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