xref: /linux/include/linux/rcu_node_tree.h (revision cdd38c5f1ce4398ec58fec95904b75824daab7b5)
1 /* SPDX-License-Identifier: GPL-2.0+ */
2 /*
3  * RCU node combining tree definitions.  These are used to compute
4  * global attributes while avoiding common-case global contention.  A key
5  * property that these computations rely on is a tournament-style approach
6  * where only one of the tasks contending a lower level in the tree need
7  * advance to the next higher level.  If properly configured, this allows
8  * unlimited scalability while maintaining a constant level of contention
9  * on the root node.
10  *
11  * This seemingly RCU-private file must be available to SRCU users
12  * because the size of the TREE SRCU srcu_struct structure depends
13  * on these definitions.
14  *
15  * Copyright IBM Corporation, 2017
16  *
17  * Author: Paul E. McKenney <paulmck@linux.ibm.com>
18  */
19 
20 #ifndef __LINUX_RCU_NODE_TREE_H
21 #define __LINUX_RCU_NODE_TREE_H
22 
23 #include <linux/math.h>
24 
25 /*
26  * Define shape of hierarchy based on NR_CPUS, CONFIG_RCU_FANOUT, and
27  * CONFIG_RCU_FANOUT_LEAF.
28  * In theory, it should be possible to add more levels straightforwardly.
29  * In practice, this did work well going from three levels to four.
30  * Of course, your mileage may vary.
31  */
32 
33 #ifdef CONFIG_RCU_FANOUT
34 #define RCU_FANOUT CONFIG_RCU_FANOUT
35 #else /* #ifdef CONFIG_RCU_FANOUT */
36 # ifdef CONFIG_64BIT
37 # define RCU_FANOUT 64
38 # else
39 # define RCU_FANOUT 32
40 # endif
41 #endif /* #else #ifdef CONFIG_RCU_FANOUT */
42 
43 #ifdef CONFIG_RCU_FANOUT_LEAF
44 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
45 #else /* #ifdef CONFIG_RCU_FANOUT_LEAF */
46 #define RCU_FANOUT_LEAF 16
47 #endif /* #else #ifdef CONFIG_RCU_FANOUT_LEAF */
48 
49 #define RCU_FANOUT_1	      (RCU_FANOUT_LEAF)
50 #define RCU_FANOUT_2	      (RCU_FANOUT_1 * RCU_FANOUT)
51 #define RCU_FANOUT_3	      (RCU_FANOUT_2 * RCU_FANOUT)
52 #define RCU_FANOUT_4	      (RCU_FANOUT_3 * RCU_FANOUT)
53 
54 #if NR_CPUS <= RCU_FANOUT_1
55 #  define RCU_NUM_LVLS	      1
56 #  define NUM_RCU_LVL_0	      1
57 #  define NUM_RCU_NODES	      NUM_RCU_LVL_0
58 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0 }
59 #  define RCU_NODE_NAME_INIT  { "rcu_node_0" }
60 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0" }
61 #elif NR_CPUS <= RCU_FANOUT_2
62 #  define RCU_NUM_LVLS	      2
63 #  define NUM_RCU_LVL_0	      1
64 #  define NUM_RCU_LVL_1	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
65 #  define NUM_RCU_NODES	      (NUM_RCU_LVL_0 + NUM_RCU_LVL_1)
66 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
67 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1" }
68 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1" }
69 #elif NR_CPUS <= RCU_FANOUT_3
70 #  define RCU_NUM_LVLS	      3
71 #  define NUM_RCU_LVL_0	      1
72 #  define NUM_RCU_LVL_1	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
73 #  define NUM_RCU_LVL_2	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
74 #  define NUM_RCU_NODES	      (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2)
75 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
76 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
77 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
78 #elif NR_CPUS <= RCU_FANOUT_4
79 #  define RCU_NUM_LVLS	      4
80 #  define NUM_RCU_LVL_0	      1
81 #  define NUM_RCU_LVL_1	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)
82 #  define NUM_RCU_LVL_2	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
83 #  define NUM_RCU_LVL_3	      DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
84 #  define NUM_RCU_NODES	      (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
85 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
86 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
87 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
88 #else
89 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
90 #endif /* #if (NR_CPUS) <= RCU_FANOUT_1 */
91 
92 #endif /* __LINUX_RCU_NODE_TREE_H */
93