403 lines
12 KiB
C
403 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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#if !defined(CONFIG_PREEMPT_RT)
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#define PREEMPT_LOCK_OFFSET PREEMPT_DISABLE_OFFSET
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#else
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#define PREEMPT_LOCK_OFFSET 0
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#endif
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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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