Linux kernel timers

1
Linux kernel timers

• How can a module cause a function to be invoked
  at some future moment in time?

2
Hardware timer-counters

• IA32 systems several hardware timers
• External devices, or internal to processor
• Examples
• 8254 Programmable Interval Timer (IRQ0)
• MC146818 Real Time Clock w/CMOS (IRQ8)
• Pentiums 64-bit Time Stamp Counter register
• Local APICs programmable Counter register
• (optional) 8254 Watchdog Timer (NMI)
• (optional) Network Interface Card (IRQ varies)

3
Linux Software Timers

• Easy for device-driver programmers to use
• Used mainly for detecting device lockups
• But could also be used for other purposes
• The driver-writer merely needs to
• 1) define a customized timeout-function
• 2) allocate and initialize a kernel-structure
• 3) call standard routines for timer-control

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Software Timers Data-Structure
struct timer_list
struct list_head list
unsigned long expires
unsigned long data
void (function)( unsigned long )
Use the init_timer() routine to initialize the
list field
5
Defining your timeout action

• void my_action( unsigned long dataptr )
• / perform your desired actions / / then
  reschedule timer expiration /
• next_timeout jiffies 5HZ
• mod_timer( my_timer, next_timeout )

6
Assigning structures attributes

• struct timer_list my_timer // declared
  global
• int init_module( void )
• init_timer( my_timer ) // for list setup
• my_timer.data (unsigned long)my_data
• my_timer.function my_action
• my_timer.expires jiffies 5 HZ
• add_timer( my_timer )
• return SUCCESS

7
Removing your timer safely

• void cleanup_module( void )
• int
• status del_timer_sync( my_timer )
• // 1 means my_timer was queued
• // 0 means my_timer wasnt queued

8
Kernels dynamic-timer handling

• Need to deal efficiently with many timers
• One big linked list would NOT be efficient
• Linux adopts a clever partitioning structure
• Uses special structures (called tvecs)
• Timers partitioned by their expiration-times
• Result reduces amount of list-processing

9
Groups of timer lists
Array of tvec structures
(lt 0xFF ) ( lt 0x3FFF ) ( lt 0xFFFFF ) (
lt 0x3FFFFFF ) ( lt 0xFFFFFFFF )
10
In-Class Exercise

• Modify the mytimer.c demo (on website)
• Change expiration-frequency (to 5 secs)
• Print timers messages on users console
• (See our announce.c demo for how-to)