Interrupts and Bottom Halves in Linux Kernel

Learn how the Linux kernel handles interrupts, including vector numbers, saving process context, and stack switching to call a common interrupt handler.

The lecture explains linux kernel interrupt statistics, showing per-interrupt counts and steps, plus attributes like edge or level triggering and the registered end-of-interrupt name.

Learn to monitor interrupts in the Linux kernel by using the watch command to observe updates in real time, including keyboard and mouse activity, at configurable intervals.

Explore PCI interrupts in the Linux kernel, and practical approaches to device interrupt handling and programmability within kernel space.

Explore requesting an interrupt handler in the Linux kernel, focusing on driver involvement, configuration space, and return values indicating success or failure.

Analyze how a Linux keyboard driver handles keyboard interrupts within the kernel, showcasing input processing and interrupt-driven design.

Discover how a Linux driver logs each typed character by processing keyboard scan codes and release events within the Linux kernel, framed by interrupts and bottom halves.

Explore ethernet interrupts in a Linux driver, and learn how interrupt events are captured and processed within the Linux kernel.

Learn how to implement a Linux driver for mouse interrupts inside the Linux kernel, addressing interrupt handling and bottom-half processing.

Explore building a Linux kernel keylogger driver that captures keyboard scancodes, converts them to ASCII, and logs keystrokes in a circular buffer using interrupt handling.

Add sysfs support for the keylogger driver by defining a keylogger attribute, exposing a file that triggers a read function to output ascii data from the buffer.

Analyze the return value of interrupt handlers in the Linux kernel, and how interrupts and bottom halves coordinate with devices and interfaces.

Explain interrupt flags, including the iq flag that allows sharing an interrupt with other devices. If sharing isn't enabled and another driver holds it, the request returns the EBC error.

Register all irqs for the linux driver by requesting each irq from 0 to 255 and setting unique identification flags. Check which irqs are shared or private and report failures.

Explore other interrupt flags in the Linux kernel, including how flags indicate timer events, shared items, and device busy status, and how to interpret interrupt request outcomes.

Explore the irqf_nobalancing flag that disables interrupt load balancing across processors. It prevents cpu affinity changes, keeping a given interrupt on a single CPU.

Explore SMP IRQ affinity in the Linux kernel and learn how to assign interrupts to specific CPUs or CPU groups. See how affinity masks represented in hexadecimal control the distribution.

Explore how shared interrupts require identifying the triggering device; a null last argument hinders this identification, so you must pass a unique device id to distinguish sources.

Register multiple devices and handle interrupts in the Linux kernel, printing the irq number and device identifier in the irq handler, and implement logic to differentiate between devices.

Enable and disable interrupts on the current processor using local control mechanisms, flags, and assembly instructions. Learn how these operations affect interrupt handling in the Linux kernel.

Disabling interrupts guarantees non-preemptive execution on the local processor, but does not protect against concurrent access from other processors; use locks to prevent simultaneous data access.

Demonstrate a Linux kernel module example that disables and enables interrupts on the current process, revealing the impact on delay, process behavior, and visibility of interrupt handling within the kernel.

Learn to save interrupt flags before disabling local interrupts and restore them to the previous state, ensuring safe and predictable interrupt handling in the Linux kernel.

Disable a specific interrupt line within the Linux kernel to mask interrupts across processors while honoring ongoing handlers, illustrated with keyboard and network controller examples.

Disable_irq_nosync disables an interrupt line without waiting for a currently running irq handler to complete, illustrating asynchronous behavior and potential delays in interrupt handling.

Understand that every disable_irq call requires a matching enable_irq call, and failing to pair them (e.g., disabling twice but enabling once) prevents proper enabling of interrupts.

Explore what happens when you disable a shared interrupt line used by multiple devices, including its effect on all devices, legacy drivers, and how often a function is called.

Learn to determine whether interrupts on the local processor are disabled using a macro and a function called disabler, which reports the disabled status.

Understand interrupt context in the Linux kernel and how it differs from process context; learn that sleeping is not allowed in interrupt handlers and how system calls relate to context.

Discover a macro to detect if code runs in interrupt context or process context, and see examples that print and verify the current context during kernel operations.

Learn how to use in_interrupt to allocate memory within kernel contexts, controlling flags and behavior across process context and interrupt handling.

The lecture demonstrates the effects of adding a delay inside an interrupt handler, showing how delays induce busy waiting and that such delays should not be done.

Learn to print a call trace inside an interrupt handler to verify which function calls the handler and trace the caller path in the Linux kernel.

Examine how the current macro behaves inside an interrupt handler. It points to the active process during interrupt processing.

The lecture shows what happens when sleep is called in an interrupt handler, revealing how sleeping blocks the system and disables input such as the mouse, making the OS unresponsive.

Explain why interrupt handlers must execute quickly with small context to minimize latency, and introduce splitting work into a fast top half and a deferred bottom half.

Explore how the Linux kernel handles network interrupts using top and bottom halves, detailing the top half’s packet acknowledgment and memory copy, and the bottom half’s deferred processing with workers.

Explore how threaded irqs offer an alternative to the formal bottom half mechanism, reducing interrupt time by pushing processing to a dedicated thread.

This example demonstrates threaded IRQs in the Linux kernel. It shows how a coordinator selects the correct handler function and how threads get created and executed during interrupts.

Explore how a kernel module prints the execution context of threaded irqs, distinguishing interrupt context from process context and examining interrupt function arguments.

Explore the irqf_oneshot flag in the Linux kernel interrupts, its mandatory use with a primary handler, and its role in enabling one-shot jobs and maintaining safe, ordered execution.

Print a call trace for threaded irqs by tracing the call stack as a thread is scheduled and switched, including assembly and source paths.

Print the pid and process name in threaded irqs to reveal how process context appears during interrupt handling.

Identify whether interrupts are enabled in threaded irqs by calling a verification function and analyzing outcome consistency within the linux kernel context.

Explore how the Linux kernel uses bottom halves and software interrupts to handle high-priority hardware events, via static compile-time structures like the softirq_action array and enum sizing.

Examine the statistics of softirqs in the Linux kernel, focusing on how software interrupt counters are collected, displayed, and used to gauge system performance.

Explore softirqs methods in the linux kernel, including registering handlers with function pointers, per-cpu bitmaps, and masking to safely signal and execute bottom halves across cpus.

Create a new softirq in the Linux kernel by adding an entry, registering it, and assigning its priority. Understand per-CPU execution and locking to ensure safe, scalable concurrency.

Examine a Linux kernel driver example of softirq and bottom halves, including function calls, module use, symbol exports, and building an image with a Makefile on a Raspberry Pi.

Assess whether software runs in interrupt context or process context when handling softirq in the Linux kernel. Explore implications for function calls, sleeping, and correctness in context verification.

Discover how to verify whether interrupts are enabled or disabled within a softirq handler using a specific macro, and understand how this state affects preemption in the Linux kernel.

Explore the value of current in a softirq handler on the Linux kernel, and its relation to the interactive process and the swapper on the local processor context.

Learn how to print a call trace from a softirq handler in the Linux kernel, illustrating how interrupts and bottom halves are traced and debugged.

Check processor id in interrupt and softirq handlers to understand how the Linux kernel handles work across CPUs.

Observe how introducing a four-second delay in a softirq handler affects interrupt processing and preemption in the Linux kernel, revealing that softirq execution can be delayed and limited in count.

Learn how to determine if softirq runs in hard irq or soft irq context by examining function behavior when it is called from both softirq and hardirq paths.

Explore softirq in the Linux kernel and how an enum entry enables early execution after a handler returns. See how softirq can run in parallel on multiple CPUs.

Explain how soft interrupts can monopolize the CPU if not executed promptly. Show how ksoftirqd and related mechanisms prevent long execution, balancing interrupts and software processing.

Trace how activity from a process is executed and how ksoftirqd handles that work in the Linux kernel.

Learn how to find out pending softirqs in the Linux kernel by inspecting and printing the softirq mask, understanding bit states, and verifying changes as processes run.

Explore when pending softirqs are executed in the Linux kernel, including how the system checks, calls, and finishes related functions after a task completes.

Explore a macro that tells whether code is running in softirq context in the Linux kernel and how the function is invoked from software.

Learn how to disable and enable softirqs to protect critical regions, manage process context, and keep the local processor from being interrupted.

Examine whether interrupts remain enabled when a spinlock is used on the local processor, by observing the enablement and disablement of interrupts during spinlock operations.

Protect shared data by applying locking between user context and softirqs using spin and bottom-half disabling. Ensure safe, non-concurrent access across cpus in critical regions.

Explore the Linux kernel's bottom halves, a software-based mechanism using function pointers to run after interrupts. Learn how task_struct fields like next and state organize the bottom half.

The lecture explains the tasklet state and count fields, showing how a scheduled but not yet run task is tracked and how these fields prevent concurrent execution on multiprocessor systems.

Explore the Linux kernel tasklet API, covering static and dynamic initialization, declaring a tasklet with a function and its argument, and enabling or disabling via non-zero or zero state.

Explore a simple Linux kernel tasklet example. It demonstrates enabling and disabling a tasklet, printing speed and content values, and illustrating macro-driven state changes.

Learn how the Linux kernel schedules tasklets by priority, managing linked lists of task structures, setting task states, and dispatching high-priority work efficiently.

Explore how a tasklet scheduling example demonstrates static initialization and tracks state changes as a kernel task is scheduled and executed.

Demonstrate dynamic initialization and scheduling of tasklets in the Linux kernel by allocating memory, passing a function as an argument, and triggering the basket function through state-based scheduling.

Explain the steps performed by tasklet_schedule, including checking the next step, potentially scheduling immediately, disabling interrupts to protect the task, and restoring state after execution.

The tasklet softirq handler disables local interrupts, clears the per-processor task list, iterates entries, and executes scheduled tasks when content is non-zero, then re-enables interrupts.

The kernel avoids running the same tasklet on multiple processors by using a per-tasklet flag; a processor sets it to claim the tasklet, and others skip if it is set.

Clarify whether a tasklet handler can sleep or block in the Linux kernel, showing that you cannot sleep in a tasklet unless you avoid semaphores or other blocking functions.

Explore how to determine whether interrupts are enabled when a tasklet runs by using a function to check enabled versus disabled states and its impact on an independent user process.

Explore the context in which a tasklet handler runs, comparing process context to the broader execution context within the Linux kernel.

Explore how to print the PID and process name in a tasklet handler within the Linux kernel. Relate this to interrupts and bottom halves to understand kernel execution context.

Learn how to print a call trace inside a tasklet handler to debug interrupts and bottom halves in the Linux kernel.

Learn how to determine if a tasklet runs in hard or soft irq context and how to inspect and manage context during interrupt handling in the Linux kernel.

Analyze how Linux handles interrupts and bottom halves by printing the pending softirqs bitmask, illustrating bitmask values, scheduling, and state transitions.

Understand how calling tasklet_schedule twice does not trigger multiple runs of the bottom halves: if a tasklet is already scheduled or pending, it runs only once.

Explore how to check the processor id in tasklet and irq handlers, showing that both run on the same CPU, with a keyboard example to illustrate bottom-half handling.

Understand the execution order between softirq and tasklet in the Linux kernel, and how priorities determine which will run first.

Learn how to enable and disable tasklets in the Linux kernel, including two methods for disabling and understanding the impact of long waits during tasklet execution.

Analyze the tasklet_kill function in the Linux kernel, noting that it kills a tasklet only after completion and should not be used in sleeping contexts.

Explore how to use tasklet_hi_schedule to schedule high-priority bottom halves in the Linux kernel, illustrating priority-driven execution between urgent tasks and regular work.

Explore how to disable the bottom half with local_bh_disable and observe its effect on tasklets and bottom half execution in the Linux kernel.

Compare tasklets and softirqs, and explore how these soft targets are allocated and executed across different processes within the Linux kernel.

Explore how Linux kernel uses bottom halves and work queues, where work items hold function pointers enqueued for execution by per-CPU and global worker pools.

Explore how a target determines when a work item can run, based on the parameters passed and the work item's attributes.

Explore workqueues APIs by creating and initializing work items, assigning the function to execute, and scheduling work using the work structure in the Linux kernel.

Queue and schedule work items in the Linux kernel using the work structure and queue APIs, targeting a specific CPU or any available CPU, with cancellation and guarantees.

Use Linux kernel workqueue API to create system work or your own, initialize a work item, schedule it asynchronously, and note that execution may not stay on the same CPU.

Investigate queuing the same work twice in the Linux kernel, showing that the first enqueue executes while the second is deferred or blocked until completion.

Explore how queue_work_on routes work to a specific CPU and how the worker function executes on different processors, with CPU parameters altering behavior.

Examine how to schedule and locate work items in the Linux kernel, assigning tasks to specific work queues and invoking work functions for execution.

Learn how to pass private data to a work function in the Linux kernel by enclosing it in the work structure and accessing it inside the work function.

Print the PID and process name for the task executing a work function, and explore how a work item runs within a process context in the Linux kernel.

Discover how to determine whether a kernel work function runs in an interrupt context or a process context, and how bottom halves execute across contexts using the available option.

Explore whether interrupts are enabled during work function execution in the Linux kernel, and learn how to use locking when sharing data between the work function and other contexts.

Explore how the Linux kernel cancels pending work, what happens when items are already running, and how cancellation prevents future execution while ongoing tasks may continue.

The lecture demonstrates how a 10-second delay in a Linux kernel work function affects cancellation, waiting, and whether the work item executes.

Compare two work items' scheduling: initialise and queue them, observe which item runs first on the CPU, and determine whether their execution order differs.

Explore how flushing work items during interrupts and scheduling tasks in the Linux kernel's bottom halves ensures correct execution order and completion.

learn how delayed work binds a work item to a timer, with a specified expiry, to execute after the delay in the linux kernel.

Demonstrates delayed work in the Linux kernel, scheduling a function with a five-second delay and showing it executes on a separate context and expires at the end of the data.

Learn how to flush delayed work in the Linux kernel by canceling timers, scheduling execution, and ensuring work runs timely on a given CPU.

Explore how cancelling delayed work in the Linux kernel prevents scheduled functions from executing, highlighting the effects of timely cancellation on CPU tasks.

Explore the differences between softirqs, tasklets, and workqueues in the Linux kernel, focusing on execution context, preemption, sleep behavior, and when to use each for scalability.

Explore kworker behavior in the Linux kernel, showing how processes execute under cpu affinity naming conventions and how to inspect a process stack to determine what a task is doing.

Explore the dedicated workqueues API, including how flags govern work item assignment and execution, and how max concurrent items limit simultaneous processing to prevent delays.

Explore the wq_unbound flag in linux kernel workqueues, where work items run on any available process and are executed promptly by unbounded workers, with the work function as the context.

Demonstrate queuing two work items in a Linux kernel workqueue with max_active set to 1, showing that only one work item executes at a time until completion.

Explore queuing two work items on a Linux kernel workqueue with max active set to 2, illustrating how two tasks are scheduled and run concurrently.

Explore how the wq_highpri workqueue flag elevates tasks with negative nice values to high priority, affecting execution timing and bottom-half processing in the Linux kernel.

Queue two work items in the WQ_HIGHPRI workqueue to observe how interrupts trigger and how bottom-half tasks execute.

Compare two workqueues to show how item priority influences which task executes first and how higher priority can determine the first to run.

Explore queue_delayed_work with delayed work to show how work items handle expiry and execute in the Linux kernel, highlighting networking context.

Explore to_delayed_work within the linux kernel, focusing on interrupts and bottom halves and how delayed tasks are scheduled and executed.

Learn to schedule periodic tasks in the Linux kernel using workqueue by configuring and executing a work function at regular intervals.

Demonstrate how the WQ_SYSFS flag affects visibility and execution of workqueue tasks in the Linux kernel, showing how flags control on which CPUs tasks run and how they’re displayed.

Learn how cpumask governs which cpus execute delayed work items, and how the associated work function is scheduled and run on the chosen cpu.

Examine other workqueue flags in the linux kernel, focusing on suspend and resume behavior, blocking and wakeups, and how flags coordinate tasks (A, B, C) under workload.

Flush workqueue shows how to ensure all queued work completes in the Linux kernel after an incident, using native functions and final status prints.

Determine the number of unbound workqueues in the Linux kernel's interrupt handling by analyzing maximum and active states for inbound work, within the bottom halves framework.

Explore how the alloc_ordered_workqueue enforces serial execution of work items, ensuring only one task runs at a time and the next item follows after the previous completes.

Explore what happens when you call queue_work in the Linux kernel, including how a work item is queued, how flags influence the operation, and how the worker wakes up.