110 lines
5.4 KiB
ReStructuredText
110 lines
5.4 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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=============================================
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Porting an architecture to support PREEMPT_RT
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=============================================
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:Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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This list outlines the architecture specific requirements that must be
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implemented in order to enable PREEMPT_RT. Once all required features are
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implemented, ARCH_SUPPORTS_RT can be selected in architecture’s Kconfig to make
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PREEMPT_RT selectable.
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Many prerequisites (genirq support for example) are enforced by the common code
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and are omitted here.
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The optional features are not strictly required but it is worth to consider
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them.
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Requirements
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------------
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Forced threaded interrupts
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CONFIG_IRQ_FORCED_THREADING must be selected. Any interrupts that must
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remain in hard-IRQ context must be marked with IRQF_NO_THREAD. This
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requirement applies for instance to clocksource event interrupts,
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perf interrupts and cascading interrupt-controller handlers.
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PREEMPTION support
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Kernel preemption must be supported and requires that
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CONFIG_ARCH_NO_PREEMPT remain unselected. Scheduling requests, such as those
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issued from an interrupt or other exception handler, must be processed
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immediately.
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POSIX CPU timers and KVM
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POSIX CPU timers must expire from thread context rather than directly within
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the timer interrupt. This behavior is enabled by setting the configuration
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option CONFIG_HAVE_POSIX_CPU_TIMERS_TASK_WORK.
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When KVM is enabled, CONFIG_KVM_XFER_TO_GUEST_WORK must also be set to ensure
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that any pending work, such as POSIX timer expiration, is handled before
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transitioning into guest mode.
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Hard-IRQ and Soft-IRQ stacks
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Soft interrupts are handled in the thread context in which they are raised. If
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a soft interrupt is triggered from hard-IRQ context, its execution is deferred
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to the ksoftirqd thread. Preemption is never disabled during soft interrupt
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handling, which makes soft interrupts preemptible.
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If an architecture provides a custom __do_softirq() implementation that uses a
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separate stack, it must select CONFIG_HAVE_SOFTIRQ_ON_OWN_STACK. The
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functionality should only be enabled when CONFIG_SOFTIRQ_ON_OWN_STACK is set.
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FPU and SIMD access in kernel mode
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FPU and SIMD registers are typically not used in kernel mode and are therefore
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not saved during kernel preemption. As a result, any kernel code that uses
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these registers must be enclosed within a kernel_fpu_begin() and
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kernel_fpu_end() section.
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The kernel_fpu_begin() function usually invokes local_bh_disable() to prevent
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interruptions from softirqs and to disable regular preemption. This allows the
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protected code to run safely in both thread and softirq contexts.
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On PREEMPT_RT kernels, however, kernel_fpu_begin() must not call
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local_bh_disable(). Instead, it should use preempt_disable(), since softirqs
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are always handled in thread context under PREEMPT_RT. In this case, disabling
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preemption alone is sufficient.
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The crypto subsystem operates on memory pages and requires users to "walk and
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map" these pages while processing a request. This operation must occur outside
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the kernel_fpu_begin()/ kernel_fpu_end() section because it requires preemption
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to be enabled. These preemption points are generally sufficient to avoid
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excessive scheduling latency.
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Exception handlers
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Exception handlers, such as the page fault handler, typically enable interrupts
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early, before invoking any generic code to process the exception. This is
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necessary because handling a page fault may involve operations that can sleep.
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Enabling interrupts is especially important on PREEMPT_RT, where certain
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locks, such as spinlock_t, become sleepable. For example, handling an
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invalid opcode may result in sending a SIGILL signal to the user task. A
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debug excpetion will send a SIGTRAP signal.
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In both cases, if the exception occurred in user space, it is safe to enable
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interrupts early. Sending a signal requires both interrupts and kernel
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preemption to be enabled.
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Optional features
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-----------------
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Timer and clocksource
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A high-resolution clocksource and clockevents device are recommended. The
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clockevents device should support the CLOCK_EVT_FEAT_ONESHOT feature for
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optimal timer behavior. In most cases, microsecond-level accuracy is
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sufficient
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Lazy preemption
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This mechanism allows an in-kernel scheduling request for non-real-time tasks
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to be delayed until the task is about to return to user space. It helps avoid
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preempting a task that holds a sleeping lock at the time of the scheduling
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request.
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With CONFIG_GENERIC_IRQ_ENTRY enabled, supporting this feature requires
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defining a bit for TIF_NEED_RESCHED_LAZY, preferably near TIF_NEED_RESCHED.
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Serial console with NBCON
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With PREEMPT_RT enabled, all console output is handled by a dedicated thread
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rather than directly from the context in which printk() is invoked. This design
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allows printk() to be safely used in atomic contexts.
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However, this also means that if the kernel crashes and cannot switch to the
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printing thread, no output will be visible preventing the system from printing
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its final messages.
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There are exceptions for immediate output, such as during panic() handling. To
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support this, the console driver must implement new-style lock handling. This
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involves setting the CON_NBCON flag in console::flags and providing
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implementations for the write_atomic, write_thread, device_lock, and
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device_unlock callbacks.
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