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 device level
- by programming the device control registers
- at the PIC level
- PIC can be programmed to disable a given IRQ line
- at the CPU level; for example, on x86 one can use the following instructions:
- 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
Handling an interrupt request
CPU checks the 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 the kernel interrupt handler
Returning from an interrupt
- pop the eror code (in case of an abort)
- call IRET
- pops values from the stack and restore the following register: CS, EIP, EFLAGS
- if privilege level changed returns to the old stack and old privilege level
Interrupt handling in Linux
IRQ nesting in Linux
- an exception (e.g. page fault, system call) can not preempt an interrupt; if that occurs it is considered a bug
- an interrupt can preempt an exception or other interrupts; however, only one level of interrupt nesting is allowed
Interrupt/Exception nesting
Interrupt context
- it runs as a result of an IRQ (not of an exception)
- there is no well defined process context associated
- not allowed to trigger a context switch (no sleep, schedule, or user memory access)
Deferrable actions in Linux
- softIRQ
- runs in interrupt context
- statically allocated
- same handler may run in parallel on multiple cores
- tasklet
- runs in interrupt context
- can be dynamically allocated
- same handler runs are serialized
- workqueues
- run in process context
Soft IRQs
Soft IRQ APIs:
- initialize:
open_softirq()- activation:
raise_softirq()- masking:
local_bh_disable(),local_bh_enable()Once activated, the callback function
do_softirq()runs either:
- after an interrupt handler or
- from the ksoftirqd kernel thread
ksoftirqd
- minimum priority kernel thread
- runs softirqs after certain limits are reached
- tries to achieve good latency and avoid process starvation
Types of soft IRQ
- HI_SOFTIRQ
- TIMER_SOFTIRQ
- NET_TX_SOFTIRQ
- NET_RX_SOFTIRQ
- BLOCK_SOFTIRQ
- IRQ_POLL_SOFTIRQ
- TASKLET_SOFTIRQ
- SCHED_SOFTIRQ
- HRTIMER_SOFTIRQ,
- RCU_SOFTIRQ
Tasklets
Tasklets are a dynamic type (not limited to a fixed number) of deferred work running in interrupt context.
Tasklets API:
- initialization:
tasklet_init()- activation:
tasklet_schedule()- masking:
tasklet_disable(),tasklet_enable()
Tasklets are implemented on top of two dedicated softirqs:
TASKLET_SOFITIRQ and HI_SOFTIRQ
Tasklets are also serialized, i.e. the same tasklet can only execute on one processor.
Workqueues
Workqueues are a type of deferred work that runs in process context.
They are implemented on top of kernel threads.
Workqueues API:
- init:
INIT_WORK- activation:
schedule_work()
Timers
Timers are implemented on top of the
TIMER_SOFTIRQTimer API:
- initialization:
setup_timer()- activation:
mod_timer()