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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 : 39 : static int convert_prio(int prio)
29 : : {
30 : 39 : int cpupri;
31 : :
32 : 39 : if (prio == CPUPRI_INVALID)
33 : : cpupri = CPUPRI_INVALID;
34 [ + - - - ]: 26 : else if (prio == MAX_PRIO)
35 : : cpupri = CPUPRI_IDLE;
36 [ - + - - ]: 26 : else if (prio >= MAX_RT_PRIO)
37 : : cpupri = CPUPRI_NORMAL;
38 : : else
39 : 0 : cpupri = MAX_RT_PRIO - prio + 1;
40 : :
41 : 39 : 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 : 0 : 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 : 0 : int idx = 0;
66 [ # # ]: 0 : int task_pri = convert_prio(p->prio);
67 : :
68 [ # # ]: 0 : BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
69 : :
70 [ # # ]: 0 : for (idx = 0; idx < task_pri; idx++) {
71 : 0 : struct cpupri_vec *vec = &cp->pri_to_cpu[idx];
72 : 0 : int skip = 0;
73 : :
74 [ # # ]: 0 : if (!atomic_read(&(vec)->count))
75 : 0 : 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 : 0 : smp_rmb();
95 : :
96 : : /* Need to do the rmb for every iteration */
97 : 0 : if (skip)
98 : 0 : continue;
99 : :
100 [ # # ]: 0 : if (cpumask_any_and(p->cpus_ptr, vec->mask) >= nr_cpu_ids)
101 : 0 : continue;
102 : :
103 [ # # ]: 0 : if (lowest_mask) {
104 : 0 : int cpu;
105 : :
106 [ # # ]: 0 : 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 [ # # ]: 0 : if (cpumask_empty(lowest_mask))
117 : 0 : continue;
118 : :
119 [ # # ]: 0 : if (!fitness_fn)
120 : : return 1;
121 : :
122 : : /* Ensure the capacity of the CPUs fit the task */
123 [ # # ]: 0 : for_each_cpu(cpu, lowest_mask) {
124 [ # # ]: 0 : if (!fitness_fn(p, cpu))
125 : 0 : 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 [ # # ]: 0 : if (cpumask_empty(lowest_mask))
133 : 0 : 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 : 39 : void cpupri_set(struct cpupri *cp, int cpu, int newpri)
153 : : {
154 : 39 : int *currpri = &cp->cpu_to_pri[cpu];
155 : 39 : int oldpri = *currpri;
156 : 39 : int do_mb = 0;
157 : :
158 [ + + ]: 39 : newpri = convert_prio(newpri);
159 : :
160 [ - + ]: 39 : BUG_ON(newpri >= CPUPRI_NR_PRIORITIES);
161 : :
162 [ + - ]: 39 : 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 [ + + ]: 39 : if (likely(newpri != CPUPRI_INVALID)) {
172 : 26 : struct cpupri_vec *vec = &cp->pri_to_cpu[newpri];
173 : :
174 : 26 : 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 : 26 : smp_mb__before_atomic();
181 : 26 : atomic_inc(&(vec)->count);
182 : 26 : do_mb = 1;
183 : : }
184 [ + + ]: 39 : if (likely(oldpri != CPUPRI_INVALID)) {
185 : 13 : 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 : 13 : if (do_mb)
200 : 13 : 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 : 13 : atomic_dec(&(vec)->count);
207 : 13 : smp_mb__after_atomic();
208 : 13 : cpumask_clear_cpu(cpu, vec->mask);
209 : : }
210 : :
211 : 39 : *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 : 26 : int cpupri_init(struct cpupri *cp)
221 : : {
222 : 26 : int i;
223 : :
224 [ + + ]: 2678 : for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) {
225 : 2652 : struct cpupri_vec *vec = &cp->pri_to_cpu[i];
226 : :
227 : 2652 : atomic_set(&vec->count, 0);
228 : 2652 : if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL))
229 : : goto cleanup;
230 : : }
231 : :
232 : 26 : cp->cpu_to_pri = kcalloc(nr_cpu_ids, sizeof(int), GFP_KERNEL);
233 [ + - ]: 26 : if (!cp->cpu_to_pri)
234 : 0 : goto cleanup;
235 : :
236 [ + + ]: 52 : for_each_possible_cpu(i)
237 : 26 : cp->cpu_to_pri[i] = CPUPRI_INVALID;
238 : :
239 : : return 0;
240 : :
241 : : cleanup:
242 : 0 : 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 : 0 : void cpupri_cleanup(struct cpupri *cp)
252 : : {
253 : 0 : int i;
254 : :
255 : 0 : kfree(cp->cpu_to_pri);
256 : 0 : for (i = 0; i < CPUPRI_NR_PRIORITIES; i++)
257 : : free_cpumask_var(cp->pri_to_cpu[i].mask);
258 : 0 : }
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