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CSC 660: Advanced Operating Systems Slide #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
CSC 660: Advanced Operating Systems Slide #3
How can hardware communicate with CPU?
Busy WaitIssue hardware request.Wait in tight loop until receives answer.
PollingIssue hardware request.Periodically check hardware status.
InterruptsIssue hardware request.Hardware signals CPU when answer ready.
CSC 660: Advanced Operating Systems Slide #4
Types of Interrupts
SynchronousProduced by CPU while executing instructions.Issues only after finishing execution of an instr.Often called exceptions.Ex: page faults, system calls, divide by zero
AsynchronousGenerated 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 Slide #5
Programmable Interrupt Controller
PIC connectsHardware devices that issue IRQs.
CPU: INTR pin and data bus.
PIC features15 IRQ lines
Sharing and dynamic assignment of IRQs.
Masking (disabling) of selected IRQs.
CPU masking of all maskable interrupts: cli, sti.
APIC: Advanced PICHandles multiprocessor systems.
CSC 660: Advanced Operating Systems Slide #6
Interrupt VectorsVector 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 Slide #7
IRQ ExampleIRQ 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 Slide #8
IRQ Handling
1. Monitor IRQ lines for raised signals.If multiple IRQs raised, select lowest # IRQ.
2. If raised signal detected1. 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.
CSC 660: Advanced Operating Systems Slide #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().
CSC 660: Advanced Operating Systems Slide #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;}
CSC 660: Advanced Operating Systems Slide #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
CSC 660: Advanced Operating Systems Slide #12
Top and Bottom Halves
Top HalfThe 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 HalfRuns in process context with interrupts enabled.
Performs most work required. Can sleep.
Ex: copies network data to memory buffers.
CSC 660: Advanced Operating Systems Slide #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.
CSC 660: Advanced Operating Systems Slide #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.
CSC 660: Advanced Operating Systems Slide #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
CSC 660: Advanced Operating Systems Slide #16
Writing an Interrupt Handler
Differentiating between devicesPre-2.0: irq
Current: dev_id
RegistersPointer to registers before interrupt occurred.
Return ValuesIRQ_NONE: Interrupt not for handler.
IRQ_HANDLED: Interrupted handled.
irqreturn_t ih(int irq,void *devid,struct pt_regs *r)
CSC 660: Advanced Operating Systems Slide #17
RTC Handlerirqreturn_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;}
CSC 660: Advanced Operating Systems Slide #18
Interrupt Control
Disable/Enable Local Interruptslocal_irq_disable();/* interrupts are disabled */local_irq_enable();
Saving and Restoring IRQ stateUseful 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 Slide #19
Interrupt Control
Disabling Specific InterruptsFor 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 Slide #20
Bottom Halves
Perform most work required by interrupt.Run in process context with interrupts enabled.
Three forms of deferring workSoftIRQs
Tasklets
Work Queues
CSC 660: Advanced Operating Systems Slide #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.
CSC 660: Advanced Operating Systems Slide #22
SoftIRQ Handlers
Prototypevoid softirq_handler(struct softirq_action *)
Callingmy_softirq->action(my_softirq);
Pre-emptionSoftIRQs don’t pre-empt other softIRQs.
Interrupt handlers can pre-empt softIRQs.
Another softIRQ can run on other CPUs.
CSC 660: Advanced Operating Systems Slide #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.
CSC 660: Advanced Operating Systems Slide #24
Current SoftIRQs
SoftIRQ 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 Slide #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()
CSC 660: Advanced Operating Systems Slide #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).
CSC 660: Advanced Operating Systems Slide #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 */
};
CSC 660: Advanced Operating Systems Slide #28
Work Queue Data Structures
worker thread
work_struct
cpu_workqueue_struct 1/CPU
workqueue_struct 1/thread type
work_struct
work_struct1/deferrable function
CSC 660: Advanced Operating Systems Slide #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.
CSC 660: Advanced Operating Systems Slide #30
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, data3. Removes entry from list, clears pending4. Invokes function
CSC 660: Advanced Operating Systems Slide #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 Half Context Serialization
Softirq Interrupt None
Tasklet Interrupt Against same tasklet
Work queues Process None
CSC 660: Advanced Operating Systems Slide #32
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 responsibilitiesUpdating uptime, system time, kernel stats.Rescheduling if current has exhausted time slice.Balancing scheduler runqueues.Running dynamic timers.
CSC 660: Advanced Operating Systems Slide #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 > btime_before(a,b) a < btime_after_eq(a,b): a >= btime_before_eq(a,b): a <= b
CSC 660: Advanced Operating Systems Slide #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.
CSC 660: Advanced Operating Systems Slide #35
References1. Daniel P. Bovet and Marco Cesati, Understanding the
Linux Kernel, 3rd edition, O’Reilly, 2005.2. Johnathan Corbet et. al., Linux Device Drivers, 3rd edition,
O’Reilly, 2005.3. Robert Love, Linux Kernel Development, 2nd 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.