CSC 660 Advanced OS Interrupts CSC 660 Advanced

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CSC 660: Advanced OS Interrupts CSC 660: Advanced Operating Systems 1

CSC 660: Advanced OS Interrupts CSC 660: Advanced Operating Systems 1

Topics 1. 2. 3. 4. 5. 6. 7. 8. 9. Types of Interrupts PIC

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

How can hardware communicate with CPU? Busy Wait Issue hardware request. Wait in tight

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. CSC 660: Advanced Operating Systems 3

Types of Interrupts Synchronous Produced by CPU while executing instructions. Issues only after finishing

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 CSC 660: Advanced Operating Systems 4

Programmable Interrupt Controller PIC connects Hardware devices that issue IRQs. CPU: INTR pin and

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. CSC 660: Advanced Operating Systems 5

Interrupt Vectors Vector Range Use 0 -19 Nonmaskable interrupts and exceptions. 20 -31 Intel-reserved

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

IRQ Example IRQ INT Hardware Device 0 32 Timer 1 33 Keyboard 2 34

IRQ Example IRQ INT Hardware Device 0 32 Timer 1 33 Keyboard 2 34 PIC Cascading 3 35 Second serial port 4 36 First serial port 6 38 Floppy Disk 8 40 System Clock 10 42 Network Interface 11 43 USB port, sound card 12 44 PS/2 Mouse 13 45 Math Coprocessor 14 46 EIDE first controller 15 47 EIDE second controller CSC 660: Advanced Operating Systems 7

IRQ Handling 1. Monitor IRQ lines for raised signals. If multiple IRQs raised, select

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

do_IRQ 1. 2. 3. 4. 5. 6. 7. 8. 9. Kernel jumps to entry

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

handle_IRQ_event() fastcall int handle_IRQ_event(unsigned int irq, struct pt_regs *regs, struct irqaction *action) { int

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; } CSC 660: Advanced Operating Systems 10

Interrupt Handlers Function kernel runs in response to interrupt. More than one handler can

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 CSC 660: Advanced Operating Systems 11

Top and Bottom Halves Top Half The interrupt handler. Current interrupt disabled, possibly all

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. CSC 660: Advanced Operating Systems 12

Interrupt Context Not associated with a process. Cannot sleep: no task to reschedule. current

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. CSC 660: Advanced Operating Systems 13

Registering a Handler request_irq() Register an interrupt handler on a given line. free_irq() Unregister

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. CSC 660: Advanced Operating Systems 14

Registering a Handler int request_irq(unsigned int irq, irqreturn_t (*handler)(int, void *, struct pt_regs *),

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 CSC 660: Advanced Operating Systems 15

Writing an Interrupt Handler irqreturn_t ih(int irq, void *devid, struct pt_regs *r) Differentiating between

Writing an Interrupt Handler irqreturn_t ih(int irq, void *devid, struct pt_regs *r) 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. CSC 660: Advanced Operating Systems 16

RTC Handler irqreturn_t rtc_interrupt(int irq, void *dev_id, struct pt_regs *regs) { spin_lock (&rtc_lock); rtc_irq_data

RTC Handler irqreturn_t rtc_interrupt(int irq, void *dev_id, struct pt_regs *regs) { spin_lock (&rtc_lock); rtc_irq_data += 0 x 100; rtc_irq_data &= ~0 xff; 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; } CSC 660: Advanced Operating Systems 17

Interrupt Control Disable/Enable Local Interrupts local_irq_disable(); /* interrupts are disabled */ local_irq_enable(); Saving and

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 */ CSC 660: Advanced Operating Systems 18

Interrupt Control Disabling Specific Interrupts For legacy hardware, avoid for shared IRQ lines. disable_irq(irq)

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… CSC 660: Advanced Operating Systems 19

Bottom Halves Perform most work required by interrupt. Run in process context with interrupts

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

Soft. IRQs Statically allocated at compile time. Only 32 soft. IRQs can exist (only

Soft. IRQs Statically allocated at compile time. Only 32 soft. IRQs 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 Soft. IRQs. All tasklets use one Soft. IRQ. Dynamically allocated. CSC 660: Advanced Operating Systems 21

Soft. IRQ Handlers Prototype void softirq_handler(struct softirq_action *) Calling my_softirq->action(my_softirq); Pre-emption Soft. IRQs don’t

Soft. IRQ Handlers Prototype void softirq_handler(struct softirq_action *) Calling my_softirq->action(my_softirq); Pre-emption Soft. IRQs don’t pre-empt other soft. IRQs. Interrupt handlers can pre-empt soft. IRQs. Another soft. IRQ can run on other CPUs. CSC 660: Advanced Operating Systems 22

Executing Soft. IRQs Interrupt handler marks soft. IRQ. Called raising the softirq. Soft. IRQs

Executing Soft. IRQs Interrupt handler marks soft. IRQ. Called raising the softirq. Soft. IRQs checked for execution: In return from hardware interrupt code. In ksoftirq kernel thread. In any code that explicitly checks for soft. IRQs. do_softirq() Loops over all soft. IRQs. CSC 660: Advanced Operating Systems 23

Current Soft. IRQs Soft. IRQ Priority Description HI 0 High priority tasklets. TIMER 1

Current Soft. IRQs Soft. IRQ Priority Description HI 0 High priority tasklets. TIMER 1 Timer bottom half. NET_TX 2 Send network packets. NET_RX 3 Receive network packets. SCSI 4 SCSI bottom half. TASKLET 5 Tasklets. CSC 660: Advanced Operating Systems 24

Tasklets • Implemented as soft. IRQs. – Linked list of tasklet_struct objects. • Two

Tasklets • Implemented as soft. IRQs. – 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() CSC 660: Advanced Operating Systems 25

ksoftirqd Soft. IRQs may occur at high frequencies. Soft. IRQs may re-raise themselves. Kernel

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

Work Queues Defer work into a kernel thread. Execute in process context. One thread

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 */ }; CSC 660: Advanced Operating Systems 27

Work Queue Data Structures worker thread cpu_workqueue_struct 1/CPU workqueue_struct 1/thread type work_struct CSC 660:

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

Worker Thread Each thread runs worker_thread() 1. 2. 3. 4. 5. Marks self as

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

run_workqueue() 1. Loops through list of work_structs struct work_struct { unsigned long pending; struct

run_workqueue() 1. Loops through list of work_structs 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 CSC 660: Advanced Operating Systems 30

Which Bottom Half to Use? 1. If needs to sleep, use work queue. 2.

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 Half Softirq Context Interrupt Serialization None Tasklet Interrupt Against same tasklet Work queues Process None CSC 660: Advanced Operating Systems 31

Timer Interrupt Executed HZ times a second. #define HZ 1000 /* <asm/param. h> */

Timer Interrupt Executed HZ times a second. #define HZ 1000 /* <asm/param. h> */ 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. CSC 660: Advanced Operating Systems 32

Jiffies = number of ticks since boot. extern unsigned long volatile jiffies; Incremented each

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 CSC 660: Advanced Operating Systems 33

Timer Interrupt Handler 1. Increments jiffies. 2. Update resource usages (sys + user time.

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. CSC 660: Advanced Operating Systems 34

References 1. 2. 3. 4. 5. 6. Daniel P. Bovet and Marco Cesati, Understanding

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