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![Thomas Gleixner](/assets/img/avatar_default.png)
RT requires the softirq processing and local bottomhalf disabled regions to be preemptible. Using the normal preempt count based serialization is therefore not possible because this implicitely disables preemption. RT kernels use a per CPU local lock to serialize bottomhalfs. As local_bh_disable() can nest the lock can only be acquired on the outermost invocation of local_bh_disable() and released when the nest count becomes zero. Tasks which hold the local lock can be preempted so its required to keep track of the nest count per task. Add a RT only counter to task struct and adjust the relevant macros in preempt.h. Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Tested-by: Paul E. McKenney <paulmck@kernel.org> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lore.kernel.org/r/20210309085726.983627589@linutronix.de
398 lines
12 KiB
C
398 lines
12 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef __LINUX_PREEMPT_H
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#define __LINUX_PREEMPT_H
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/*
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* include/linux/preempt.h - macros for accessing and manipulating
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* preempt_count (used for kernel preemption, interrupt count, etc.)
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*/
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#include <linux/linkage.h>
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#include <linux/list.h>
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/*
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* We put the hardirq and softirq counter into the preemption
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* counter. The bitmask has the following meaning:
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*
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* - bits 0-7 are the preemption count (max preemption depth: 256)
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* - bits 8-15 are the softirq count (max # of softirqs: 256)
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*
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* The hardirq count could in theory be the same as the number of
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* interrupts in the system, but we run all interrupt handlers with
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* interrupts disabled, so we cannot have nesting interrupts. Though
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* there are a few palaeontologic drivers which reenable interrupts in
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* the handler, so we need more than one bit here.
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*
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* PREEMPT_MASK: 0x000000ff
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* SOFTIRQ_MASK: 0x0000ff00
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* HARDIRQ_MASK: 0x000f0000
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* NMI_MASK: 0x00f00000
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* PREEMPT_NEED_RESCHED: 0x80000000
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*/
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#define PREEMPT_BITS 8
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#define SOFTIRQ_BITS 8
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#define HARDIRQ_BITS 4
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#define NMI_BITS 4
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#define PREEMPT_SHIFT 0
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#define SOFTIRQ_SHIFT (PREEMPT_SHIFT + PREEMPT_BITS)
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#define HARDIRQ_SHIFT (SOFTIRQ_SHIFT + SOFTIRQ_BITS)
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#define NMI_SHIFT (HARDIRQ_SHIFT + HARDIRQ_BITS)
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#define __IRQ_MASK(x) ((1UL << (x))-1)
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#define PREEMPT_MASK (__IRQ_MASK(PREEMPT_BITS) << PREEMPT_SHIFT)
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#define SOFTIRQ_MASK (__IRQ_MASK(SOFTIRQ_BITS) << SOFTIRQ_SHIFT)
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#define HARDIRQ_MASK (__IRQ_MASK(HARDIRQ_BITS) << HARDIRQ_SHIFT)
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#define NMI_MASK (__IRQ_MASK(NMI_BITS) << NMI_SHIFT)
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#define PREEMPT_OFFSET (1UL << PREEMPT_SHIFT)
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#define SOFTIRQ_OFFSET (1UL << SOFTIRQ_SHIFT)
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#define HARDIRQ_OFFSET (1UL << HARDIRQ_SHIFT)
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#define NMI_OFFSET (1UL << NMI_SHIFT)
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#define SOFTIRQ_DISABLE_OFFSET (2 * SOFTIRQ_OFFSET)
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#define PREEMPT_DISABLED (PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
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/*
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* Disable preemption until the scheduler is running -- use an unconditional
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* value so that it also works on !PREEMPT_COUNT kernels.
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*
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* Reset by start_kernel()->sched_init()->init_idle()->init_idle_preempt_count().
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*/
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#define INIT_PREEMPT_COUNT PREEMPT_OFFSET
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/*
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* Initial preempt_count value; reflects the preempt_count schedule invariant
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* which states that during context switches:
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*
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* preempt_count() == 2*PREEMPT_DISABLE_OFFSET
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*
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* Note: PREEMPT_DISABLE_OFFSET is 0 for !PREEMPT_COUNT kernels.
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* Note: See finish_task_switch().
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*/
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#define FORK_PREEMPT_COUNT (2*PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
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/* preempt_count() and related functions, depends on PREEMPT_NEED_RESCHED */
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#include <asm/preempt.h>
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#define nmi_count() (preempt_count() & NMI_MASK)
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#define hardirq_count() (preempt_count() & HARDIRQ_MASK)
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#ifdef CONFIG_PREEMPT_RT
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# define softirq_count() (current->softirq_disable_cnt & SOFTIRQ_MASK)
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#else
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# define softirq_count() (preempt_count() & SOFTIRQ_MASK)
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#endif
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#define irq_count() (nmi_count() | hardirq_count() | softirq_count())
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/*
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* Macros to retrieve the current execution context:
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*
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* in_nmi() - We're in NMI context
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* in_hardirq() - We're in hard IRQ context
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* in_serving_softirq() - We're in softirq context
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* in_task() - We're in task context
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*/
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#define in_nmi() (nmi_count())
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#define in_hardirq() (hardirq_count())
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#define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET)
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#define in_task() (!(in_nmi() | in_hardirq() | in_serving_softirq()))
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/*
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* The following macros are deprecated and should not be used in new code:
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* in_irq() - Obsolete version of in_hardirq()
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* in_softirq() - We have BH disabled, or are processing softirqs
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* in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled
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*/
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#define in_irq() (hardirq_count())
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#define in_softirq() (softirq_count())
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#define in_interrupt() (irq_count())
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/*
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* The preempt_count offset after preempt_disable();
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*/
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#if defined(CONFIG_PREEMPT_COUNT)
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# define PREEMPT_DISABLE_OFFSET PREEMPT_OFFSET
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#else
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# define PREEMPT_DISABLE_OFFSET 0
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#endif
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/*
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* The preempt_count offset after spin_lock()
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*/
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#define PREEMPT_LOCK_OFFSET PREEMPT_DISABLE_OFFSET
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/*
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* The preempt_count offset needed for things like:
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*
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* spin_lock_bh()
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*
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* Which need to disable both preemption (CONFIG_PREEMPT_COUNT) and
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* softirqs, such that unlock sequences of:
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*
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* spin_unlock();
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* local_bh_enable();
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*
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* Work as expected.
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*/
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#define SOFTIRQ_LOCK_OFFSET (SOFTIRQ_DISABLE_OFFSET + PREEMPT_LOCK_OFFSET)
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/*
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* Are we running in atomic context? WARNING: this macro cannot
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* always detect atomic context; in particular, it cannot know about
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* held spinlocks in non-preemptible kernels. Thus it should not be
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* used in the general case to determine whether sleeping is possible.
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* Do not use in_atomic() in driver code.
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*/
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#define in_atomic() (preempt_count() != 0)
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/*
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* Check whether we were atomic before we did preempt_disable():
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* (used by the scheduler)
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*/
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#define in_atomic_preempt_off() (preempt_count() != PREEMPT_DISABLE_OFFSET)
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#if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_TRACE_PREEMPT_TOGGLE)
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extern void preempt_count_add(int val);
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extern void preempt_count_sub(int val);
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#define preempt_count_dec_and_test() \
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({ preempt_count_sub(1); should_resched(0); })
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#else
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#define preempt_count_add(val) __preempt_count_add(val)
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#define preempt_count_sub(val) __preempt_count_sub(val)
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#define preempt_count_dec_and_test() __preempt_count_dec_and_test()
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#endif
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#define __preempt_count_inc() __preempt_count_add(1)
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#define __preempt_count_dec() __preempt_count_sub(1)
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#define preempt_count_inc() preempt_count_add(1)
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#define preempt_count_dec() preempt_count_sub(1)
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#ifdef CONFIG_PREEMPT_COUNT
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#define preempt_disable() \
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do { \
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preempt_count_inc(); \
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barrier(); \
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} while (0)
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#define sched_preempt_enable_no_resched() \
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do { \
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barrier(); \
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preempt_count_dec(); \
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} while (0)
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#define preempt_enable_no_resched() sched_preempt_enable_no_resched()
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#define preemptible() (preempt_count() == 0 && !irqs_disabled())
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#ifdef CONFIG_PREEMPTION
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#define preempt_enable() \
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do { \
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barrier(); \
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if (unlikely(preempt_count_dec_and_test())) \
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__preempt_schedule(); \
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} while (0)
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#define preempt_enable_notrace() \
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do { \
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barrier(); \
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if (unlikely(__preempt_count_dec_and_test())) \
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__preempt_schedule_notrace(); \
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} while (0)
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#define preempt_check_resched() \
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do { \
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if (should_resched(0)) \
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__preempt_schedule(); \
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} while (0)
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#else /* !CONFIG_PREEMPTION */
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#define preempt_enable() \
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do { \
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barrier(); \
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preempt_count_dec(); \
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} while (0)
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#define preempt_enable_notrace() \
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do { \
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barrier(); \
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__preempt_count_dec(); \
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} while (0)
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#define preempt_check_resched() do { } while (0)
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#endif /* CONFIG_PREEMPTION */
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#define preempt_disable_notrace() \
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do { \
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__preempt_count_inc(); \
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barrier(); \
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} while (0)
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#define preempt_enable_no_resched_notrace() \
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do { \
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barrier(); \
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__preempt_count_dec(); \
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} while (0)
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#else /* !CONFIG_PREEMPT_COUNT */
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/*
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* Even if we don't have any preemption, we need preempt disable/enable
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* to be barriers, so that we don't have things like get_user/put_user
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* that can cause faults and scheduling migrate into our preempt-protected
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* region.
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*/
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#define preempt_disable() barrier()
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#define sched_preempt_enable_no_resched() barrier()
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#define preempt_enable_no_resched() barrier()
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#define preempt_enable() barrier()
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#define preempt_check_resched() do { } while (0)
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#define preempt_disable_notrace() barrier()
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#define preempt_enable_no_resched_notrace() barrier()
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#define preempt_enable_notrace() barrier()
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#define preemptible() 0
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#endif /* CONFIG_PREEMPT_COUNT */
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#ifdef MODULE
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/*
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* Modules have no business playing preemption tricks.
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*/
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#undef sched_preempt_enable_no_resched
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#undef preempt_enable_no_resched
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#undef preempt_enable_no_resched_notrace
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#undef preempt_check_resched
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#endif
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#define preempt_set_need_resched() \
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do { \
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set_preempt_need_resched(); \
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} while (0)
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#define preempt_fold_need_resched() \
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do { \
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if (tif_need_resched()) \
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set_preempt_need_resched(); \
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} while (0)
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#ifdef CONFIG_PREEMPT_NOTIFIERS
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struct preempt_notifier;
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/**
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* preempt_ops - notifiers called when a task is preempted and rescheduled
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* @sched_in: we're about to be rescheduled:
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* notifier: struct preempt_notifier for the task being scheduled
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* cpu: cpu we're scheduled on
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* @sched_out: we've just been preempted
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* notifier: struct preempt_notifier for the task being preempted
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* next: the task that's kicking us out
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*
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* Please note that sched_in and out are called under different
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* contexts. sched_out is called with rq lock held and irq disabled
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* while sched_in is called without rq lock and irq enabled. This
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* difference is intentional and depended upon by its users.
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*/
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struct preempt_ops {
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void (*sched_in)(struct preempt_notifier *notifier, int cpu);
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void (*sched_out)(struct preempt_notifier *notifier,
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struct task_struct *next);
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};
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/**
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* preempt_notifier - key for installing preemption notifiers
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* @link: internal use
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* @ops: defines the notifier functions to be called
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*
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* Usually used in conjunction with container_of().
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*/
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struct preempt_notifier {
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struct hlist_node link;
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struct preempt_ops *ops;
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};
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void preempt_notifier_inc(void);
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void preempt_notifier_dec(void);
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void preempt_notifier_register(struct preempt_notifier *notifier);
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void preempt_notifier_unregister(struct preempt_notifier *notifier);
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static inline void preempt_notifier_init(struct preempt_notifier *notifier,
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struct preempt_ops *ops)
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{
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INIT_HLIST_NODE(¬ifier->link);
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notifier->ops = ops;
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}
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#endif
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#ifdef CONFIG_SMP
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/*
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* Migrate-Disable and why it is undesired.
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*
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* When a preempted task becomes elegible to run under the ideal model (IOW it
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* becomes one of the M highest priority tasks), it might still have to wait
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* for the preemptee's migrate_disable() section to complete. Thereby suffering
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* a reduction in bandwidth in the exact duration of the migrate_disable()
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* section.
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*
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* Per this argument, the change from preempt_disable() to migrate_disable()
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* gets us:
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*
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* - a higher priority tasks gains reduced wake-up latency; with preempt_disable()
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* it would have had to wait for the lower priority task.
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*
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* - a lower priority tasks; which under preempt_disable() could've instantly
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* migrated away when another CPU becomes available, is now constrained
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* by the ability to push the higher priority task away, which might itself be
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* in a migrate_disable() section, reducing it's available bandwidth.
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*
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* IOW it trades latency / moves the interference term, but it stays in the
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* system, and as long as it remains unbounded, the system is not fully
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* deterministic.
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*
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*
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* The reason we have it anyway.
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*
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* PREEMPT_RT breaks a number of assumptions traditionally held. By forcing a
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* number of primitives into becoming preemptible, they would also allow
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* migration. This turns out to break a bunch of per-cpu usage. To this end,
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* all these primitives employ migirate_disable() to restore this implicit
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* assumption.
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*
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* This is a 'temporary' work-around at best. The correct solution is getting
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* rid of the above assumptions and reworking the code to employ explicit
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* per-cpu locking or short preempt-disable regions.
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*
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* The end goal must be to get rid of migrate_disable(), alternatively we need
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* a schedulability theory that does not depend on abritrary migration.
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*
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*
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* Notes on the implementation.
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*
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* The implementation is particularly tricky since existing code patterns
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* dictate neither migrate_disable() nor migrate_enable() is allowed to block.
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* This means that it cannot use cpus_read_lock() to serialize against hotplug,
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* nor can it easily migrate itself into a pending affinity mask change on
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* migrate_enable().
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*
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*
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* Note: even non-work-conserving schedulers like semi-partitioned depends on
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* migration, so migrate_disable() is not only a problem for
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* work-conserving schedulers.
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*
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*/
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extern void migrate_disable(void);
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extern void migrate_enable(void);
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#else
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static inline void migrate_disable(void) { }
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static inline void migrate_enable(void) { }
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#endif /* CONFIG_SMP */
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#endif /* __LINUX_PREEMPT_H */
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