Timer Management (Slideshow)

Original Timer API

  • “Timer wheel”

  • relies on jiffies (ticks per second, CONFIG_HZ)

  • updated by periodic timer tick

  • ⟶ not power friendly

  • ⟶ not very fine grained (100 - 1000 Hz)

  • But simple!

Attention

Timer callbacks run in atomic context!

jiffies

  • Representation of now

  • Updated by timer tick

  • Nowadays, jiffies is maintained by the hrtimer subsystem

#include <linux/jiffies.h>

extern unsigned long volatile jiffies;

extern unsigned int jiffies_to_msecs(const unsigned long j);
extern unsigned int jiffies_to_usecs(const unsigned long j);
u64 jiffies_to_nsecs(const unsigned long j);
unsigned long msecs_to_jiffies(const unsigned int m);
unsigned long usecs_to_jiffies(const unsigned int u);
long timespec64_to_jiffies(const struct timespec64 *value);
void jiffies_to_timespec64(const unsigned long jiffies, struct timespec64 *value);

“Timer Wheel” Methods

#include <linux/timer.h>

struct timer_list my_timer;
void timerfunc(struct timer_list *timer) { ... }

void timer_setup(struct timer_list* timer, timerfunc, unsigned flags);
void add_timer(struct timer_list* timer);
int del_timer(struct timer_list *timer)
int del_timer_sync(struct timer_list *timer)

  • del_timer_sync() waits until handler has finished running on any CPU

  • my_timer.expires: set to absolute monotonic jiffies (expires = number of jiffies from boot into the future)

Better Documentation