-signal safe, so it cannot use difftime().
*/
static inline double timespec_diff(const struct timespec after, const struct timespec before)
{
    return (double)(after.tv_sec - before.tv_sec)
         + (double)(after.tv_nsec - before.tv_nsec) / 1000000000.0;
}

/* Add positive seconds to a timespec, nothing if seconds is negative.
 * This must be async-signal safe.
*/
static inline void timespec_add(struct timespec *const to, const double seconds)
{
    if (to && seconds > 0.0) {
        long  s = (long)seconds;
        long  ns = (long)(0.5 + 1000000000.0 * (seconds - (double)s));

        /* Adjust for rounding errors. */
        if (ns < 0L)
            ns = 0L;
        else
        if (ns > 999999999L)
            ns = 999999999L;

        to->tv_sec += (time_t)s;
        to->tv_nsec += ns;

        if (to->tv_nsec >= 1000000000L) {
            to->tv_nsec -= 1000000000L;
            to->tv_sec++;
        }
    }
}

/* Set the timespec to the specified number of seconds, or zero if negative seconds.
*/
static inline void timespec_set(struct timespec *const to, const double seconds)
{
    if (to) {
        if (seconds > 0.0) {
            const long  s = (long)seconds;
            long       ns = (long)(0.5 + 1000000000.0 * (seconds - (double)s));

            if (ns < 0L)
                ns = 0L;
            else
            if (ns > 999999999L)
                ns = 999999999L;

            to->tv_sec = (time_t)s;
            to->tv_nsec = ns;

        } else {
            to->tv_sec = (time_t)0;
            to->tv_nsec = 0L;
        }
    }
}


/* Return nonzero if the timeout has occurred.
*/
static inline int timeout_passed(const int timeout)
{
    if (timeout >= 0 && timeout < TIMEOUTS) {
        const int  state = __sync_or_and_fetch(&timeout_state[timeout], 0);

        /* Refers to an unused timeout? */
        if (!(state & TIMEOUT_USED))
            return -1;

        /* Not armed? */
        if (!(state & TIMEOUT_ARMED))
            return -1;

        /* Return 1 if timeout passed, 0 otherwise. */
        return (state & TIMEOUT_PASSED) ? 1 : 0;

    } else {
        /* Invalid timeout number. */
        return -1;
    }
}

/* Release the timeout.
 * Returns 0 if the timeout had not fired yet, 1 if it had.
*/
static inline int timeout_unset(const int timeout)
{
    if (timeout >= 0 && timeout < TIMEOUTS) {
        /* Obtain the current timeout state to 'state',
         * then clear all but the TIMEOUT_PASSED flag
         * for the specified timeout.
         * Thanks to Bylos for catching this bug. */
        const int  state = __sync_fetch_and_and(&timeout_state[timeout], TIMEOUT_PASSED);

        /* Invalid timeout? */
        if (!(state & TIMEOUT_USED))
            return -1;

        /* Not armed? */
        if (!(state & TIMEOUT_ARMED))
            return -1;

        /* Return 1 if passed, 0 otherwise. */
        return (state & TIMEOUT_PASSED) ? 1 : 0;

    } else {
        /* Invalid timeout number. */
        return -1;
    }
}


int timeout_set(const double seconds)
{
    struct timespec   now, then;
    struct itimerspec when;
    double            next;
    int               timeout, i;

    /* Timeout must be in the future. */
    if (seconds <= 0.0)
        return -1;

    /* Get current time, */
    if (clock_gettime(CLOCK_REALTIME, &now))
        return -1;

    /* and calculate when the timeout should fire. */
    then = now;
    timespec_add(&then, seconds);

    /* Find an unused timeout. */
    for (timeout = 0; timeout < TIMEOUTS; timeout++)
        if (!(__sync_fetch_and_or(&timeout_state[timeout], TIMEOUT_USED) & TIMEOUT_USED))
            break;

    /* No unused timeouts? */
    if (timeout >= TIMEOUTS)
        return -1;

    /* Clear all but TIMEOUT_USED from the state, */
    __sync_and_and_fetch(&timeout_state[timeout], TIMEOUT_USED);

    /* update the timeout details, */
    timeout_time[timeout] = then;

    /* and mark the timeout armable. */
    __sync_or_and_fetch(&timeout_state[timeout], TIMEOUT_ARMED);

    /* How long till the next timeout? */
    next = seconds;
    for (i = 0; i < TIMEOUTS; i++)
        if ((__sync_fetch_and_or(&timeout_state[i], 0) & (TIMEOUT_USED | TIMEOUT_ARMED | TIMEOUT_PASSED)) == (TIMEOUT_USED | TIMEOUT_ARMED)) {
            const double secs = timespec_diff(timeout_time[i], now);
            if (secs >= 0.0 && secs < next)
                next = secs;
        }

    /* Calculate duration when to fire the timeout next, */
    timespec_set(&when.it_value, next);
    when.it_interval.tv_sec = 0;
    when.it_interval.tv_nsec = 0L;

    /* and arm the timer. */
    if (timer_settime(timeout_timer, 0, &when, NULL)) {
        /* Failed. */
        __sync_and_and_fetch(&timeout_state[timeout], 0);
        return -1;
    }

    /* Return the timeout number. */
    return timeout;
}


static void timeout_signal_handler(int signum __attribute__((unused)), siginfo_t *info, void *context __attribute__((unused)))
{
    struct timespec   now;
    struct itimerspec when;
    int               saved_errno, i;
    double            next;

    /* Not a timer signal? */
    if (!info || info->si_code != SI_TIMER)
        return;

    /* Save errno; some of the functions used may modify errno. */
    saved_errno = errno;

    if (clock_gettime(CLOCK_REALTIME, &now)) {
        errno = saved_errno;
        return;
    }

    /* Assume no next timeout. */
    next = -1.0;

    /* Check all timeouts that are used and armed, but not passed yet. */
    for (i = 0; i < TIMEOUTS; i++)
        if ((__sync_or_and_fetch(&timeout_state[i], 0) & (TIMEOUT_USED | TIMEOUT_ARMED | TIMEOUT_PASSED)) == (TIMEOUT_USED | TIMEOUT_ARMED)) {
            const double  seconds = timespec_diff(timeout_time[i], now);
            if (seconds <= 0.0) {
                /* timeout [i] fires! */
                __sync_or_and_fetch(&timeout_state[i], TIMEOUT_PASSED);

            } else
            if (next <= 0.0 || seconds < next) {
                /* This is the soonest timeout in the future. */
                next = seconds;
            }
        }

    /* Note: timespec_set() will set the time to zero if next <= 0.0,
     *       which in turn will disarm the timer.
     * The timer is one-shot; it_interval == 0.
    */
    timespec_set(&when.it_value, next);
    when.it_interval.tv_sec = 0;
    when.it_interval.tv_nsec = 0L;
    timer_settime(timeout_timer, 0, &when, NULL);

    /* Restore errno. */
    errno = saved_errno;
}


int timeout_init(void)
{
    struct sigaction  act;
    struct sigevent   evt;
    struct itimerspec arm;

    /* Install timeout_signal_handler. */
    sigemptyset(&act.sa_mask);
    act.sa_sigaction = timeout_signal_handler;
    act.sa_flags = SA_SIGINFO;
    if (sigaction(TIMEOUT_SIGNAL, &act, NULL))
        return errno;

    /* Create a timer that will signal to timeout_signal_handler. */
    evt.sigev_notify = SIGEV_SIGNAL;
    evt.sigev_signo = TIMEOUT_SIGNAL;
    evt.sigev_value.sival_ptr = NULL;
    if (timer_create(CLOCK_REALTIME, &evt, &timeout_timer))
        return errno;

    /* Disarm the timeout timer (for now). */
    arm.it_value.tv_sec = 0;
    arm.it_value.tv_nsec = 0L;
    arm.it_interval.tv_sec = 0;
    arm.it_interval.tv_nsec = 0L;
    if (timer_settime(timeout_timer, 0, &arm, NULL))
        return errno;

    return 0;
}

int timeout_done(void)
{
    struct sigaction  act;
    struct itimerspec arm;
    int               errors = 0;

    /* Ignore the timeout signals. */
    sigemptyset(&act.sa_mask);
    act.sa_handler = SIG_IGN;
    if (sigaction(TIMEOUT_SIGNAL, &act, NULL))
        if (!errors) errors = errno;

    /* Disarm any current timeouts. */
    arm.it_value.tv_sec = 0;
    arm.it_value.tv_nsec = 0L;
    arm.it_interval.tv_sec = 0;
    arm.it_interval.tv_nsec = 0;
    if (timer_settime(timeout_timer, 0, &arm, NULL))
        if (!errors) errors = errno;

    /* Destroy the timer itself. */
    if (timer_delete(timeout_timer))
        if (!errors) errors = errno;

    /* If any errors occurred, set errno. */
    if (errors)
        errno = errors;

    /* Return 0 if success, errno otherwise. */
    return errors;
}

Remember to include the rt library when compiling, i.e. use gcc -W -Wall *source*.c -lrt -o *binary* to compile.

The idea is that the main program first calls timeout_init() to install all the necessary handlers et cetera, and may call timeout_done() to deistall it before exiting (or in a child process after fork()ing).

To set a timeout, you call timeout_set(seconds). The return value is a timeout descriptor. Currently there is just a flag you can check using timeout_passed(), but the delivery of the timeout signal also interrupts any blocking I/O calls. Thus, you can expect the timeout to interrupt any blocking I/O call.

If you want to do anything more than set a flag at timeout, you cannot do it in the signal handler; remember, in a signal handler, you're limited to async-signal safe functions. The easiest way around that is to use a separate thread with an endless loop over sigwaitinfo(), with the TIMEOUT_SIGNAL signal blocked in all other threads. That way the dedicated thread is guaranteed to catch the signal, but at the same time, is not limited to async-signal safe functions. It can, for example, do much more work, or even send a signal to a specific thread using pthread_kill(). (As long as that signal has a handler, even one with an empty body, its delivery will interrupt any blocking I/O call in that thread.)

Here is a simple example main() for using the timeouts. It is silly, and relies on fgets() not retrying (when interrupted by a signal), but it seems to work.

#include <string.h>
#include <stdio.h>

int main(void)
{
    char    buffer[1024], *line;
    int t1, t2, warned1;

    if (timeout_init()) {
        fprintf(stderr, "timeout_init(): %s.\n", strerror(errno));
        return 1;
    }

    printf("You have five seconds to type something.\n");
    t1 = timeout_set(2.5); warned1 = 0;
    t2 = timeout_set(5.0);
    line = NULL;

    while (1) {

        if (timeout_passed(t1)) {
            /* Print only the first time we notice. */
            if (!warned1++)
                printf("\nTwo and a half seconds left, buddy.\n");
        }

        if (timeout_passed(t2)) {
            printf("\nAw, just forget it, then.\n");
            break;
        }

        line = fgets(buffer, sizeof buffer, stdin);
        if (line) {
            printf("\nOk, you typed: %s\n", line);
            break;
        }
    }

    /* The two timeouts are no longer needed. */
    timeout_unset(t1);
    timeout_unset(t2);

    /* Note: 'line' is non-NULL if the user did type a line. */

    if (timeout_done()) {
        fprintf(stderr, "timeout_done(): %s.\n", strerror(errno));
        return 1;
    }

    return 0;
}