1 /* 2 * kernel/sched/cpupri.c 3 * 4 * CPU priority management 5 * 6 * Copyright (C) 2007-2008 Novell 7 * 8 * Author: Gregory Haskins <ghaskins@novell.com> 9 * 10 * This code tracks the priority of each CPU so that global migration 11 * decisions are easy to calculate. Each CPU can be in a state as follows: 12 * 13 * (INVALID), IDLE, NORMAL, RT1, ... RT99 14 * 15 * going from the lowest priority to the highest. CPUs in the INVALID state 16 * are not eligible for routing. The system maintains this state with 17 * a 2 dimensional bitmap (the first for priority class, the second for cpus 18 * in that class). Therefore a typical application without affinity 19 * restrictions can find a suitable CPU with O(1) complexity (e.g. two bit 20 * searches). For tasks with affinity restrictions, the algorithm has a 21 * worst case complexity of O(min(102, nr_domcpus)), though the scenario that 22 * yields the worst case search is fairly contrived. 23 * 24 * This program is free software; you can redistribute it and/or 25 * modify it under the terms of the GNU General Public License 26 * as published by the Free Software Foundation; version 2 27 * of the License. 28 */ 29 30 #include <linux/gfp.h> 31 #include <linux/sched.h> 32 #include <linux/sched/rt.h> 33 #include "cpupri.h" 34 35 /* Convert between a 140 based task->prio, and our 102 based cpupri */ 36 static int convert_prio(int prio) 37 { 38 int cpupri; 39 40 if (prio == CPUPRI_INVALID) 41 cpupri = CPUPRI_INVALID; 42 else if (prio == MAX_PRIO) 43 cpupri = CPUPRI_IDLE; 44 else if (prio >= MAX_RT_PRIO) 45 cpupri = CPUPRI_NORMAL; 46 else 47 cpupri = MAX_RT_PRIO - prio + 1; 48 49 return cpupri; 50 } 51 52 /** 53 * cpupri_find - find the best (lowest-pri) CPU in the system 54 * @cp: The cpupri context 55 * @p: The task 56 * @lowest_mask: A mask to fill in with selected CPUs (or NULL) 57 * 58 * Note: This function returns the recommended CPUs as calculated during the 59 * current invocation. By the time the call returns, the CPUs may have in 60 * fact changed priorities any number of times. While not ideal, it is not 61 * an issue of correctness since the normal rebalancer logic will correct 62 * any discrepancies created by racing against the uncertainty of the current 63 * priority configuration. 64 * 65 * Returns: (int)bool - CPUs were found 66 */ 67 int cpupri_find(struct cpupri *cp, struct task_struct *p, 68 struct cpumask *lowest_mask) 69 { 70 int idx = 0; 71 int task_pri = convert_prio(p->prio); 72 73 if (task_pri >= MAX_RT_PRIO) 74 return 0; 75 76 for (idx = 0; idx < task_pri; idx++) { 77 struct cpupri_vec *vec = &cp->pri_to_cpu[idx]; 78 int skip = 0; 79 80 if (!atomic_read(&(vec)->count)) 81 skip = 1; 82 /* 83 * When looking at the vector, we need to read the counter, 84 * do a memory barrier, then read the mask. 85 * 86 * Note: This is still all racey, but we can deal with it. 87 * Ideally, we only want to look at masks that are set. 88 * 89 * If a mask is not set, then the only thing wrong is that we 90 * did a little more work than necessary. 91 * 92 * If we read a zero count but the mask is set, because of the 93 * memory barriers, that can only happen when the highest prio 94 * task for a run queue has left the run queue, in which case, 95 * it will be followed by a pull. If the task we are processing 96 * fails to find a proper place to go, that pull request will 97 * pull this task if the run queue is running at a lower 98 * priority. 99 */ 100 smp_rmb(); 101 102 /* Need to do the rmb for every iteration */ 103 if (skip) 104 continue; 105 106 if (cpumask_any_and(&p->cpus_allowed, vec->mask) >= nr_cpu_ids) 107 continue; 108 109 if (lowest_mask) { 110 cpumask_and(lowest_mask, &p->cpus_allowed, vec->mask); 111 112 /* 113 * We have to ensure that we have at least one bit 114 * still set in the array, since the map could have 115 * been concurrently emptied between the first and 116 * second reads of vec->mask. If we hit this 117 * condition, simply act as though we never hit this 118 * priority level and continue on. 119 */ 120 if (cpumask_any(lowest_mask) >= nr_cpu_ids) 121 continue; 122 } 123 124 return 1; 125 } 126 127 return 0; 128 } 129 130 /** 131 * cpupri_set - update the cpu priority setting 132 * @cp: The cpupri context 133 * @cpu: The target cpu 134 * @newpri: The priority (INVALID-RT99) to assign to this CPU 135 * 136 * Note: Assumes cpu_rq(cpu)->lock is locked 137 * 138 * Returns: (void) 139 */ 140 void cpupri_set(struct cpupri *cp, int cpu, int newpri) 141 { 142 int *currpri = &cp->cpu_to_pri[cpu]; 143 int oldpri = *currpri; 144 int do_mb = 0; 145 146 newpri = convert_prio(newpri); 147 148 BUG_ON(newpri >= CPUPRI_NR_PRIORITIES); 149 150 if (newpri == oldpri) 151 return; 152 153 /* 154 * If the cpu was currently mapped to a different value, we 155 * need to map it to the new value then remove the old value. 156 * Note, we must add the new value first, otherwise we risk the 157 * cpu being missed by the priority loop in cpupri_find. 158 */ 159 if (likely(newpri != CPUPRI_INVALID)) { 160 struct cpupri_vec *vec = &cp->pri_to_cpu[newpri]; 161 162 cpumask_set_cpu(cpu, vec->mask); 163 /* 164 * When adding a new vector, we update the mask first, 165 * do a write memory barrier, and then update the count, to 166 * make sure the vector is visible when count is set. 167 */ 168 smp_mb__before_atomic_inc(); 169 atomic_inc(&(vec)->count); 170 do_mb = 1; 171 } 172 if (likely(oldpri != CPUPRI_INVALID)) { 173 struct cpupri_vec *vec = &cp->pri_to_cpu[oldpri]; 174 175 /* 176 * Because the order of modification of the vec->count 177 * is important, we must make sure that the update 178 * of the new prio is seen before we decrement the 179 * old prio. This makes sure that the loop sees 180 * one or the other when we raise the priority of 181 * the run queue. We don't care about when we lower the 182 * priority, as that will trigger an rt pull anyway. 183 * 184 * We only need to do a memory barrier if we updated 185 * the new priority vec. 186 */ 187 if (do_mb) 188 smp_mb__after_atomic_inc(); 189 190 /* 191 * When removing from the vector, we decrement the counter first 192 * do a memory barrier and then clear the mask. 193 */ 194 atomic_dec(&(vec)->count); 195 smp_mb__after_atomic_inc(); 196 cpumask_clear_cpu(cpu, vec->mask); 197 } 198 199 *currpri = newpri; 200 } 201 202 /** 203 * cpupri_init - initialize the cpupri structure 204 * @cp: The cpupri context 205 * 206 * Returns: -ENOMEM if memory fails. 207 */ 208 int cpupri_init(struct cpupri *cp) 209 { 210 int i; 211 212 memset(cp, 0, sizeof(*cp)); 213 214 for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) { 215 struct cpupri_vec *vec = &cp->pri_to_cpu[i]; 216 217 atomic_set(&vec->count, 0); 218 if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL)) 219 goto cleanup; 220 } 221 222 for_each_possible_cpu(i) 223 cp->cpu_to_pri[i] = CPUPRI_INVALID; 224 return 0; 225 226 cleanup: 227 for (i--; i >= 0; i--) 228 free_cpumask_var(cp->pri_to_cpu[i].mask); 229 return -ENOMEM; 230 } 231 232 /** 233 * cpupri_cleanup - clean up the cpupri structure 234 * @cp: The cpupri context 235 */ 236 void cpupri_cleanup(struct cpupri *cp) 237 { 238 int i; 239 240 for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) 241 free_cpumask_var(cp->pri_to_cpu[i].mask); 242 } 243