Sage-Tips

What is Local_irq_save?

local_irq_save(flags); This code disables all interrupts at the level of CPU IRQ interface. Interrupts are passed to CPU IRQ interface after they have passed priority & routing pipeline of the interrupt controller and have been asserted to the CPU.

What is Tasklets?

Tasklets are a deferral scheme that you can schedule for a registered function to run later. The top half (the interrupt handler) performs a small amount of work, and then schedules the tasklet to execute later at the bottom half.

Why does Linux make use of Tasklets?

Tasklet in Linux Kernel Tasklets are used to queue up work to be done at a later time. Tasklets can be run in parallel, but the same tasklet cannot be run on multiple CPUs at the same time. Also, each tasklet will run only on the CPU that schedules it, to optimize cache usage.

What is SoftIrq in Linux?

SoftIrqs allow the critical part of servicing hardware interrupts to be as short as possible; instead of having to process the entire hw interrupt on the spot, the important data is read off the device into RAM or otherwise, and then a SoftIrq is started to finish the work.

Which instruction is mainly used for disabling IRQ interrupt?

You can disable IRQ exceptions within a privileged mode by setting the CPSR I flag. When the I flag is cleared to 0, the processor checks for a LOW level on the output of the nIRQ register at the end of each instruction.

Why are interrupts not disabled in interrupt handling?

An interrupt should be generally cleared the moment you enter ISR. If it’s not cleared, the hardware keeps interrupting the CPU and the safety code inside ISR will never execute. Also the CPU needs to maintain a stack of its current execution state. A recurring interrupt makes it a complex situation.

What are the differences between Softirq Tasklet and Workqueue?

It is similar to tasklets with some differences. Workqueue functions run in the context of a kernel process, but tasklet functions run in the software interrupt context. This means that workqueue functions must not be atomic as tasklet functions.

When should I use Tasklets?

1) The Tasklet are used in interrupt context. All the tasklet code must be atomic,so all rules that are applied on atomic context are applied to it. For eg. They cannot sleep(as they cannot be reschecduled) or hold a lock for long time.

What is difference between Tasklet and Workqueue?

A tasklet always runs in the interrupt context, that means when a tasklet executes it is as if the processor is executing an interrupt service routine. On the other hand a workqueue executes as a normal process and not a interrupt, hence a worqueue can go to sleep it can hold semaphores.

What is difference between Tasklet and Softirq?

Softirqs of the same type can run concurrently on several CPUs. Tasklets differ from softirqs because a tasklet is always serialized with respect to itself; in other words, a tasklet cannot be executed by two CPUs at the same time. However, different tasklets can be executed concurrently on several CPUs.

What is difference between Softirq and Tasklet?

Why FIQ is faster than IRQ?

FIQ has higher priority than IRQ. This means that when the core takes an FIQ exception, it automatically masks out IRQs. An IRQ cannot interrupt the FIQ handler. The opposite is not true – the IRQ does not mask FIQs and so the FIQ handler (if used) can interrupt the IRQ.

What is the difference between disable_IRQ() and local_IRQ_save()?

I found that the disable_irq () disables a specific interrupt, on the other hand local_irq_save () disables all the interrupt. So I wonder the meaning of the above code. This makes sense, because interrupts are disabled at different levels. This code disables (masks) interrupt in interrupt controller.

Is spin_lock_irqsave an interrupt routine?

Is there another interrupt routine that could interrupt us? spin_lock_irqsave is basically used to save the interrupt state before taking the spin lock, this is because spin lock disables the interrupt, when the lock is taken in interrupt context, and re-enables it when while unlocking.

What does locallocal_BH_disable actually do?

local_bh_disable disables the processing of bottom halves (softirqs). Softirqs are processed on either, interrupt return path, or by the ksoftirqd- (per cpu)-thread that will be woken up if the system suffers of heavy softirq-load.