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1/* SPDX-License-Identifier: GPL-2.0 */
2#ifndef __LINUX_PREEMPT_H
3#define __LINUX_PREEMPT_H
4
5/*
6 * include/linux/preempt.h - macros for accessing and manipulating
7 * preempt_count (used for kernel preemption, interrupt count, etc.)
8 */
9
10#include <linux/linkage.h>
11#include <linux/list.h>
12
13/*
14 * We put the hardirq and softirq counter into the preemption
15 * counter. The bitmask has the following meaning:
16 *
17 * - bits 0-7 are the preemption count (max preemption depth: 256)
18 * - bits 8-15 are the softirq count (max # of softirqs: 256)
19 *
20 * The hardirq count could in theory be the same as the number of
21 * interrupts in the system, but we run all interrupt handlers with
22 * interrupts disabled, so we cannot have nesting interrupts. Though
23 * there are a few palaeontologic drivers which reenable interrupts in
24 * the handler, so we need more than one bit here.
25 *
26 * PREEMPT_MASK: 0x000000ff
27 * SOFTIRQ_MASK: 0x0000ff00
28 * HARDIRQ_MASK: 0x000f0000
29 * NMI_MASK: 0x00f00000
30 * PREEMPT_NEED_RESCHED: 0x80000000
31 */
32#define PREEMPT_BITS 8
33#define SOFTIRQ_BITS 8
34#define HARDIRQ_BITS 4
35#define NMI_BITS 4
36
37#define PREEMPT_SHIFT 0
38#define SOFTIRQ_SHIFT (PREEMPT_SHIFT + PREEMPT_BITS)
39#define HARDIRQ_SHIFT (SOFTIRQ_SHIFT + SOFTIRQ_BITS)
40#define NMI_SHIFT (HARDIRQ_SHIFT + HARDIRQ_BITS)
41
42#define __IRQ_MASK(x) ((1UL << (x))-1)
43
44#define PREEMPT_MASK (__IRQ_MASK(PREEMPT_BITS) << PREEMPT_SHIFT)
45#define SOFTIRQ_MASK (__IRQ_MASK(SOFTIRQ_BITS) << SOFTIRQ_SHIFT)
46#define HARDIRQ_MASK (__IRQ_MASK(HARDIRQ_BITS) << HARDIRQ_SHIFT)
47#define NMI_MASK (__IRQ_MASK(NMI_BITS) << NMI_SHIFT)
48
49#define PREEMPT_OFFSET (1UL << PREEMPT_SHIFT)
50#define SOFTIRQ_OFFSET (1UL << SOFTIRQ_SHIFT)
51#define HARDIRQ_OFFSET (1UL << HARDIRQ_SHIFT)
52#define NMI_OFFSET (1UL << NMI_SHIFT)
53
54#define SOFTIRQ_DISABLE_OFFSET (2 * SOFTIRQ_OFFSET)
55
56#define PREEMPT_DISABLED (PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
57
58/*
59 * Disable preemption until the scheduler is running -- use an unconditional
60 * value so that it also works on !PREEMPT_COUNT kernels.
61 *
62 * Reset by start_kernel()->sched_init()->init_idle()->init_idle_preempt_count().
63 */
64#define INIT_PREEMPT_COUNT PREEMPT_OFFSET
65
66/*
67 * Initial preempt_count value; reflects the preempt_count schedule invariant
68 * which states that during context switches:
69 *
70 * preempt_count() == 2*PREEMPT_DISABLE_OFFSET
71 *
72 * Note: PREEMPT_DISABLE_OFFSET is 0 for !PREEMPT_COUNT kernels.
73 * Note: See finish_task_switch().
74 */
75#define FORK_PREEMPT_COUNT (2*PREEMPT_DISABLE_OFFSET + PREEMPT_ENABLED)
76
77/* preempt_count() and related functions, depends on PREEMPT_NEED_RESCHED */
78#include <asm/preempt.h>
79
80#define nmi_count() (preempt_count() & NMI_MASK)
81#define hardirq_count() (preempt_count() & HARDIRQ_MASK)
82#define softirq_count() (preempt_count() & SOFTIRQ_MASK)
83#define irq_count() (nmi_count() | hardirq_count() | softirq_count())
84
85/*
86 * Macros to retrieve the current execution context:
87 *
88 * in_nmi() - We're in NMI context
89 * in_hardirq() - We're in hard IRQ context
90 * in_serving_softirq() - We're in softirq context
91 * in_task() - We're in task context
92 */
93#define in_nmi() (nmi_count())
94#define in_hardirq() (hardirq_count())
95#define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET)
96#define in_task() (!(in_nmi() | in_hardirq() | in_serving_softirq()))
97
98/*
99 * The following macros are deprecated and should not be used in new code:
100 * in_irq() - Obsolete version of in_hardirq()
101 * in_softirq() - We have BH disabled, or are processing softirqs
102 * in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled
103 */
104#define in_irq() (hardirq_count())
105#define in_softirq() (softirq_count())
106#define in_interrupt() (irq_count())
107
108/*
109 * The preempt_count offset after preempt_disable();
110 */
111#if defined(CONFIG_PREEMPT_COUNT)
112# define PREEMPT_DISABLE_OFFSET PREEMPT_OFFSET
113#else
114# define PREEMPT_DISABLE_OFFSET 0
115#endif
116
117/*
118 * The preempt_count offset after spin_lock()
119 */
120#define PREEMPT_LOCK_OFFSET PREEMPT_DISABLE_OFFSET
121
122/*
123 * The preempt_count offset needed for things like:
124 *
125 * spin_lock_bh()
126 *
127 * Which need to disable both preemption (CONFIG_PREEMPT_COUNT) and
128 * softirqs, such that unlock sequences of:
129 *
130 * spin_unlock();
131 * local_bh_enable();
132 *
133 * Work as expected.
134 */
135#define SOFTIRQ_LOCK_OFFSET (SOFTIRQ_DISABLE_OFFSET + PREEMPT_LOCK_OFFSET)
136
137/*
138 * Are we running in atomic context? WARNING: this macro cannot
139 * always detect atomic context; in particular, it cannot know about
140 * held spinlocks in non-preemptible kernels. Thus it should not be
141 * used in the general case to determine whether sleeping is possible.
142 * Do not use in_atomic() in driver code.
143 */
144#define in_atomic() (preempt_count() != 0)
145
146/*
147 * Check whether we were atomic before we did preempt_disable():
148 * (used by the scheduler)
149 */
150#define in_atomic_preempt_off() (preempt_count() != PREEMPT_DISABLE_OFFSET)
151
152#if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_TRACE_PREEMPT_TOGGLE)
153extern void preempt_count_add(int val);
154extern void preempt_count_sub(int val);
155#define preempt_count_dec_and_test() \
156 ({ preempt_count_sub(1); should_resched(0); })
157#else
158#define preempt_count_add(val) __preempt_count_add(val)
159#define preempt_count_sub(val) __preempt_count_sub(val)
160#define preempt_count_dec_and_test() __preempt_count_dec_and_test()
161#endif
162
163#define __preempt_count_inc() __preempt_count_add(1)
164#define __preempt_count_dec() __preempt_count_sub(1)
165
166#define preempt_count_inc() preempt_count_add(1)
167#define preempt_count_dec() preempt_count_sub(1)
168
169#ifdef CONFIG_PREEMPT_COUNT
170
171#define preempt_disable() \
172do { \
173 preempt_count_inc(); \
174 barrier(); \
175} while (0)
176
177#define sched_preempt_enable_no_resched() \
178do { \
179 barrier(); \
180 preempt_count_dec(); \
181} while (0)
182
183#define preempt_enable_no_resched() sched_preempt_enable_no_resched()
184
185#define preemptible() (preempt_count() == 0 && !irqs_disabled())
186
187#ifdef CONFIG_PREEMPTION
188#define preempt_enable() \
189do { \
190 barrier(); \
191 if (unlikely(preempt_count_dec_and_test())) \
192 __preempt_schedule(); \
193} while (0)
194
195#define preempt_enable_notrace() \
196do { \
197 barrier(); \
198 if (unlikely(__preempt_count_dec_and_test())) \
199 __preempt_schedule_notrace(); \
200} while (0)
201
202#define preempt_check_resched() \
203do { \
204 if (should_resched(0)) \
205 __preempt_schedule(); \
206} while (0)
207
208#else /* !CONFIG_PREEMPTION */
209#define preempt_enable() \
210do { \
211 barrier(); \
212 preempt_count_dec(); \
213} while (0)
214
215#define preempt_enable_notrace() \
216do { \
217 barrier(); \
218 __preempt_count_dec(); \
219} while (0)
220
221#define preempt_check_resched() do { } while (0)
222#endif /* CONFIG_PREEMPTION */
223
224#define preempt_disable_notrace() \
225do { \
226 __preempt_count_inc(); \
227 barrier(); \
228} while (0)
229
230#define preempt_enable_no_resched_notrace() \
231do { \
232 barrier(); \
233 __preempt_count_dec(); \
234} while (0)
235
236#else /* !CONFIG_PREEMPT_COUNT */
237
238/*
239 * Even if we don't have any preemption, we need preempt disable/enable
240 * to be barriers, so that we don't have things like get_user/put_user
241 * that can cause faults and scheduling migrate into our preempt-protected
242 * region.
243 */
244#define preempt_disable() barrier()
245#define sched_preempt_enable_no_resched() barrier()
246#define preempt_enable_no_resched() barrier()
247#define preempt_enable() barrier()
248#define preempt_check_resched() do { } while (0)
249
250#define preempt_disable_notrace() barrier()
251#define preempt_enable_no_resched_notrace() barrier()
252#define preempt_enable_notrace() barrier()
253#define preemptible() 0
254
255#endif /* CONFIG_PREEMPT_COUNT */
256
257#ifdef MODULE
258/*
259 * Modules have no business playing preemption tricks.
260 */
261#undef sched_preempt_enable_no_resched
262#undef preempt_enable_no_resched
263#undef preempt_enable_no_resched_notrace
264#undef preempt_check_resched
265#endif
266
267#define preempt_set_need_resched() \
268do { \
269 set_preempt_need_resched(); \
270} while (0)
271#define preempt_fold_need_resched() \
272do { \
273 if (tif_need_resched()) \
274 set_preempt_need_resched(); \
275} while (0)
276
277#ifdef CONFIG_PREEMPT_NOTIFIERS
278
279struct preempt_notifier;
280
281/**
282 * preempt_ops - notifiers called when a task is preempted and rescheduled
283 * @sched_in: we're about to be rescheduled:
284 * notifier: struct preempt_notifier for the task being scheduled
285 * cpu: cpu we're scheduled on
286 * @sched_out: we've just been preempted
287 * notifier: struct preempt_notifier for the task being preempted
288 * next: the task that's kicking us out
289 *
290 * Please note that sched_in and out are called under different
291 * contexts. sched_out is called with rq lock held and irq disabled
292 * while sched_in is called without rq lock and irq enabled. This
293 * difference is intentional and depended upon by its users.
294 */
295struct preempt_ops {
296 void (*sched_in)(struct preempt_notifier *notifier, int cpu);
297 void (*sched_out)(struct preempt_notifier *notifier,
298 struct task_struct *next);
299};
300
301/**
302 * preempt_notifier - key for installing preemption notifiers
303 * @link: internal use
304 * @ops: defines the notifier functions to be called
305 *
306 * Usually used in conjunction with container_of().
307 */
308struct preempt_notifier {
309 struct hlist_node link;
310 struct preempt_ops *ops;
311};
312
313void preempt_notifier_inc(void);
314void preempt_notifier_dec(void);
315void preempt_notifier_register(struct preempt_notifier *notifier);
316void preempt_notifier_unregister(struct preempt_notifier *notifier);
317
318static inline void preempt_notifier_init(struct preempt_notifier *notifier,
319 struct preempt_ops *ops)
320{
321 INIT_HLIST_NODE(¬ifier->link);
322 notifier->ops = ops;
323}
324
325#endif
326
327#ifdef CONFIG_SMP
328
329/*
330 * Migrate-Disable and why it is undesired.
331 *
332 * When a preempted task becomes elegible to run under the ideal model (IOW it
333 * becomes one of the M highest priority tasks), it might still have to wait
334 * for the preemptee's migrate_disable() section to complete. Thereby suffering
335 * a reduction in bandwidth in the exact duration of the migrate_disable()
336 * section.
337 *
338 * Per this argument, the change from preempt_disable() to migrate_disable()
339 * gets us:
340 *
341 * - a higher priority tasks gains reduced wake-up latency; with preempt_disable()
342 * it would have had to wait for the lower priority task.
343 *
344 * - a lower priority tasks; which under preempt_disable() could've instantly
345 * migrated away when another CPU becomes available, is now constrained
346 * by the ability to push the higher priority task away, which might itself be
347 * in a migrate_disable() section, reducing it's available bandwidth.
348 *
349 * IOW it trades latency / moves the interference term, but it stays in the
350 * system, and as long as it remains unbounded, the system is not fully
351 * deterministic.
352 *
353 *
354 * The reason we have it anyway.
355 *
356 * PREEMPT_RT breaks a number of assumptions traditionally held. By forcing a
357 * number of primitives into becoming preemptible, they would also allow
358 * migration. This turns out to break a bunch of per-cpu usage. To this end,
359 * all these primitives employ migirate_disable() to restore this implicit
360 * assumption.
361 *
362 * This is a 'temporary' work-around at best. The correct solution is getting
363 * rid of the above assumptions and reworking the code to employ explicit
364 * per-cpu locking or short preempt-disable regions.
365 *
366 * The end goal must be to get rid of migrate_disable(), alternatively we need
367 * a schedulability theory that does not depend on abritrary migration.
368 *
369 *
370 * Notes on the implementation.
371 *
372 * The implementation is particularly tricky since existing code patterns
373 * dictate neither migrate_disable() nor migrate_enable() is allowed to block.
374 * This means that it cannot use cpus_read_lock() to serialize against hotplug,
375 * nor can it easily migrate itself into a pending affinity mask change on
376 * migrate_enable().
377 *
378 *
379 * Note: even non-work-conserving schedulers like semi-partitioned depends on
380 * migration, so migrate_disable() is not only a problem for
381 * work-conserving schedulers.
382 *
383 */
384extern void migrate_disable(void);
385extern void migrate_enable(void);
386
387#else
388
389static inline void migrate_disable(void) { }
390static inline void migrate_enable(void) { }
391
392#endif /* CONFIG_SMP */
393
394#endif /* __LINUX_PREEMPT_H */