Linux kernel timersHardware timer-countersLinux Software TimersSoftware Timer’s Data-StructureDefining your “timeout” actionAssigning structure’s attributesRemoving your timer safelyKernel’s dynamic-timer handlingGroups of timer listsIn-Class ExerciseLinux kernel timersHow can a module cause a function to be invoked at some future moment in time?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)•Pentium’s 64-bit Time Stamp Counter register•Local APIC’s programmable Counter register•(optional) 8254 Watchdog Timer (NMI)•(optional) Network Interface Card (IRQ varies)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-structure3) call standard routines for timer-controlSoftware Timer’s Data-Structurestruct list_head list;unsigned long expires;unsigned long data;void (*function)( unsigned long );struct timer_listUse the ‘init_timer()’ routine to initialize the ‘list’ fieldDefining your “timeout” actionvoid my_action( unsigned long dataptr ){/* …perform your desired actions… *//* then reschedule timer expiration */next_timeout = jiffies + 5*HZ;mod_timer( &my_timer, next_timeout );}Assigning structure’s attributesstruct timer_list my_timer; // declared globalint 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;}Removing your timer safelyvoid cleanup_module( void ){intstatus = del_timer_sync( &my_timer );// 1 means ‘my_timer’ was queued// 0 means ‘my_timer’ wasn’t queued}Kernel’s 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 ‘tvec’s)•Timers partitioned by their expiration-times•Result: reduces amount of list-processingGroups of timer listsArray of ‘tvec’ structures(< 0xFF ) ( < 0x3FFF ) ( < 0xFFFFF ) ( < 0x3FFFFFF ) ( < 0xFFFFFFFF )In-Class Exercise•Modify the ‘mytimer.c’ demo (on website)•Change expiration-frequency (to 5 secs)•Print timer’s messages on user’s console•(See our ‘announce.c’ demo for
View Full Document
Unlocking...