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), IDLE, NORMAL, RT1, ... RT99 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(102, nr_domcpus)), though the scenario that 23 * yields the worst case search is fairly contrived. 24 */ 25 #include "sched.h" 26 27 /* Convert between a 140 based task->prio, and our 102 based cpupri */ 28 static int convert_prio(int prio) 29 { 30 int cpupri; 31 32 if (prio == CPUPRI_INVALID) 33 cpupri = CPUPRI_INVALID; 34 else if (prio == MAX_PRIO) 35 cpupri = CPUPRI_IDLE; 36 else if (prio >= MAX_RT_PRIO) 37 cpupri = CPUPRI_NORMAL; 38 else 39 cpupri = MAX_RT_PRIO - prio + 1; 40 41 return cpupri; 42 } 43 44 /** 45 * cpupri_find - find the best (lowest-pri) CPU in the system 46 * @cp: The cpupri context 47 * @p: The task 48 * @lowest_mask: A mask to fill in with selected CPUs (or NULL) 49 * @fitness_fn: A pointer to a function to do custom checks whether the CPU 50 * fits a specific criteria so that we only return those CPUs. 51 * 52 * Note: This function returns the recommended CPUs as calculated during the 53 * current invocation. By the time the call returns, the CPUs may have in 54 * fact changed priorities any number of times. While not ideal, it is not 55 * an issue of correctness since the normal rebalancer logic will correct 56 * any discrepancies created by racing against the uncertainty of the current 57 * priority configuration. 58 * 59 * Return: (int)bool - CPUs were found 60 */ 61 int cpupri_find(struct cpupri *cp, struct task_struct *p, 62 struct cpumask *lowest_mask, 63 bool (*fitness_fn)(struct task_struct *p, int cpu)) 64 { 65 int idx = 0; 66 int task_pri = convert_prio(p->prio); 67 68 BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES); 69 70 for (idx = 0; idx < task_pri; idx++) { 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 racey, 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 continue; 99 100 if (cpumask_any_and(p->cpus_ptr, vec->mask) >= nr_cpu_ids) 101 continue; 102 103 if (lowest_mask) { 104 int cpu; 105 106 cpumask_and(lowest_mask, p->cpus_ptr, vec->mask); 107 108 /* 109 * We have to ensure that we have at least one bit 110 * still set in the array, since the map could have 111 * been concurrently emptied between the first and 112 * second reads of vec->mask. If we hit this 113 * condition, simply act as though we never hit this 114 * priority level and continue on. 115 */ 116 if (cpumask_empty(lowest_mask)) 117 continue; 118 119 if (!fitness_fn) 120 return 1; 121 122 /* Ensure the capacity of the CPUs fit the task */ 123 for_each_cpu(cpu, lowest_mask) { 124 if (!fitness_fn(p, cpu)) 125 cpumask_clear_cpu(cpu, lowest_mask); 126 } 127 128 /* 129 * If no CPU at the current priority can fit the task 130 * continue looking 131 */ 132 if (cpumask_empty(lowest_mask)) 133 continue; 134 } 135 136 return 1; 137 } 138 139 return 0; 140 } 141 142 /** 143 * cpupri_set - update the CPU priority setting 144 * @cp: The cpupri context 145 * @cpu: The target CPU 146 * @newpri: The priority (INVALID-RT99) to assign to this CPU 147 * 148 * Note: Assumes cpu_rq(cpu)->lock is locked 149 * 150 * Returns: (void) 151 */ 152 void cpupri_set(struct cpupri *cp, int cpu, int newpri) 153 { 154 int *currpri = &cp->cpu_to_pri[cpu]; 155 int oldpri = *currpri; 156 int do_mb = 0; 157 158 newpri = convert_prio(newpri); 159 160 BUG_ON(newpri >= CPUPRI_NR_PRIORITIES); 161 162 if (newpri == oldpri) 163 return; 164 165 /* 166 * If the CPU was currently mapped to a different value, we 167 * need to map it to the new value then remove the old value. 168 * Note, we must add the new value first, otherwise we risk the 169 * cpu being missed by the priority loop in cpupri_find. 170 */ 171 if (likely(newpri != CPUPRI_INVALID)) { 172 struct cpupri_vec *vec = &cp->pri_to_cpu[newpri]; 173 174 cpumask_set_cpu(cpu, vec->mask); 175 /* 176 * When adding a new vector, we update the mask first, 177 * do a write memory barrier, and then update the count, to 178 * make sure the vector is visible when count is set. 179 */ 180 smp_mb__before_atomic(); 181 atomic_inc(&(vec)->count); 182 do_mb = 1; 183 } 184 if (likely(oldpri != CPUPRI_INVALID)) { 185 struct cpupri_vec *vec = &cp->pri_to_cpu[oldpri]; 186 187 /* 188 * Because the order of modification of the vec->count 189 * is important, we must make sure that the update 190 * of the new prio is seen before we decrement the 191 * old prio. This makes sure that the loop sees 192 * one or the other when we raise the priority of 193 * the run queue. We don't care about when we lower the 194 * priority, as that will trigger an rt pull anyway. 195 * 196 * We only need to do a memory barrier if we updated 197 * the new priority vec. 198 */ 199 if (do_mb) 200 smp_mb__after_atomic(); 201 202 /* 203 * When removing from the vector, we decrement the counter first 204 * do a memory barrier and then clear the mask. 205 */ 206 atomic_dec(&(vec)->count); 207 smp_mb__after_atomic(); 208 cpumask_clear_cpu(cpu, vec->mask); 209 } 210 211 *currpri = newpri; 212 } 213 214 /** 215 * cpupri_init - initialize the cpupri structure 216 * @cp: The cpupri context 217 * 218 * Return: -ENOMEM on memory allocation failure. 219 */ 220 int cpupri_init(struct cpupri *cp) 221 { 222 int i; 223 224 for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) { 225 struct cpupri_vec *vec = &cp->pri_to_cpu[i]; 226 227 atomic_set(&vec->count, 0); 228 if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL)) 229 goto cleanup; 230 } 231 232 cp->cpu_to_pri = kcalloc(nr_cpu_ids, sizeof(int), GFP_KERNEL); 233 if (!cp->cpu_to_pri) 234 goto cleanup; 235 236 for_each_possible_cpu(i) 237 cp->cpu_to_pri[i] = CPUPRI_INVALID; 238 239 return 0; 240 241 cleanup: 242 for (i--; i >= 0; i--) 243 free_cpumask_var(cp->pri_to_cpu[i].mask); 244 return -ENOMEM; 245 } 246 247 /** 248 * cpupri_cleanup - clean up the cpupri structure 249 * @cp: The cpupri context 250 */ 251 void cpupri_cleanup(struct cpupri *cp) 252 { 253 int i; 254 255 kfree(cp->cpu_to_pri); 256 for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) 257 free_cpumask_var(cp->pri_to_cpu[i].mask); 258 } 259