5.7.6. How to delay execution

• Linux timekeeping: HZ, jiffies

• sometimes we need to delay execution, for example
  • short: device initialization, timeouts, ...
  • longer: need to do something at certain time

bad: burning CPU cycles (don't use)

#define DELAY_MS 100
unsigned long j = jiffies + DELAY_MS * HZ / 1000;

while (time_before(jiffies, j1))
      cpu_relax();

• cpu_relax means not doing much. Calls architecture dependent code, actually only a memory barrier on powerpc.

slightly better but still wasteful:

while (time_before(jiffies, j1))
      schedule();

• others can run, but system stays loaded (we stay in run queue of the scheduler)

recommended for small (busy waiting) delays

void ndelay(unsigned long nsecs);
void udelay(unsigned long usecs);
void mdelay(unsigned long msecs);

better: sleep instead of busy waiting if possible:

void msleep(unsigned int ms);

/* 
 * interruptible version (for waitqueues)
 * return 0 or if interrupted remaining time
 */
unsigned long msleep_interruptible(unsigned int ms);

void ssleep(unsigned int sec);

• Interesting: msleep(1) can sleep for up to 20ms lkml thread. Proposed solution: implement msleep with hrtimers.

longer delays: use waitqueues!:

wait_queue_head_t wait;
init_waitqueue_head(&wait);

/* condition is always false=0, delay in jiffies  */
wait_event_interruptible_timeout(wait, 0, delay);