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 * 50 * Note: This function returns the recommended CPUs as calculated during the 51 * current invocation. By the time the call returns, the CPUs may have in 52 * fact changed priorities any number of times. While not ideal, it is not 53 * an issue of correctness since the normal rebalancer logic will correct 54 * any discrepancies created by racing against the uncertainty of the current 55 * priority configuration. 56 * 57 * Return: (int)bool - CPUs were found 58 */ 59 int cpupri_find(struct cpupri *cp, struct task_struct *p, 60 struct cpumask *lowest_mask) 61 { 62 int idx = 0; 63 int task_pri = convert_prio(p->prio); 64 65 BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES); 66 67 for (idx = 0; idx < task_pri; idx++) { 68 struct cpupri_vec *vec = &cp->pri_to_cpu[idx]; 69 int skip = 0; 70 71 if (!atomic_read(&(vec)->count)) 72 skip = 1; 73 /* 74 * When looking at the vector, we need to read the counter, 75 * do a memory barrier, then read the mask. 76 * 77 * Note: This is still all racey, but we can deal with it. 78 * Ideally, we only want to look at masks that are set. 79 * 80 * If a mask is not set, then the only thing wrong is that we 81 * did a little more work than necessary. 82 * 83 * If we read a zero count but the mask is set, because of the 84 * memory barriers, that can only happen when the highest prio 85 * task for a run queue has left the run queue, in which case, 86 * it will be followed by a pull. If the task we are processing 87 * fails to find a proper place to go, that pull request will 88 * pull this task if the run queue is running at a lower 89 * priority. 90 */ 91 smp_rmb(); 92 93 /* Need to do the rmb for every iteration */ 94 if (skip) 95 continue; 96 97 if (cpumask_any_and(p->cpus_ptr, vec->mask) >= nr_cpu_ids) 98 continue; 99 100 if (lowest_mask) { 101 cpumask_and(lowest_mask, p->cpus_ptr, vec->mask); 102 103 /* 104 * We have to ensure that we have at least one bit 105 * still set in the array, since the map could have 106 * been concurrently emptied between the first and 107 * second reads of vec->mask. If we hit this 108 * condition, simply act as though we never hit this 109 * priority level and continue on. 110 */ 111 if (cpumask_any(lowest_mask) >= nr_cpu_ids) 112 continue; 113 } 114 115 return 1; 116 } 117 118 return 0; 119 } 120 121 /** 122 * cpupri_set - update the CPU priority setting 123 * @cp: The cpupri context 124 * @cpu: The target CPU 125 * @newpri: The priority (INVALID-RT99) to assign to this CPU 126 * 127 * Note: Assumes cpu_rq(cpu)->lock is locked 128 * 129 * Returns: (void) 130 */ 131 void cpupri_set(struct cpupri *cp, int cpu, int newpri) 132 { 133 int *currpri = &cp->cpu_to_pri[cpu]; 134 int oldpri = *currpri; 135 int do_mb = 0; 136 137 newpri = convert_prio(newpri); 138 139 BUG_ON(newpri >= CPUPRI_NR_PRIORITIES); 140 141 if (newpri == oldpri) 142 return; 143 144 /* 145 * If the CPU was currently mapped to a different value, we 146 * need to map it to the new value then remove the old value. 147 * Note, we must add the new value first, otherwise we risk the 148 * cpu being missed by the priority loop in cpupri_find. 149 */ 150 if (likely(newpri != CPUPRI_INVALID)) { 151 struct cpupri_vec *vec = &cp->pri_to_cpu[newpri]; 152 153 cpumask_set_cpu(cpu, vec->mask); 154 /* 155 * When adding a new vector, we update the mask first, 156 * do a write memory barrier, and then update the count, to 157 * make sure the vector is visible when count is set. 158 */ 159 smp_mb__before_atomic(); 160 atomic_inc(&(vec)->count); 161 do_mb = 1; 162 } 163 if (likely(oldpri != CPUPRI_INVALID)) { 164 struct cpupri_vec *vec = &cp->pri_to_cpu[oldpri]; 165 166 /* 167 * Because the order of modification of the vec->count 168 * is important, we must make sure that the update 169 * of the new prio is seen before we decrement the 170 * old prio. This makes sure that the loop sees 171 * one or the other when we raise the priority of 172 * the run queue. We don't care about when we lower the 173 * priority, as that will trigger an rt pull anyway. 174 * 175 * We only need to do a memory barrier if we updated 176 * the new priority vec. 177 */ 178 if (do_mb) 179 smp_mb__after_atomic(); 180 181 /* 182 * When removing from the vector, we decrement the counter first 183 * do a memory barrier and then clear the mask. 184 */ 185 atomic_dec(&(vec)->count); 186 smp_mb__after_atomic(); 187 cpumask_clear_cpu(cpu, vec->mask); 188 } 189 190 *currpri = newpri; 191 } 192 193 /** 194 * cpupri_init - initialize the cpupri structure 195 * @cp: The cpupri context 196 * 197 * Return: -ENOMEM on memory allocation failure. 198 */ 199 int cpupri_init(struct cpupri *cp) 200 { 201 int i; 202 203 for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) { 204 struct cpupri_vec *vec = &cp->pri_to_cpu[i]; 205 206 atomic_set(&vec->count, 0); 207 if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL)) 208 goto cleanup; 209 } 210 211 cp->cpu_to_pri = kcalloc(nr_cpu_ids, sizeof(int), GFP_KERNEL); 212 if (!cp->cpu_to_pri) 213 goto cleanup; 214 215 for_each_possible_cpu(i) 216 cp->cpu_to_pri[i] = CPUPRI_INVALID; 217 218 return 0; 219 220 cleanup: 221 for (i--; i >= 0; i--) 222 free_cpumask_var(cp->pri_to_cpu[i].mask); 223 return -ENOMEM; 224 } 225 226 /** 227 * cpupri_cleanup - clean up the cpupri structure 228 * @cp: The cpupri context 229 */ 230 void cpupri_cleanup(struct cpupri *cp) 231 { 232 int i; 233 234 kfree(cp->cpu_to_pri); 235 for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) 236 free_cpumask_var(cp->pri_to_cpu[i].mask); 237 } 238