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