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