Kernel timing issues: An introduction to the use of kernel timers and work queues includes Kernel timing issues, Kernel timers, jiffies, jiffies overflow, Kernel timer syntax, A kernel-timer caution, Programming syntax. | Kernel timing issues An introduction to the use of kernel timers and work queues Kernel timers • Linux offers a facility that lets drivers put a process to sleep until a fixed amount of time has elapsed (as measured in jiffies) • When the timer expires, a driver-defined action will be performed, which can ‘wake up’ the process that was put to sleep, or could perform some alternative action (for example, the kernel timer could re-start) jiffies • • • • • • • unsigned long volatile jiffies; global kernel variable (used by scheduler) initialized to zero when system reboots gets incremented during a timer interrupt so it counts ‘clock-ticks’ since cpu restart ‘tick-frequency’ is a ‘configuration’ option On our machines: HZ=250 (in ‘.config’) jiffies overflow • Won’t overflow for at least 16 months • Linux kernel got modified to ‘fix’ overflow • Now the declaration is in ‘linux/’: unsigned long long jiffies_64; and a new instruction in ‘do_timer()’ (*(u64*)&jiffies_64)++; which compiles to assembly language as add $1, jiffies+0 adc $0, jiffies+4 Kernel timer syntax • Declare a timer: struct timer_list mytimer; • Initialize this timer: init_timer( &mytimer ); = mytimeraction; = (unsigned long)mydata; = • Install this timer: add_timer( &mytimer ); • Modify this timer: mod_timer( &mytimer, ); • Delete this timer: del_timer( &mytimer ); • Delete it safely: del_timer_sync( .
