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 */
convert_prio(int prio)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
__cpupri_find(struct cpupri * cp,struct task_struct * p,struct cpumask * lowest_mask,int idx)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
cpupri_find(struct cpupri * cp,struct task_struct * p,struct cpumask * lowest_mask)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 */
cpupri_find_fitness(struct cpupri * cp,struct task_struct * p,struct cpumask * lowest_mask,bool (* fitness_fn)(struct task_struct * p,int cpu))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 */
cpupri_set(struct cpupri * cp,int cpu,int newpri)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 */
cpupri_init(struct cpupri * cp)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 */
cpupri_cleanup(struct cpupri * cp)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