Interrupts
- Interrupts and exceptions (x86)
- Interrupts and exceptions (Linux)
- Deferrable work
- Timers
Interrupts
- synchronous, generated by executing an instruction
- asynchronous, generated by an external event
- maskable
- can be ignored
- signalled via INT pin
- non-maskable
- cannot be ignored
- signalled via NMI pin
Exceptions
- processor detected
- faults
- traps
- aborts
- programmed
- int n
Hardware (PIC)
Hardware (APIC)
Enabling/disabling the interrupts
- at the PIC level
- programming the PIC
- PIC can be programmed to disable a given IRQ line
- at the CPU level
- cli (CLear Interrupt flag)
- sti (SeT Interrupt flag)
Interrupt Descriptor Table
- it is used as a jump table by the CPU when a given vector is triggered
- it is an array of 256 x 8 bytes entries
- may reside anywhere in physical memory
- processor locates IDT by the means of IDTR
Linux IRQ vector layout
Interrupt descriptor table entry (gate)
Interrupt handler address
Interrupt handler stack
Interrupt execution
check current privilege level
if need to change privilege level
- change stack with the one associated with new privilege
- save old stack information on the new stack
save EFLAGS, CS, EIP on stack
save error code on stack in case of an abort
execute interrupt handler
Returning from an interrupt
- pop the eror code (in case of an abort)
- use IRET
- pops CS, EIP, EFLAGS
- if privilege level changed returns to the old stack and old privilege level
Interrupt/Exception nesting
Interrupt context
- runs as a result of an IRQ (not of an exception)
- there is no ‘process’ context associated
- can’t trigger a context switch
- no sleep
- no schedule
- no user memory access
Interrupt handling
Deferrable actions in Linux
- implemented using deferrable functions
- softIRQ
- runs in interrupt context
- statically allocated
- same handler may run in parallel on multiple cores
- tasklet
- runs in interrupt context
- can be dinamically allocated
- same handler runs are serialized
- workqueues
- run in process context
Soft IRQ
- init: open_softirq()
- activation: raise_softirq()
- execution: do_softirq()
- it runs
- after an interrupt handler
- from the ksoftirqd kernel thread
Types of soft IRQ
- HI_SOFTIRQ
- TIMER_SOFTIRQ
- NET_TX_SOFTIRQ
- NET_RX_SOFTIRQ
- BLOCK_SOFTIRQ
- TASKLET_SOFTIRQ
- HRTIMER_SOFTIRQ
ksoftirqd
- minimum priority kernel thread
- runs self raised softirqs
- trade-off between softirqs and process/kernel threads
Tasklets
- implemented on top of soft IRQ
- TASKLET_SOFITIRQ, HI_SOFTIRQ
- init: tasklet_init
- activation: tasklet_schedule
- masking: tasklet_disable(), tasklet_enable()
Workqueues
- implemented on top of kernel threads
- TASKLET_SOFITIRQ, HI_SOFTIRQ
- init: INIT_WORK
- activation: schedule_work()
Timers
- implemented on top of TIMER_SOFTIRQ
- init: setup_timer
- activation: mod_timer