1 // SPDX-License-Identifier: GPL-2.0-only 2 /* 3 * kernel/sched/cpupri.c 4 * 5 * CPU priority management 6 * 7 * Copyright (C) 2007-2008 Novell 8 * 9 * Author: Gregory Haskins <ghaskins@novell.com> 10 * 11 * This code tracks the priority of each CPU so that global migration 12 * decisions are easy to calculate. Each CPU can be in a state as follows: 13 * 14 * (INVALID), NORMAL, RT1, ... RT99, HIGHER 15 * 16 * going from the lowest priority to the highest. CPUs in the INVALID state 17 * are not eligible for routing. The system maintains this state with 18 * a 2 dimensional bitmap (the first for priority class, the second for CPUs 19 * in that class). Therefore a typical application without affinity 20 * restrictions can find a suitable CPU with O(1) complexity (e.g. two bit 21 * searches). For tasks with affinity restrictions, the algorithm has a 22 * worst case complexity of O(min(101, nr_domcpus)), though the scenario that 23 * yields the worst case search is fairly contrived. 24 */ 25 #include "sched.h" 26 27 /* 28 * p->rt_priority p->prio newpri cpupri 29 * 30 * -1 -1 (CPUPRI_INVALID) 31 * 32 * 99 0 (CPUPRI_NORMAL) 33 * 34 * 1 98 98 1 35 * ... 36 * 49 50 50 49 37 * 50 49 49 50 38 * ... 39 * 99 0 0 99 40 * 41 * 100 100 (CPUPRI_HIGHER) 42 */ 43 static int convert_prio(int prio) 44 { 45 int cpupri; 46 47 switch (prio) { 48 case CPUPRI_INVALID: 49 cpupri = CPUPRI_INVALID; /* -1 */ 50 break; 51 52 case 0 ... 98: 53 cpupri = MAX_RT_PRIO-1 - prio; /* 1 ... 99 */ 54 break; 55 56 case MAX_RT_PRIO-1: 57 cpupri = CPUPRI_NORMAL; /* 0 */ 58 break; 59 60 case MAX_RT_PRIO: 61 cpupri = CPUPRI_HIGHER; /* 100 */ 62 break; 63 } 64 65 return cpupri; 66 } 67 68 static inline int __cpupri_find(struct cpupri *cp, struct task_struct *p, 69 struct cpumask *lowest_mask, int idx) 70 { 71 struct cpupri_vec *vec = &cp->pri_to_cpu[idx]; 72 int skip = 0; 73 74 if (!atomic_read(&(vec)->count)) 75 skip = 1; 76 /* 77 * When looking at the vector, we need to read the counter, 78 * do a memory barrier, then read the mask. 79 * 80 * Note: This is still all racy, but we can deal with it. 81 * Ideally, we only want to look at masks that are set. 82 * 83 * If a mask is not set, then the only thing wrong is that we 84 * did a little more work than necessary. 85 * 86 * If we read a zero count but the mask is set, because of the 87 * memory barriers, that can only happen when the highest prio 88 * task for a run queue has left the run queue, in which case, 89 * it will be followed by a pull. If the task we are processing 90 * fails to find a proper place to go, that pull request will 91 * pull this task if the run queue is running at a lower 92 * priority. 93 */ 94 smp_rmb(); 95 96 /* Need to do the rmb for every iteration */ 97 if (skip) 98 return 0; 99 100 if (cpumask_any_and(&p->cpus_mask, vec->mask) >= nr_cpu_ids) 101 return 0; 102 103 if (lowest_mask) { 104 cpumask_and(lowest_mask, &p->cpus_mask, vec->mask); 105 cpumask_and(lowest_mask, lowest_mask, cpu_active_mask); 106 107 /* 108 * We have to ensure that we have at least one bit 109 * still set in the array, since the map could have 110 * been concurrently emptied between the first and 111 * second reads of vec->mask. If we hit this 112 * condition, simply act as though we never hit this 113 * priority level and continue on. 114 */ 115 if (cpumask_empty(lowest_mask)) 116 return 0; 117 } 118 119 return 1; 120 } 121 122 int cpupri_find(struct cpupri *cp, struct task_struct *p, 123 struct cpumask *lowest_mask) 124 { 125 return cpupri_find_fitness(cp, p, lowest_mask, NULL); 126 } 127 128 /** 129 * cpupri_find_fitness - find the best (lowest-pri) CPU in the system 130 * @cp: The cpupri context 131 * @p: The task 132 * @lowest_mask: A mask to fill in with selected CPUs (or NULL) 133 * @fitness_fn: A pointer to a function to do custom checks whether the CPU 134 * fits a specific criteria so that we only return those CPUs. 135 * 136 * Note: This function returns the recommended CPUs as calculated during the 137 * current invocation. By the time the call returns, the CPUs may have in 138 * fact changed priorities any number of times. While not ideal, it is not 139 * an issue of correctness since the normal rebalancer logic will correct 140 * any discrepancies created by racing against the uncertainty of the current 141 * priority configuration. 142 * 143 * Return: (int)bool - CPUs were found 144 */ 145 int cpupri_find_fitness(struct cpupri *cp, struct task_struct *p, 146 struct cpumask *lowest_mask, 147 bool (*fitness_fn)(struct task_struct *p, int cpu)) 148 { 149 int task_pri = convert_prio(p->prio); 150 int idx, cpu; 151 152 WARN_ON_ONCE(task_pri >= CPUPRI_NR_PRIORITIES); 153 154 for (idx = 0; idx < task_pri; idx++) { 155 156 if (!__cpupri_find(cp, p, lowest_mask, idx)) 157 continue; 158 159 if (!lowest_mask || !fitness_fn) 160 return 1; 161 162 /* Ensure the capacity of the CPUs fit the task */ 163 for_each_cpu(cpu, lowest_mask) { 164 if (!fitness_fn(p, cpu)) 165 cpumask_clear_cpu(cpu, lowest_mask); 166 } 167 168 /* 169 * If no CPU at the current priority can fit the task 170 * continue looking 171 */ 172 if (cpumask_empty(lowest_mask)) 173 continue; 174 175 return 1; 176 } 177 178 /* 179 * If we failed to find a fitting lowest_mask, kick off a new search 180 * but without taking into account any fitness criteria this time. 181 * 182 * This rule favours honouring priority over fitting the task in the 183 * correct CPU (Capacity Awareness being the only user now). 184 * The idea is that if a higher priority task can run, then it should 185 * run even if this ends up being on unfitting CPU. 186 * 187 * The cost of this trade-off is not entirely clear and will probably 188 * be good for some workloads and bad for others. 189 * 190 * The main idea here is that if some CPUs were over-committed, we try 191 * to spread which is what the scheduler traditionally did. Sys admins 192 * must do proper RT planning to avoid overloading the system if they 193 * really care. 194 */ 195 if (fitness_fn) 196 return cpupri_find(cp, p, lowest_mask); 197 198 return 0; 199 } 200 201 /** 202 * cpupri_set - update the CPU priority setting 203 * @cp: The cpupri context 204 * @cpu: The target CPU 205 * @newpri: The priority (INVALID,NORMAL,RT1-RT99,HIGHER) to assign to this CPU 206 * 207 * Note: Assumes cpu_rq(cpu)->lock is locked 208 * 209 * Returns: (void) 210 */ 211 void cpupri_set(struct cpupri *cp, int cpu, int newpri) 212 { 213 int *currpri = &cp->cpu_to_pri[cpu]; 214 int oldpri = *currpri; 215 int do_mb = 0; 216 217 newpri = convert_prio(newpri); 218 219 BUG_ON(newpri >= CPUPRI_NR_PRIORITIES); 220 221 if (newpri == oldpri) 222 return; 223 224 /* 225 * If the CPU was currently mapped to a different value, we 226 * need to map it to the new value then remove the old value. 227 * Note, we must add the new value first, otherwise we risk the 228 * cpu being missed by the priority loop in cpupri_find. 229 */ 230 if (likely(newpri != CPUPRI_INVALID)) { 231 struct cpupri_vec *vec = &cp->pri_to_cpu[newpri]; 232 233 cpumask_set_cpu(cpu, vec->mask); 234 /* 235 * When adding a new vector, we update the mask first, 236 * do a write memory barrier, and then update the count, to 237 * make sure the vector is visible when count is set. 238 */ 239 smp_mb__before_atomic(); 240 atomic_inc(&(vec)->count); 241 do_mb = 1; 242 } 243 if (likely(oldpri != CPUPRI_INVALID)) { 244 struct cpupri_vec *vec = &cp->pri_to_cpu[oldpri]; 245 246 /* 247 * Because the order of modification of the vec->count 248 * is important, we must make sure that the update 249 * of the new prio is seen before we decrement the 250 * old prio. This makes sure that the loop sees 251 * one or the other when we raise the priority of 252 * the run queue. We don't care about when we lower the 253 * priority, as that will trigger an rt pull anyway. 254 * 255 * We only need to do a memory barrier if we updated 256 * the new priority vec. 257 */ 258 if (do_mb) 259 smp_mb__after_atomic(); 260 261 /* 262 * When removing from the vector, we decrement the counter first 263 * do a memory barrier and then clear the mask. 264 */ 265 atomic_dec(&(vec)->count); 266 smp_mb__after_atomic(); 267 cpumask_clear_cpu(cpu, vec->mask); 268 } 269 270 *currpri = newpri; 271 } 272 273 /** 274 * cpupri_init - initialize the cpupri structure 275 * @cp: The cpupri context 276 * 277 * Return: -ENOMEM on memory allocation failure. 278 */ 279 int cpupri_init(struct cpupri *cp) 280 { 281 int i; 282 283 for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) { 284 struct cpupri_vec *vec = &cp->pri_to_cpu[i]; 285 286 atomic_set(&vec->count, 0); 287 if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL)) 288 goto cleanup; 289 } 290 291 cp->cpu_to_pri = kcalloc(nr_cpu_ids, sizeof(int), GFP_KERNEL); 292 if (!cp->cpu_to_pri) 293 goto cleanup; 294 295 for_each_possible_cpu(i) 296 cp->cpu_to_pri[i] = CPUPRI_INVALID; 297 298 return 0; 299 300 cleanup: 301 for (i--; i >= 0; i--) 302 free_cpumask_var(cp->pri_to_cpu[i].mask); 303 return -ENOMEM; 304 } 305 306 /** 307 * cpupri_cleanup - clean up the cpupri structure 308 * @cp: The cpupri context 309 */ 310 void cpupri_cleanup(struct cpupri *cp) 311 { 312 int i; 313 314 kfree(cp->cpu_to_pri); 315 for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) 316 free_cpumask_var(cp->pri_to_cpu[i].mask); 317 } 318