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 /*
102 * These macro definitions avoid redundant invocations of preempt_count()
103 * because such invocations would result in redundant loads given that
104 * preempt_count() is commonly implemented with READ_ONCE().
105 */
106
107 #define nmi_count() (preempt_count() & NMI_MASK)
108 #define hardirq_count() (preempt_count() & HARDIRQ_MASK)
109 #ifdef CONFIG_PREEMPT_RT
110 # define softirq_count() (current->softirq_disable_cnt & SOFTIRQ_MASK)
111 # define irq_count() ((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | softirq_count())
112 #else
113 # define softirq_count() (preempt_count() & SOFTIRQ_MASK)
114 # define irq_count() (preempt_count() & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_MASK))
115 #endif
116
117 /*
118 * Macros to retrieve the current execution context:
119 *
120 * in_nmi() - We're in NMI context
121 * in_hardirq() - We're in hard IRQ context
122 * in_serving_softirq() - We're in softirq context
123 * in_task() - We're in task context
124 */
125 #define in_nmi() (nmi_count())
126 #define in_hardirq() (hardirq_count())
127 #define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET)
128 #ifdef CONFIG_PREEMPT_RT
129 # define in_task() (!((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | in_serving_softirq()))
130 #else
131 # define in_task() (!(preempt_count() & (NMI_MASK | HARDIRQ_MASK | SOFTIRQ_OFFSET)))
132 #endif
133
134 /*
135 * The following macros are deprecated and should not be used in new code:
136 * in_irq() - Obsolete version of in_hardirq()
137 * in_softirq() - We have BH disabled, or are processing softirqs
138 * in_interrupt() - We're in NMI,IRQ,SoftIRQ context or have BH disabled
139 */
140 #define in_irq() (hardirq_count())
141 #define in_softirq() (softirq_count())
142 #define in_interrupt() (irq_count())
143
144 /*
145 * The preempt_count offset after preempt_disable();
146 */
147 #if defined(CONFIG_PREEMPT_COUNT)
148 # define PREEMPT_DISABLE_OFFSET PREEMPT_OFFSET
149 #else
150 # define PREEMPT_DISABLE_OFFSET 0
151 #endif
152
153 /*
154 * The preempt_count offset after spin_lock()
155 */
156 #if !defined(CONFIG_PREEMPT_RT)
157 #define PREEMPT_LOCK_OFFSET PREEMPT_DISABLE_OFFSET
158 #else
159 #define PREEMPT_LOCK_OFFSET 0
160 #endif
161
162 /*
163 * The preempt_count offset needed for things like:
164 *
165 * spin_lock_bh()
166 *
167 * Which need to disable both preemption (CONFIG_PREEMPT_COUNT) and
168 * softirqs, such that unlock sequences of:
169 *
170 * spin_unlock();
171 * local_bh_enable();
172 *
173 * Work as expected.
174 */
175 #define SOFTIRQ_LOCK_OFFSET (SOFTIRQ_DISABLE_OFFSET + PREEMPT_LOCK_OFFSET)
176
177 /*
178 * Are we running in atomic context? WARNING: this macro cannot
179 * always detect atomic context; in particular, it cannot know about
180 * held spinlocks in non-preemptible kernels. Thus it should not be
181 * used in the general case to determine whether sleeping is possible.
182 * Do not use in_atomic() in driver code.
183 */
184 #define in_atomic() (preempt_count() != 0)
185
186 /*
187 * Check whether we were atomic before we did preempt_disable():
188 * (used by the scheduler)
189 */
190 #define in_atomic_preempt_off() (preempt_count() != PREEMPT_DISABLE_OFFSET)
191
192 #if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_TRACE_PREEMPT_TOGGLE)
193 extern void preempt_count_add(int val);
194 extern void preempt_count_sub(int val);
195 #define preempt_count_dec_and_test() \
196 ({ preempt_count_sub(1); should_resched(0); })
197 #else
198 #define preempt_count_add(val) __preempt_count_add(val)
199 #define preempt_count_sub(val) __preempt_count_sub(val)
200 #define preempt_count_dec_and_test() __preempt_count_dec_and_test()
201 #endif
202
203 #define __preempt_count_inc() __preempt_count_add(1)
204 #define __preempt_count_dec() __preempt_count_sub(1)
205
206 #define preempt_count_inc() preempt_count_add(1)
207 #define preempt_count_dec() preempt_count_sub(1)
208
209 #ifdef CONFIG_PREEMPT_COUNT
210
211 #define preempt_disable() \
212 do { \
213 preempt_count_inc(); \
214 barrier(); \
215 } while (0)
216
217 #define sched_preempt_enable_no_resched() \
218 do { \
219 barrier(); \
220 preempt_count_dec(); \
221 } while (0)
222
223 #define preempt_enable_no_resched() sched_preempt_enable_no_resched()
224
225 #define preemptible() (preempt_count() == 0 && !irqs_disabled())
226
227 #ifdef CONFIG_PREEMPTION
228 #define preempt_enable() \
229 do { \
230 barrier(); \
231 if (unlikely(preempt_count_dec_and_test())) \
232 __preempt_schedule(); \
233 } while (0)
234
235 #define preempt_enable_notrace() \
236 do { \
237 barrier(); \
238 if (unlikely(__preempt_count_dec_and_test())) \
239 __preempt_schedule_notrace(); \
240 } while (0)
241
242 #define preempt_check_resched() \
243 do { \
244 if (should_resched(0)) \
245 __preempt_schedule(); \
246 } while (0)
247
248 #else /* !CONFIG_PREEMPTION */
249 #define preempt_enable() \
250 do { \
251 barrier(); \
252 preempt_count_dec(); \
253 } while (0)
254
255 #define preempt_enable_notrace() \
256 do { \
257 barrier(); \
258 __preempt_count_dec(); \
259 } while (0)
260
261 #define preempt_check_resched() do { } while (0)
262 #endif /* CONFIG_PREEMPTION */
263
264 #define preempt_disable_notrace() \
265 do { \
266 __preempt_count_inc(); \
267 barrier(); \
268 } while (0)
269
270 #define preempt_enable_no_resched_notrace() \
271 do { \
272 barrier(); \
273 __preempt_count_dec(); \
274 } while (0)
275
276 #else /* !CONFIG_PREEMPT_COUNT */
277
278 /*
279 * Even if we don't have any preemption, we need preempt disable/enable
280 * to be barriers, so that we don't have things like get_user/put_user
281 * that can cause faults and scheduling migrate into our preempt-protected
282 * region.
283 */
284 #define preempt_disable() barrier()
285 #define sched_preempt_enable_no_resched() barrier()
286 #define preempt_enable_no_resched() barrier()
287 #define preempt_enable() barrier()
288 #define preempt_check_resched() do { } while (0)
289
290 #define preempt_disable_notrace() barrier()
291 #define preempt_enable_no_resched_notrace() barrier()
292 #define preempt_enable_notrace() barrier()
293 #define preemptible() 0
294
295 #endif /* CONFIG_PREEMPT_COUNT */
296
297 #ifdef MODULE
298 /*
299 * Modules have no business playing preemption tricks.
300 */
301 #undef sched_preempt_enable_no_resched
302 #undef preempt_enable_no_resched
303 #undef preempt_enable_no_resched_notrace
304 #undef preempt_check_resched
305 #endif
306
307 #define preempt_set_need_resched() \
308 do { \
309 set_preempt_need_resched(); \
310 } while (0)
311 #define preempt_fold_need_resched() \
312 do { \
313 if (tif_need_resched()) \
314 set_preempt_need_resched(); \
315 } while (0)
316
317 #ifdef CONFIG_PREEMPT_NOTIFIERS
318
319 struct preempt_notifier;
320
321 /**
322 * preempt_ops - notifiers called when a task is preempted and rescheduled
323 * @sched_in: we're about to be rescheduled:
324 * notifier: struct preempt_notifier for the task being scheduled
325 * cpu: cpu we're scheduled on
326 * @sched_out: we've just been preempted
327 * notifier: struct preempt_notifier for the task being preempted
328 * next: the task that's kicking us out
329 *
330 * Please note that sched_in and out are called under different
331 * contexts. sched_out is called with rq lock held and irq disabled
332 * while sched_in is called without rq lock and irq enabled. This
333 * difference is intentional and depended upon by its users.
334 */
335 struct preempt_ops {
336 void (*sched_in)(struct preempt_notifier *notifier, int cpu);
337 void (*sched_out)(struct preempt_notifier *notifier,
338 struct task_struct *next);
339 };
340
341 /**
342 * preempt_notifier - key for installing preemption notifiers
343 * @link: internal use
344 * @ops: defines the notifier functions to be called
345 *
346 * Usually used in conjunction with container_of().
347 */
348 struct preempt_notifier {
349 struct hlist_node link;
350 struct preempt_ops *ops;
351 };
352
353 void preempt_notifier_inc(void);
354 void preempt_notifier_dec(void);
355 void preempt_notifier_register(struct preempt_notifier *notifier);
356 void preempt_notifier_unregister(struct preempt_notifier *notifier);
357
preempt_notifier_init(struct preempt_notifier * notifier,struct preempt_ops * ops)358 static inline void preempt_notifier_init(struct preempt_notifier *notifier,
359 struct preempt_ops *ops)
360 {
361 INIT_HLIST_NODE(¬ifier->link);
362 notifier->ops = ops;
363 }
364
365 #endif
366
367 #ifdef CONFIG_SMP
368
369 /*
370 * Migrate-Disable and why it is undesired.
371 *
372 * When a preempted task becomes elegible to run under the ideal model (IOW it
373 * becomes one of the M highest priority tasks), it might still have to wait
374 * for the preemptee's migrate_disable() section to complete. Thereby suffering
375 * a reduction in bandwidth in the exact duration of the migrate_disable()
376 * section.
377 *
378 * Per this argument, the change from preempt_disable() to migrate_disable()
379 * gets us:
380 *
381 * - a higher priority tasks gains reduced wake-up latency; with preempt_disable()
382 * it would have had to wait for the lower priority task.
383 *
384 * - a lower priority tasks; which under preempt_disable() could've instantly
385 * migrated away when another CPU becomes available, is now constrained
386 * by the ability to push the higher priority task away, which might itself be
387 * in a migrate_disable() section, reducing it's available bandwidth.
388 *
389 * IOW it trades latency / moves the interference term, but it stays in the
390 * system, and as long as it remains unbounded, the system is not fully
391 * deterministic.
392 *
393 *
394 * The reason we have it anyway.
395 *
396 * PREEMPT_RT breaks a number of assumptions traditionally held. By forcing a
397 * number of primitives into becoming preemptible, they would also allow
398 * migration. This turns out to break a bunch of per-cpu usage. To this end,
399 * all these primitives employ migirate_disable() to restore this implicit
400 * assumption.
401 *
402 * This is a 'temporary' work-around at best. The correct solution is getting
403 * rid of the above assumptions and reworking the code to employ explicit
404 * per-cpu locking or short preempt-disable regions.
405 *
406 * The end goal must be to get rid of migrate_disable(), alternatively we need
407 * a schedulability theory that does not depend on abritrary migration.
408 *
409 *
410 * Notes on the implementation.
411 *
412 * The implementation is particularly tricky since existing code patterns
413 * dictate neither migrate_disable() nor migrate_enable() is allowed to block.
414 * This means that it cannot use cpus_read_lock() to serialize against hotplug,
415 * nor can it easily migrate itself into a pending affinity mask change on
416 * migrate_enable().
417 *
418 *
419 * Note: even non-work-conserving schedulers like semi-partitioned depends on
420 * migration, so migrate_disable() is not only a problem for
421 * work-conserving schedulers.
422 *
423 */
424 extern void migrate_disable(void);
425 extern void migrate_enable(void);
426
427 #else
428
migrate_disable(void)429 static inline void migrate_disable(void) { }
migrate_enable(void)430 static inline void migrate_enable(void) { }
431
432 #endif /* CONFIG_SMP */
433
434 #endif /* __LINUX_PREEMPT_H */
435