Kernel Event-Timers

1. Kernel Event-Timers Our first look at the Linux kernel’s mechanism for managing its dynamic timers

2. The timer interrupt “There is a sense in which the timer interrupt drives the entire Operating System” -- John O’Gorman

3. The ‘task_struct’ structure • The OS kernel creates a Task Control Block for each task which currently exists • In Linux this TCB has type ‘task_struct’ • There are numerous fields in a task_struct • We wrote a module which creates a ‘/proc’ file (named ‘tcbinfo’) to let us see a TCB • We wrote a demo-program (dumptcb,cpp) which displays a task’s ‘task_struct’ in hex format, by reading the ‘/proc/tcbinfo’ file

4. Timer-related fields • There are several fields in the ‘task_struct’ which pertain to time-keeping mechanisms • ‘it_real_value’ and ‘it_real_incr’ • ‘it_prof_value’ and ‘it_prof_incr’ • ‘it_virt_value’ and ‘it_virt_incr’ • ‘real_timer’ • The ‘real_timer’ field is actually a structure of type ‘struct timer_list’ (an ‘event timer’)

5. ‘struct timer_list’ • The ‘real_timer’ field in the ‘task_struct’ is an element of type ‘struct timer_list’. next prev expires data function

6. Demo: ‘realtimers.c’ • We wrote a kernel module which creates a pseudo-file (named ‘/proc/realtimers’) that shows a list of all current tasks that have an active real-time event-timer • You can compile this module (using our ‘Makefile’ script) with: $ make realtimers.o • Install it with: $ /sbin/insmod realtimers.o • Then use it with: $ cat /proc/realtimers

7. The ‘setitimer()’ function • In our lab-exercise on Friday we studied the ‘setitimer()’ library-function, used by applications to implement ‘interval timers’ • Now we can see how ‘setitimer()’ affects timer-related fields in a task’s ‘task_struct’ • Our ‘timerinfo.c’ module creates a ‘/proc’ file that displays those timer-related fields • Our ‘trytimer.cpp’ demo illustrates this

8. The kernel’s ‘TO DO’ list • A multitasking operating system has many timer-related events to keep track of, so it could have lots of work to do each time a timer-interrupt occurs • So there needs to be a very efficient way to manage all of the kernel’s event-timers • Linux employs a clever mechanism for organizing all of its active event-timers

9. Doubly-Linked Lists next prev A single-element list next next next next prev prev prev prev A multi-element list

10. timer-list vectors index vec

11. The ‘tvecs[ ]’ array tvecs tvec1 index vec[ 256 ] tvec2 index vec[ 64 ] tvec3 index vec[ 64 ] tvec4 index vec[ 64 ] tvec5 index vec[ 64 ]

12. Where is an event-timer put? • Each event-timer has an ‘expires’ field • ‘expires’ holds a future value for ‘jiffies’ • It’s a 32-bit unsigned long integer • These bits are subdivided: 32=6+6+6+6+8 • The event-timer is assigned to one of the five timer-list vectors, based on which one of five numeric intervals ‘expires’ falls into • This organizes the kernel’s ‘TO DO’ list

13. The five ‘expires’ intervals • Compute: when = expires – jiffies; • Use tvec1[ when>>0 ]: 0  when < 28 • Use tvec2[ when>>8 ]: 28 when < 214 • Use tvec3[ when>>14]: 214 when < 220 • Use tvec4[ when>>20]: 220 when < 226 • Use tvec5[ when>>26]: 226 when < 232

14. The work to be done • When a timer-interrupt occurs, the value of ‘jiffies’ gets incremented, so it is possible that some ‘event-timers’ may have expired • The kernel checks the ‘index’ field of the ‘root’ timer-list vector to quickly find the list of event-timers which might have expired • It processes the event-timers in that list. then increments the root vector’s ‘index’ (modulo 256) • If ( index == 0 ) the tvecs array is ‘cascaded’

15. In-class exercises • Try modifying the ‘trytimer.cpp’ program so that it calls ‘setitimer()’ with ITIMER_PROF instead of ITIMER_REAL. What do you observe about the ‘task_struct’ fields? • Try modifying the ‘trytimer.cpp’ program so it calls ‘setitimer()’ with ITIMER_VIRTUAL instead of ITIMER_REAL. What do you observe about the ‘task_struct’ fields?

16. Exercises (continued) • Try installing your own signal-handlers for the SIGALRM, the SIGVTALRM, and the SIGPROF signals, to see the effects of using ‘setitimer()’ with the three different types of interval timers. • Use our ‘realtimers.c’ module to look at the list of all current tasks that have an active ‘event-timer’