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