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Linux/kernel/sched/cpupri.c

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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         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  * Return: (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         BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
 74 
 75         for (idx = 0; idx < task_pri; idx++) {
 76                 struct cpupri_vec *vec  = &cp->pri_to_cpu[idx];
 77                 int skip = 0;
 78 
 79                 if (!atomic_read(&(vec)->count))
 80                         skip = 1;
 81                 /*
 82                  * When looking at the vector, we need to read the counter,
 83                  * do a memory barrier, then read the mask.
 84                  *
 85                  * Note: This is still all racey, but we can deal with it.
 86                  *  Ideally, we only want to look at masks that are set.
 87                  *
 88                  *  If a mask is not set, then the only thing wrong is that we
 89                  *  did a little more work than necessary.
 90                  *
 91                  *  If we read a zero count but the mask is set, because of the
 92                  *  memory barriers, that can only happen when the highest prio
 93                  *  task for a run queue has left the run queue, in which case,
 94                  *  it will be followed by a pull. If the task we are processing
 95                  *  fails to find a proper place to go, that pull request will
 96                  *  pull this task if the run queue is running at a lower
 97                  *  priority.
 98                  */
 99                 smp_rmb();
100 
101                 /* Need to do the rmb for every iteration */
102                 if (skip)
103                         continue;
104 
105                 if (cpumask_any_and(&p->cpus_allowed, vec->mask) >= nr_cpu_ids)
106                         continue;
107 
108                 if (lowest_mask) {
109                         cpumask_and(lowest_mask, &p->cpus_allowed, vec->mask);
110 
111                         /*
112                          * We have to ensure that we have at least one bit
113                          * still set in the array, since the map could have
114                          * been concurrently emptied between the first and
115                          * second reads of vec->mask.  If we hit this
116                          * condition, simply act as though we never hit this
117                          * priority level and continue on.
118                          */
119                         if (cpumask_any(lowest_mask) >= nr_cpu_ids)
120                                 continue;
121                 }
122 
123                 return 1;
124         }
125 
126         return 0;
127 }
128 
129 /**
130  * cpupri_set - update the cpu priority setting
131  * @cp: The cpupri context
132  * @cpu: The target cpu
133  * @newpri: The priority (INVALID-RT99) to assign to this CPU
134  *
135  * Note: Assumes cpu_rq(cpu)->lock is locked
136  *
137  * Returns: (void)
138  */
139 void cpupri_set(struct cpupri *cp, int cpu, int newpri)
140 {
141         int *currpri = &cp->cpu_to_pri[cpu];
142         int oldpri = *currpri;
143         int do_mb = 0;
144 
145         newpri = convert_prio(newpri);
146 
147         BUG_ON(newpri >= CPUPRI_NR_PRIORITIES);
148 
149         if (newpri == oldpri)
150                 return;
151 
152         /*
153          * If the cpu was currently mapped to a different value, we
154          * need to map it to the new value then remove the old value.
155          * Note, we must add the new value first, otherwise we risk the
156          * cpu being missed by the priority loop in cpupri_find.
157          */
158         if (likely(newpri != CPUPRI_INVALID)) {
159                 struct cpupri_vec *vec = &cp->pri_to_cpu[newpri];
160 
161                 cpumask_set_cpu(cpu, vec->mask);
162                 /*
163                  * When adding a new vector, we update the mask first,
164                  * do a write memory barrier, and then update the count, to
165                  * make sure the vector is visible when count is set.
166                  */
167                 smp_mb__before_atomic_inc();
168                 atomic_inc(&(vec)->count);
169                 do_mb = 1;
170         }
171         if (likely(oldpri != CPUPRI_INVALID)) {
172                 struct cpupri_vec *vec  = &cp->pri_to_cpu[oldpri];
173 
174                 /*
175                  * Because the order of modification of the vec->count
176                  * is important, we must make sure that the update
177                  * of the new prio is seen before we decrement the
178                  * old prio. This makes sure that the loop sees
179                  * one or the other when we raise the priority of
180                  * the run queue. We don't care about when we lower the
181                  * priority, as that will trigger an rt pull anyway.
182                  *
183                  * We only need to do a memory barrier if we updated
184                  * the new priority vec.
185                  */
186                 if (do_mb)
187                         smp_mb__after_atomic_inc();
188 
189                 /*
190                  * When removing from the vector, we decrement the counter first
191                  * do a memory barrier and then clear the mask.
192                  */
193                 atomic_dec(&(vec)->count);
194                 smp_mb__after_atomic_inc();
195                 cpumask_clear_cpu(cpu, vec->mask);
196         }
197 
198         *currpri = newpri;
199 }
200 
201 /**
202  * cpupri_init - initialize the cpupri structure
203  * @cp: The cpupri context
204  *
205  * Return: -ENOMEM on memory allocation failure.
206  */
207 int cpupri_init(struct cpupri *cp)
208 {
209         int i;
210 
211         memset(cp, 0, sizeof(*cp));
212 
213         for (i = 0; i < CPUPRI_NR_PRIORITIES; i++) {
214                 struct cpupri_vec *vec = &cp->pri_to_cpu[i];
215 
216                 atomic_set(&vec->count, 0);
217                 if (!zalloc_cpumask_var(&vec->mask, GFP_KERNEL))
218                         goto cleanup;
219         }
220 
221         for_each_possible_cpu(i)
222                 cp->cpu_to_pri[i] = CPUPRI_INVALID;
223         return 0;
224 
225 cleanup:
226         for (i--; i >= 0; i--)
227                 free_cpumask_var(cp->pri_to_cpu[i].mask);
228         return -ENOMEM;
229 }
230 
231 /**
232  * cpupri_cleanup - clean up the cpupri structure
233  * @cp: The cpupri context
234  */
235 void cpupri_cleanup(struct cpupri *cp)
236 {
237         int i;
238 
239         for (i = 0; i < CPUPRI_NR_PRIORITIES; i++)
240                 free_cpumask_var(cp->pri_to_cpu[i].mask);
241 }
242 

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