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) 161 extern void preempt_count_add(int val); 162 extern 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() \ 180 do { \ 181 preempt_count_inc(); \ 182 barrier(); \ 183 } while (0) 184 185 #define sched_preempt_enable_no_resched() \ 186 do { \ 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() \ 197 do { \ 198 barrier(); \ 199 if (unlikely(preempt_count_dec_and_test())) \ 200 __preempt_schedule(); \ 201 } while (0) 202 203 #define preempt_enable_notrace() \ 204 do { \ 205 barrier(); \ 206 if (unlikely(__preempt_count_dec_and_test())) \ 207 __preempt_schedule_notrace(); \ 208 } while (0) 209 210 #define preempt_check_resched() \ 211 do { \ 212 if (should_resched(0)) \ 213 __preempt_schedule(); \ 214 } while (0) 215 216 #else /* !CONFIG_PREEMPTION */ 217 #define preempt_enable() \ 218 do { \ 219 barrier(); \ 220 preempt_count_dec(); \ 221 } while (0) 222 223 #define preempt_enable_notrace() \ 224 do { \ 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() \ 233 do { \ 234 __preempt_count_inc(); \ 235 barrier(); \ 236 } while (0) 237 238 #define preempt_enable_no_resched_notrace() \ 239 do { \ 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() \ 276 do { \ 277 set_preempt_need_resched(); \ 278 } while (0) 279 #define preempt_fold_need_resched() \ 280 do { \ 281 if (tif_need_resched()) \ 282 set_preempt_need_resched(); \ 283 } while (0) 284 285 #ifdef CONFIG_PREEMPT_NOTIFIERS 286 287 struct 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 */ 303 struct 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 */ 316 struct preempt_notifier { 317 struct hlist_node link; 318 struct preempt_ops *ops; 319 }; 320 321 void preempt_notifier_inc(void); 322 void preempt_notifier_dec(void); 323 void preempt_notifier_register(struct preempt_notifier *notifier); 324 void preempt_notifier_unregister(struct preempt_notifier *notifier); 325 326 static 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 */ 392 extern void migrate_disable(void); 393 extern void migrate_enable(void); 394 395 #else 396 397 static inline void migrate_disable(void) { } 398 static inline void migrate_enable(void) { } 399 400 #endif /* CONFIG_SMP */ 401 402 #endif /* __LINUX_PREEMPT_H */ 403