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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 : : static int convert_prio(int prio)
29 : : {
30 : : int cpupri;
31 : :
32 [ + + # # ]: 2898 : if (prio == CPUPRI_INVALID)
33 : : cpupri = CPUPRI_INVALID;
34 [ + - # # ]: 2070 : else if (prio == MAX_PRIO)
35 : : cpupri = CPUPRI_IDLE;
36 [ + + # # ]: 2070 : else if (prio >= MAX_RT_PRIO)
37 : : cpupri = CPUPRI_NORMAL;
38 : : else
39 : 207 : 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 : 0 : int cpupri_find(struct cpupri *cp, struct task_struct *p,
60 : : struct cpumask *lowest_mask)
61 : : {
62 : : int idx = 0;
63 : 0 : int task_pri = convert_prio(p->prio);
64 : :
65 [ # # ]: 0 : BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
66 : :
67 [ # # ]: 0 : for (idx = 0; idx < task_pri; idx++) {
68 : : struct cpupri_vec *vec = &cp->pri_to_cpu[idx];
69 : : int skip = 0;
70 : :
71 [ # # ]: 0 : 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 : 0 : smp_rmb();
92 : :
93 : : /* Need to do the rmb for every iteration */
94 [ # # ]: 0 : if (skip)
95 : 0 : continue;
96 : :
97 [ # # ]: 0 : if (cpumask_any_and(p->cpus_ptr, vec->mask) >= nr_cpu_ids)
98 : 0 : continue;
99 : :
100 [ # # ]: 0 : if (lowest_mask) {
101 : 0 : 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 [ # # ]: 0 : if (cpumask_any(lowest_mask) >= nr_cpu_ids)
112 : 0 : 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 : 2898 : void cpupri_set(struct cpupri *cp, int cpu, int newpri)
132 : : {
133 : 2898 : int *currpri = &cp->cpu_to_pri[cpu];
134 : 2898 : int oldpri = *currpri;
135 : : int do_mb = 0;
136 : :
137 : : newpri = convert_prio(newpri);
138 : :
139 [ - + ]: 2898 : BUG_ON(newpri >= CPUPRI_NR_PRIORITIES);
140 : :
141 [ + - ]: 2898 : if (newpri == oldpri)
142 : 2898 : 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 [ + + ]: 2898 : if (likely(newpri != CPUPRI_INVALID)) {
151 : : struct cpupri_vec *vec = &cp->pri_to_cpu[newpri];
152 : :
153 : 2070 : 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 : 2070 : smp_mb__before_atomic();
160 : 2070 : atomic_inc(&(vec)->count);
161 : : do_mb = 1;
162 : : }
163 [ + + ]: 2898 : 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 [ + + ]: 1242 : if (do_mb)
179 : 414 : 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 : 1242 : atomic_dec(&(vec)->count);
186 : 1242 : smp_mb__after_atomic();
187 : 1242 : cpumask_clear_cpu(cpu, vec->mask);
188 : : }
189 : :
190 : 2898 : *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 : 414 : int cpupri_init(struct cpupri *cp)
200 : : {
201 : : int i;
202 : :
203 [ + + ]: 42642 : for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) {
204 : : struct cpupri_vec *vec = &cp->pri_to_cpu[i];
205 : :
206 : 42228 : atomic_set(&vec->count, 0);
207 : 42228 : if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL))
208 : : goto cleanup;
209 : : }
210 : :
211 : 828 : cp->cpu_to_pri = kcalloc(nr_cpu_ids, sizeof(int), GFP_KERNEL);
212 [ + - ]: 414 : if (!cp->cpu_to_pri)
213 : : goto cleanup;
214 : :
215 [ + + ]: 2070 : for_each_possible_cpu(i)
216 : 1656 : 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 : 0 : void cpupri_cleanup(struct cpupri *cp)
231 : : {
232 : : int i;
233 : :
234 : 0 : 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 : 0 : }
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