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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 [ + + ][ + - ]: 4141 : if (prio == CPUPRI_INVALID)
41 : : cpupri = CPUPRI_INVALID;
42 [ + - ][ + - ]: 2822 : else if (prio == MAX_PRIO)
43 : : cpupri = CPUPRI_IDLE;
44 [ + + ][ + - ]: 2822 : else if (prio >= MAX_RT_PRIO)
45 : : cpupri = CPUPRI_NORMAL;
46 : : else
47 : 904 : 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 : : * Return: (int)bool - CPUs were found
66 : : */
67 : 0 : int cpupri_find(struct cpupri *cp, struct task_struct *p,
68 : : struct cpumask *lowest_mask)
69 : : {
70 : : int idx = 0;
71 : 298 : int task_pri = convert_prio(p->prio);
72 : :
73 [ + + ]: 298 : if (task_pri >= MAX_RT_PRIO)
74 : : return 0;
75 : :
76 [ + + ]: 697 : for (idx = 0; idx < task_pri; idx++) {
77 : 690 : struct cpupri_vec *vec = &cp->pri_to_cpu[idx];
78 : : int skip = 0;
79 : :
80 [ + + ]: 690 : 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 : 690 : smp_rmb();
101 : :
102 : : /* Need to do the rmb for every iteration */
103 [ + + ]: 988 : if (skip)
104 : 439 : continue;
105 : :
106 [ - + ]: 251 : if (cpumask_any_and(&p->cpus_allowed, vec->mask) >= nr_cpu_ids)
107 : 0 : continue;
108 : :
109 [ + + ]: 251 : 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 [ - + ]: 103 : if (cpumask_any(lowest_mask) >= nr_cpu_ids)
121 : 0 : 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 : 0 : void cpupri_set(struct cpupri *cp, int cpu, int newpri)
141 : : {
142 : : int *currpri = &cp->cpu_to_pri[cpu];
143 : 3843 : int oldpri = *currpri;
144 : : int do_mb = 0;
145 : :
146 : : newpri = convert_prio(newpri);
147 : :
148 [ - + ]: 3843 : BUG_ON(newpri >= CPUPRI_NR_PRIORITIES);
149 : :
150 [ + - ]: 3843 : if (newpri == oldpri)
151 : 3845 : 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 [ + + ]: 3843 : if (likely(newpri != CPUPRI_INVALID)) {
160 : 2523 : 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 : 2525 : smp_mb__before_atomic_inc();
169 : 2525 : atomic_inc(&(vec)->count);
170 : : do_mb = 1;
171 : : }
172 [ + + ]: 7688 : if (likely(oldpri != CPUPRI_INVALID)) {
173 : 2522 : 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 [ + + ]: 2522 : if (do_mb)
188 : 1202 : 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 : 2522 : atomic_dec(&(vec)->count);
195 : 2522 : smp_mb__after_atomic_inc();
196 : : cpumask_clear_cpu(cpu, vec->mask);
197 : : }
198 : :
199 : 3845 : *currpri = newpri;
200 : : }
201 : :
202 : : /**
203 : : * cpupri_init - initialize the cpupri structure
204 : : * @cp: The cpupri context
205 : : *
206 : : * Return: -ENOMEM on memory allocation failure.
207 : : */
208 : 0 : int cpupri_init(struct cpupri *cp)
209 : : {
210 : : int i;
211 : :
212 : 159 : memset(cp, 0, sizeof(*cp));
213 : :
214 [ + + ]: 16536 : for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) {
215 : 16218 : struct cpupri_vec *vec = &cp->pri_to_cpu[i];
216 : :
217 : 16218 : atomic_set(&vec->count, 0);
218 : : if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL))
219 : : goto cleanup;
220 : : }
221 : :
222 [ + + ]: 954 : for_each_possible_cpu(i)
223 : 795 : 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 : 0 : 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 : 159 : }
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