1. CSC 660: Advanced Operating SystemsSlide #1 CSC 660: Advanced OS Interrupts

2. CSC 660: Advanced Operating SystemsSlide #2 Topics 1.Types of Interrupts 2.PIC and IRQs 3.Interrupt Handlers 4.Top Halves and Bottom Halves 5.Enabling/Disabling Interrupts 6.SoftIRQs 7.Tasklets 8.Work Queues 9.Timer Interrupts

3. CSC 660: Advanced Operating SystemsSlide #3 How can hardware communicate with CPU? Busy Wait Issue hardware request. Wait in tight loop until receives answer. Polling Issue hardware request. Periodically check hardware status. Interrupts Issue hardware request. Hardware signals CPU when answer ready.

4. CSC 660: Advanced Operating SystemsSlide #4 Types of Interrupts Synchronous Produced by CPU while executing instructions. Issues only after finishing execution of an instr. Often called exceptions. Ex: page faults, system calls, divide by zero Asynchronous Generated by other hardware devices. Occur at arbitrary times, including while CPU is busy executing an instruction. Ex: I/O, timer interrupts

5. CSC 660: Advanced Operating SystemsSlide #5 Programmable Interrupt Controller PIC connects Hardware devices that issue IRQs. CPU: INTR pin and data bus. PIC features 15 IRQ lines Sharing and dynamic assignment of IRQs. Masking (disabling) of selected IRQs. CPU masking of all maskable interrupts: cli, sti. APIC: Advanced PIC Handles multiprocessor systems.

6. CSC 660: Advanced Operating SystemsSlide #6 Interrupt Vectors Vector RangeUse 0-19Nonmaskable interrupts and exceptions. 20-31Intel-reserved 32-127External interrupts (IRQs) 128System Call exception 129-238External interrupts (IRQs) 239Local APIC timer interrupt 240Local APIC thermal interrupt 241-250Reserved by Linux for future use 251-253Interprocessor interrupts 254Local APIC error interrupt 255Local APIC suprious interrupt

7. CSC 660: Advanced Operating SystemsSlide #7 IRQ Example IRQINTHardware Device 032Timer 133Keyboard 234PIC Cascading 335Second serial port 436First serial port 638Floppy Disk 840System Clock 1042Network Interface 1143USB port, sound card 1244PS/2 Mouse 1345Math Coprocessor 1446EIDE first controller 1547EIDE second controller

8. CSC 660: Advanced Operating SystemsSlide #8 IRQ Handling 1.Monitor IRQ lines for raised signals. If multiple IRQs raised, select lowest # IRQ. 2.If raised signal detected 1.Converts raised signal into vector (0-255). 2.Stores vector in I/O port, allowing CPU to read. 3.Sends raised signal to CPU INTR pin. 4.Waits for CPU to acknowledge interrupt. 5.Kernel runs do_IRQ(). 6.Clears INTR line. 3.Goto step 1.

9. CSC 660: Advanced Operating SystemsSlide #9 do_IRQ 1.Kernel jumps to entry point in entry.S. 2.Entry point saves registers, calls do_IRQ(). 3.Finds IRQ number in saved %EAX register. 4.Looks up IRQ descriptor using IRQ #. 5.Acknowledges receipt of interrupt. 6.Disables interrupt delivery on line. 7.Calls handle_IRQ_event() to run handlers. 8.Cleans up and returns. 9.Jumps to ret_from_intr().

10. CSC 660: Advanced Operating SystemsSlide #10 handle_IRQ_event() fastcall int handle_IRQ_event(unsigned int irq, struct pt_regs *regs, struct irqaction *action) { int ret, retval = 0, status = 0; if (!(action->flags & SA_INTERRUPT)) local_irq_enable(); do { ret = action->handler(irq, action->dev_id, regs); if (ret == IRQ_HANDLED) status |= action->flags; retval |= ret; action = action->next; } while (action); if (status & SA_SAMPLE_RANDOM) add_interrupt_randomness(irq); local_irq_disable(); return retval; }

11. CSC 660: Advanced Operating SystemsSlide #11 Interrupt Handlers Function kernel runs in response to interrupt. More than one handler can exist per IRQ. Must run quickly. Resume execution of interrupted code. How to deal with high work interrupts? Ex: network, hard disk

12. CSC 660: Advanced Operating SystemsSlide #12 Top and Bottom Halves Top Half The interrupt handler. Current interrupt disabled, possibly all disabled. Runs in interrupt context, not process context. Can’t sleep. Acknowledges receipt of interrupt. Schedules bottom half to run later. Bottom Half Runs in process context with interrupts enabled. Performs most work required. Can sleep. Ex: copies network data to memory buffers.

13. CSC 660: Advanced Operating SystemsSlide #13 Interrupt Context Not associated with a process. Cannot sleep: no task to reschedule. current macro points to interrupted process. Shares kernel stack of interrupted process. Be very frugal in stack usage.

14. CSC 660: Advanced Operating SystemsSlide #14 Registering a Handler request_irq() Register an interrupt handler on a given line. free_irq() Unregister a given interrupt handler. Disable interrupt line if all handlers unregistered.

15. CSC 660: Advanced Operating SystemsSlide #15 Registering a Handler int request_irq(unsigned int irq, irqreturn_t (*handler)(int, void *, struct pt_regs *), unsigned long irqflags, const char * devname, void *dev_id) irqflaqs = SA_INTERRUPT | SA_SAMPLE_RANDOM | SA_SHIRQ

16. CSC 660: Advanced Operating SystemsSlide #16 Writing an Interrupt Handler Differentiating between devices Pre-2.0: irq Current: dev_id Registers Pointer to registers before interrupt occurred. Return Values IRQ_NONE : Interrupt not for handler. IRQ_HANDLED : Interrupted handled. irqreturn_t ih(int irq,void *devid,struct pt_regs *r)

17. CSC 660: Advanced Operating SystemsSlide #17 RTC Handler irqreturn_t rtc_interrupt(int irq, void *dev_id, struct pt_regs *regs) { spin_lock (&rtc_lock); rtc_irq_data += 0x100; rtc_irq_data &= ~0xff; if (rtc_status & RTC_TIMER_ON) mod_timer(&rtc_irq_timer, jiffies + HZ/rtc_freq + 2*HZ/100); spin_unlock (&rtc_lock); /* Now do the rest of the actions */ spin_lock(&rtc_task_lock); if (rtc_callback) rtc_callback->func(rtc_callback->private_data); spin_unlock(&rtc_task_lock); wake_up_interruptible(&rtc_wait); kill_fasync (&rtc_async_queue, SIGIO, POLL_IN); return IRQ_HANDLED; }

18. CSC 660: Advanced Operating SystemsSlide #18 Interrupt Control Disable/Enable Local Interrupts local_irq_disable(); /* interrupts are disabled */ local_irq_enable(); Saving and Restoring IRQ state Useful when don’t know prior IRQ state. unsigned long flags; local_irq_save(flags); /* interrupts are disabled */ local_irq_restore(flags); /* interrupts in original state */

19. CSC 660: Advanced Operating SystemsSlide #19 Interrupt Control Disabling Specific Interrupts For legacy hardware, avoid for shared IRQ lines. disable_irq(irq) enable_irq(irq) What about other processors? Disable local interrupts + spin lock. We’ll talk about spin locks next time…

20. CSC 660: Advanced Operating SystemsSlide #20 Bottom Halves Perform most work required by interrupt. Run in process context with interrupts enabled. Three forms of deferring work SoftIRQs Tasklets Work Queues

21. CSC 660: Advanced Operating SystemsSlide #21 SoftIRQs Statically allocated at compile time. Only 32 softIRQs can exist (only 6 currently used.) struct softirq_action { void (*action)(struct softirq_action *); void *data; }; static struct softirq_action softirq_vec[32]; Tasklets built on SoftIRQs. All tasklets use one SoftIRQ. Dynamically allocated.

22. CSC 660: Advanced Operating SystemsSlide #22 SoftIRQ Handlers Prototype void softirq_handler(struct softirq_action *) Calling my_softirq->action(my_softirq); Pre-emption SoftIRQs don’t pre-empt other softIRQs. Interrupt handlers can pre-empt softIRQs. Another softIRQ can run on other CPUs.

23. CSC 660: Advanced Operating SystemsSlide #23 Executing SoftIRQs Interrupt handler marks softIRQ. Called raising the softirq. SoftIRQs checked for execution: In return from hardware interrupt code. In ksoftirq kernel thread. In any code that explicitly checks for softIRQs. do_softirq() Loops over all softIRQs.

24. CSC 660: Advanced Operating SystemsSlide #24 Current SoftIRQs SoftIRQPriorityDescription HI0High priority tasklets. TIMER1Timer bottom half. NET_TX2Send network packets. NET_RX3Receive network packets. SCSI4SCSI bottom half. TASKLET5Tasklets.

25. CSC 660: Advanced Operating SystemsSlide #25 Tasklets Implemented as softIRQs. –Linked list of tasklet_struct objects. Two priorities of tasklets: –HI: tasklet_hi_schedule() –TASKLET: tasklet_schedule() Scheduled tasklets run via do_softirq() –HI action: tasklet_action() –TASKLET action: tasklet_hi_action()

26. CSC 660: Advanced Operating SystemsSlide #26 ksoftirqd SoftIRQs may occur at high frequencies. SoftIRQs may re-raise themselves. Kernel will not handle re-raised softIRQs immediately in do_softirq(). Kernel thread ksoftirq solves problem. One thread per processor. Runs at lowest priority (nice +19).

27. CSC 660: Advanced Operating SystemsSlide #27 Work Queues Defer work into a kernel thread. Execute in process context. One thread per processor: events/n. Processes can create own threads if needed. struct workqueue_struct { struct cpu_workqueue_struct cpu_wq[NR_CPUS]; const char *name; struct list_head list; /* Empty if single thread */ };

28. CSC 660: Advanced Operating SystemsSlide #28 Work Queue Data Structures worker thread work_struct cpu_workqueue_struct 1/CPU workqueue_struct 1/thread type work_struct 1/deferrable function

29. CSC 660: Advanced Operating SystemsSlide #29 Worker Thread Each thread runs worker_thread() 1.Marks self as sleeping. 2.Adds self to wait queue. 3.If linked list of work empty, schedule(). 4.Else, marks self as running, removes from queue. 5.Calls run_workqueue() to perform work.

30. CSC 660: Advanced Operating SystemsSlide #30 run_workqueue() 1.Loops through list of work_struct s struct work_struct { unsigned long pending; struct list_head entry; void (*func)(void *); void *data; void *wq_data; struct timer_list timer; }; 2.Retrieves function, func, and arg, data 3.Removes entry from list, clears pending 4.Invokes function

31. CSC 660: Advanced Operating SystemsSlide #31 Which Bottom Half to Use? 1.If needs to sleep, use work queue. 2.If doesn’t need to sleep, use tasklet. 3.What about serialization needs? Bottom HalfContextSerialization SoftirqInterruptNone TaskletInterruptAgainst same tasklet Work queuesProcessNone

32. CSC 660: Advanced Operating SystemsSlide #32 Timer Interrupt Executed HZ times a second. #define HZ 1000 /* */ Called the tick rate. Time between two interrupts is a tick. Driven by Programmable Interrupt Timer (PIT). Interrupt handler responsibilities Updating uptime, system time, kernel stats. Rescheduling if current has exhausted time slice. Balancing scheduler runqueues. Running dynamic timers.

33. CSC 660: Advanced Operating SystemsSlide #33 Jiffies Jiffies = number of ticks since boot. extern unsigned long volatile jiffies; Incremented each timer interrupt. Uptime = jiffies/HZ seconds. Convert for user space: jiffies_to_clock_t() Comparing jiffies, while avoiding overflow. time_after(a, b): a > b time_before(a,b) a < b time_after_eq(a,b): a >= b time_before_eq(a,b): a <= b

34. CSC 660: Advanced Operating SystemsSlide #34 Timer Interrupt Handler 1.Increments jiffies. 2.Update resource usages (sys + user time.) 3.Run dynamic timers. 4.Execute scheduler_tick(). 5.Update wall time. 6.Calculate load average.

35. CSC 660: Advanced Operating SystemsSlide #35 References 1.Daniel P. Bovet and Marco Cesati, Understanding the Linux Kernel, 3 rd edition, O’Reilly, 2005. 2.Johnathan Corbet et. al., Linux Device Drivers, 3 rd edition, O’Reilly, 2005. 3.Robert Love, Linux Kernel Development, 2 nd edition, Prentice-Hall, 2005. 4.Claudia Rodriguez et al, The Linux Kernel Primer, Prentice-Hall, 2005. 5.Peter Salzman et. al., Linux Kernel Module Programming Guide, version 2.6.1, 2005. 6.Andrew S. Tanenbaum, Modern Operating Systems, 3rd edition, Prentice-Hall, 2005.