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 /**
81  * interrupt_context_level - return interrupt context level
82  *
83  * Returns the current interrupt context level.
84  *  0 - normal context
85  *  1 - softirq context
86  *  2 - hardirq context
87  *  3 - NMI context
88  */
interrupt_context_level(void)89 static __always_inline unsigned char interrupt_context_level(void)
90 {
91 	unsigned long pc = preempt_count();
92 	unsigned char level = 0;
93 
94 	level += !!(pc & (NMI_MASK));
95 	level += !!(pc & (NMI_MASK | HARDIRQ_MASK));
96 	level += !!(pc & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET));
97 
98 	return level;
99 }
100 
101 #define nmi_count()	(preempt_count() & NMI_MASK)
102 #define hardirq_count()	(preempt_count() & HARDIRQ_MASK)
103 #ifdef CONFIG_PREEMPT_RT
104 # define softirq_count()	(current->softirq_disable_cnt & SOFTIRQ_MASK)
105 #else
106 # define softirq_count()	(preempt_count() & SOFTIRQ_MASK)
107 #endif
108 #define irq_count()	(nmi_count() | hardirq_count() | softirq_count())
109 
110 /*
111  * Macros to retrieve the current execution context:
112  *
113  * in_nmi()		- We're in NMI context
114  * in_hardirq()		- We're in hard IRQ context
115  * in_serving_softirq()	- We're in softirq context
116  * in_task()		- We're in task context
117  */
118 #define in_nmi()		(nmi_count())
119 #define in_hardirq()		(hardirq_count())
120 #define in_serving_softirq()	(softirq_count() & SOFTIRQ_OFFSET)
121 #define in_task()		(!(in_nmi() | in_hardirq() | in_serving_softirq()))
122 
123 /*
124  * The following macros are deprecated and should not be used in new code:
125  * in_irq()       - Obsolete version of in_hardirq()
126  * in_softirq()   - We have BH disabled, or are processing softirqs
127  * in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled
128  */
129 #define in_irq()		(hardirq_count())
130 #define in_softirq()		(softirq_count())
131 #define in_interrupt()		(irq_count())
132 
133 /*
134  * The preempt_count offset after preempt_disable();
135  */
136 #if defined(CONFIG_PREEMPT_COUNT)
137 # define PREEMPT_DISABLE_OFFSET	PREEMPT_OFFSET
138 #else
139 # define PREEMPT_DISABLE_OFFSET	0
140 #endif
141 
142 /*
143  * The preempt_count offset after spin_lock()
144  */
145 #if !defined(CONFIG_PREEMPT_RT)
146 #define PREEMPT_LOCK_OFFSET		PREEMPT_DISABLE_OFFSET
147 #else
148 /* Locks on RT do not disable preemption */
149 #define PREEMPT_LOCK_OFFSET		0
150 #endif
151 
152 /*
153  * The preempt_count offset needed for things like:
154  *
155  *  spin_lock_bh()
156  *
157  * Which need to disable both preemption (CONFIG_PREEMPT_COUNT) and
158  * softirqs, such that unlock sequences of:
159  *
160  *  spin_unlock();
161  *  local_bh_enable();
162  *
163  * Work as expected.
164  */
165 #define SOFTIRQ_LOCK_OFFSET (SOFTIRQ_DISABLE_OFFSET + PREEMPT_LOCK_OFFSET)
166 
167 /*
168  * Are we running in atomic context?  WARNING: this macro cannot
169  * always detect atomic context; in particular, it cannot know about
170  * held spinlocks in non-preemptible kernels.  Thus it should not be
171  * used in the general case to determine whether sleeping is possible.
172  * Do not use in_atomic() in driver code.
173  */
174 #define in_atomic()	(preempt_count() != 0)
175 
176 /*
177  * Check whether we were atomic before we did preempt_disable():
178  * (used by the scheduler)
179  */
180 #define in_atomic_preempt_off() (preempt_count() != PREEMPT_DISABLE_OFFSET)
181 
182 #if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_TRACE_PREEMPT_TOGGLE)
183 extern void preempt_count_add(int val);
184 extern void preempt_count_sub(int val);
185 #define preempt_count_dec_and_test() \
186 	({ preempt_count_sub(1); should_resched(0); })
187 #else
188 #define preempt_count_add(val)	__preempt_count_add(val)
189 #define preempt_count_sub(val)	__preempt_count_sub(val)
190 #define preempt_count_dec_and_test() __preempt_count_dec_and_test()
191 #endif
192 
193 #define __preempt_count_inc() __preempt_count_add(1)
194 #define __preempt_count_dec() __preempt_count_sub(1)
195 
196 #define preempt_count_inc() preempt_count_add(1)
197 #define preempt_count_dec() preempt_count_sub(1)
198 
199 #ifdef CONFIG_PREEMPT_COUNT
200 
201 #define preempt_disable() \
202 do { \
203 	preempt_count_inc(); \
204 	barrier(); \
205 } while (0)
206 
207 #define sched_preempt_enable_no_resched() \
208 do { \
209 	barrier(); \
210 	preempt_count_dec(); \
211 } while (0)
212 
213 #define preempt_enable_no_resched() sched_preempt_enable_no_resched()
214 
215 #define preemptible()	(preempt_count() == 0 && !irqs_disabled())
216 
217 #ifdef CONFIG_PREEMPTION
218 #define preempt_enable() \
219 do { \
220 	barrier(); \
221 	if (unlikely(preempt_count_dec_and_test())) \
222 		__preempt_schedule(); \
223 } while (0)
224 
225 #define preempt_enable_notrace() \
226 do { \
227 	barrier(); \
228 	if (unlikely(__preempt_count_dec_and_test())) \
229 		__preempt_schedule_notrace(); \
230 } while (0)
231 
232 #define preempt_check_resched() \
233 do { \
234 	if (should_resched(0)) \
235 		__preempt_schedule(); \
236 } while (0)
237 
238 #else /* !CONFIG_PREEMPTION */
239 #define preempt_enable() \
240 do { \
241 	barrier(); \
242 	preempt_count_dec(); \
243 } while (0)
244 
245 #define preempt_enable_notrace() \
246 do { \
247 	barrier(); \
248 	__preempt_count_dec(); \
249 } while (0)
250 
251 #define preempt_check_resched() do { } while (0)
252 #endif /* CONFIG_PREEMPTION */
253 
254 #define preempt_disable_notrace() \
255 do { \
256 	__preempt_count_inc(); \
257 	barrier(); \
258 } while (0)
259 
260 #define preempt_enable_no_resched_notrace() \
261 do { \
262 	barrier(); \
263 	__preempt_count_dec(); \
264 } while (0)
265 
266 #else /* !CONFIG_PREEMPT_COUNT */
267 
268 /*
269  * Even if we don't have any preemption, we need preempt disable/enable
270  * to be barriers, so that we don't have things like get_user/put_user
271  * that can cause faults and scheduling migrate into our preempt-protected
272  * region.
273  */
274 #define preempt_disable()			barrier()
275 #define sched_preempt_enable_no_resched()	barrier()
276 #define preempt_enable_no_resched()		barrier()
277 #define preempt_enable()			barrier()
278 #define preempt_check_resched()			do { } while (0)
279 
280 #define preempt_disable_notrace()		barrier()
281 #define preempt_enable_no_resched_notrace()	barrier()
282 #define preempt_enable_notrace()		barrier()
283 #define preemptible()				0
284 
285 #endif /* CONFIG_PREEMPT_COUNT */
286 
287 #ifdef MODULE
288 /*
289  * Modules have no business playing preemption tricks.
290  */
291 #undef sched_preempt_enable_no_resched
292 #undef preempt_enable_no_resched
293 #undef preempt_enable_no_resched_notrace
294 #undef preempt_check_resched
295 #endif
296 
297 #define preempt_set_need_resched() \
298 do { \
299 	set_preempt_need_resched(); \
300 } while (0)
301 #define preempt_fold_need_resched() \
302 do { \
303 	if (tif_need_resched()) \
304 		set_preempt_need_resched(); \
305 } while (0)
306 
307 #ifdef CONFIG_PREEMPT_NOTIFIERS
308 
309 struct preempt_notifier;
310 
311 /**
312  * preempt_ops - notifiers called when a task is preempted and rescheduled
313  * @sched_in: we're about to be rescheduled:
314  *    notifier: struct preempt_notifier for the task being scheduled
315  *    cpu:  cpu we're scheduled on
316  * @sched_out: we've just been preempted
317  *    notifier: struct preempt_notifier for the task being preempted
318  *    next: the task that's kicking us out
319  *
320  * Please note that sched_in and out are called under different
321  * contexts.  sched_out is called with rq lock held and irq disabled
322  * while sched_in is called without rq lock and irq enabled.  This
323  * difference is intentional and depended upon by its users.
324  */
325 struct preempt_ops {
326 	void (*sched_in)(struct preempt_notifier *notifier, int cpu);
327 	void (*sched_out)(struct preempt_notifier *notifier,
328 			  struct task_struct *next);
329 };
330 
331 /**
332  * preempt_notifier - key for installing preemption notifiers
333  * @link: internal use
334  * @ops: defines the notifier functions to be called
335  *
336  * Usually used in conjunction with container_of().
337  */
338 struct preempt_notifier {
339 	struct hlist_node link;
340 	struct preempt_ops *ops;
341 };
342 
343 void preempt_notifier_inc(void);
344 void preempt_notifier_dec(void);
345 void preempt_notifier_register(struct preempt_notifier *notifier);
346 void preempt_notifier_unregister(struct preempt_notifier *notifier);
347 
preempt_notifier_init(struct preempt_notifier * notifier,struct preempt_ops * ops)348 static inline void preempt_notifier_init(struct preempt_notifier *notifier,
349 				     struct preempt_ops *ops)
350 {
351 	INIT_HLIST_NODE(&notifier->link);
352 	notifier->ops = ops;
353 }
354 
355 #endif
356 
357 #ifdef CONFIG_SMP
358 
359 /*
360  * Migrate-Disable and why it is undesired.
361  *
362  * When a preempted task becomes elegible to run under the ideal model (IOW it
363  * becomes one of the M highest priority tasks), it might still have to wait
364  * for the preemptee's migrate_disable() section to complete. Thereby suffering
365  * a reduction in bandwidth in the exact duration of the migrate_disable()
366  * section.
367  *
368  * Per this argument, the change from preempt_disable() to migrate_disable()
369  * gets us:
370  *
371  * - a higher priority tasks gains reduced wake-up latency; with preempt_disable()
372  *   it would have had to wait for the lower priority task.
373  *
374  * - a lower priority tasks; which under preempt_disable() could've instantly
375  *   migrated away when another CPU becomes available, is now constrained
376  *   by the ability to push the higher priority task away, which might itself be
377  *   in a migrate_disable() section, reducing it's available bandwidth.
378  *
379  * IOW it trades latency / moves the interference term, but it stays in the
380  * system, and as long as it remains unbounded, the system is not fully
381  * deterministic.
382  *
383  *
384  * The reason we have it anyway.
385  *
386  * PREEMPT_RT breaks a number of assumptions traditionally held. By forcing a
387  * number of primitives into becoming preemptible, they would also allow
388  * migration. This turns out to break a bunch of per-cpu usage. To this end,
389  * all these primitives employ migirate_disable() to restore this implicit
390  * assumption.
391  *
392  * This is a 'temporary' work-around at best. The correct solution is getting
393  * rid of the above assumptions and reworking the code to employ explicit
394  * per-cpu locking or short preempt-disable regions.
395  *
396  * The end goal must be to get rid of migrate_disable(), alternatively we need
397  * a schedulability theory that does not depend on abritrary migration.
398  *
399  *
400  * Notes on the implementation.
401  *
402  * The implementation is particularly tricky since existing code patterns
403  * dictate neither migrate_disable() nor migrate_enable() is allowed to block.
404  * This means that it cannot use cpus_read_lock() to serialize against hotplug,
405  * nor can it easily migrate itself into a pending affinity mask change on
406  * migrate_enable().
407  *
408  *
409  * Note: even non-work-conserving schedulers like semi-partitioned depends on
410  *       migration, so migrate_disable() is not only a problem for
411  *       work-conserving schedulers.
412  *
413  */
414 extern void migrate_disable(void);
415 extern void migrate_enable(void);
416 
417 #else
418 
migrate_disable(void)419 static inline void migrate_disable(void) { }
migrate_enable(void)420 static inline void migrate_enable(void) { }
421 
422 #endif /* CONFIG_SMP */
423 
424 #endif /* __LINUX_PREEMPT_H */
425