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1 | TUNABLE FRAMEWORK |
2 | ================= | |
3 | ||
4 | Tunables is a feature in the GNU C Library that allows application authors and | |
5 | distribution maintainers to alter the runtime library behaviour to match their | |
6 | workload. | |
7 | ||
8 | The tunable framework allows modules within glibc to register variables that | |
9 | may be tweaked through an environment variable. It aims to enforce a strict | |
10 | namespace rule to bring consistency to naming of these tunable environment | |
11 | variables across the project. This document is a guide for glibc developers to | |
12 | add tunables to the framework. | |
13 | ||
14 | ADDING A NEW TUNABLE | |
15 | -------------------- | |
16 | ||
17 | The TOP_NAMESPACE macro is defined by default as 'glibc'. If distributions | |
18 | intend to add their own tunables, they should do so in a different top | |
19 | namespace by overriding the TOP_NAMESPACE macro for that tunable. Downstream | |
20 | implementations are discouraged from using the 'glibc' top namespace for | |
21 | tunables they don't already have consensus to push upstream. | |
22 | ||
44330b6d | 23 | There are three steps to adding a tunable: |
67e58f39 | 24 | |
44330b6d | 25 | 1. Add a tunable to the list and fully specify its properties: |
67e58f39 | 26 | |
44330b6d SP |
27 | For each tunable you want to add, make an entry in elf/dl-tunables.list. The |
28 | format of the file is as follows: | |
67e58f39 SP |
29 | |
30 | TOP_NAMESPACE { | |
31 | NAMESPACE1 { | |
32 | TUNABLE1 { | |
33 | # tunable attributes, one per line | |
34 | } | |
35 | # A tunable with default attributes, i.e. string variable. | |
36 | TUNABLE2 | |
37 | TUNABLE3 { | |
38 | # its attributes | |
39 | } | |
40 | } | |
41 | NAMESPACE2 { | |
42 | ... | |
43 | } | |
44 | } | |
45 | ||
46 | The list of allowed attributes are: | |
47 | ||
48 | - type: Data type. Defaults to STRING. Allowed types are: | |
ad2f35cb SP |
49 | INT_32, UINT_64, SIZE_T and STRING. Numeric types may |
50 | be in octal or hexadecimal format too. | |
67e58f39 SP |
51 | |
52 | - minval: Optional minimum acceptable value. For a string type | |
53 | this is the minimum length of the value. | |
54 | ||
55 | - maxval: Optional maximum acceptable value. For a string type | |
56 | this is the maximum length of the value. | |
57 | ||
d1310307 SP |
58 | - default: Specify an optional default value for the tunable. |
59 | ||
67e58f39 SP |
60 | - env_alias: An alias environment variable |
61 | ||
6c2b5799 SP |
62 | - security_level: Specify security level of the tunable for AT_SECURE |
63 | binaries. Valid values are: | |
65eff7fb | 64 | |
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65 | SXID_ERASE: (default) Do not read and do not pass on to |
66 | child processes. | |
67 | SXID_IGNORE: Do not read, but retain for non-AT_SECURE | |
68 | child processes. | |
65eff7fb | 69 | NONE: Read all the time. |
67e58f39 | 70 | |
dfb8e514 | 71 | 2. Use TUNABLE_GET/TUNABLE_SET/TUNABLE_SET_WITH_BOUNDS to get and set tunables. |
44330b6d SP |
72 | |
73 | 3. OPTIONAL: If tunables in a namespace are being used multiple times within a | |
74 | specific module, set the TUNABLE_NAMESPACE macro to reduce the amount of | |
75 | typing. | |
76 | ||
77 | GETTING AND SETTING TUNABLES | |
78 | ---------------------------- | |
79 | ||
80 | When the TUNABLE_NAMESPACE macro is defined, one may get tunables in that | |
81 | module using the TUNABLE_GET macro as follows: | |
82 | ||
83 | val = TUNABLE_GET (check, int32_t, TUNABLE_CALLBACK (check_callback)) | |
84 | ||
85 | where 'check' is the tunable name, 'int32_t' is the C type of the tunable and | |
86 | 'check_callback' is the function to call if the tunable got initialized to a | |
87 | non-default value. The macro returns the value as type 'int32_t'. | |
88 | ||
89 | The callback function should be defined as follows: | |
90 | ||
91 | void | |
92 | TUNABLE_CALLBACK (check_callback) (int32_t *valp) | |
93 | { | |
94 | ... | |
95 | } | |
96 | ||
97 | where it can expect the tunable value to be passed in VALP. | |
98 | ||
99 | Tunables in the module can be updated using: | |
100 | ||
61117bfa | 101 | TUNABLE_SET (check, val) |
44330b6d | 102 | |
61117bfa | 103 | where 'check' is the tunable name and 'val' is a value of same type. |
44330b6d SP |
104 | |
105 | To get and set tunables in a different namespace from that module, use the full | |
106 | form of the macros as follows: | |
107 | ||
dce452dc | 108 | val = TUNABLE_GET_FULL (glibc, cpu, hwcap_mask, uint64_t, NULL) |
44330b6d | 109 | |
61117bfa | 110 | TUNABLE_SET_FULL (glibc, cpu, hwcap_mask, val) |
44330b6d | 111 | |
dce452dc | 112 | where 'glibc' is the top namespace, 'cpu' is the tunable namespace and the |
44330b6d SP |
113 | remaining arguments are the same as the short form macros. |
114 | ||
dfb8e514 L |
115 | The minimum and maximum values can updated together with the tunable value |
116 | using: | |
117 | ||
61117bfa | 118 | TUNABLE_SET_WITH_BOUNDS (check, val, min, max) |
dfb8e514 | 119 | |
61117bfa SP |
120 | where 'check' is the tunable name, 'val' is a value of same type, 'min' and |
121 | 'max' are the minimum and maximum values of the tunable. | |
dfb8e514 L |
122 | |
123 | To set the minimum and maximum values of tunables in a different namespace | |
124 | from that module, use the full form of the macros as follows: | |
125 | ||
126 | val = TUNABLE_GET_FULL (glibc, cpu, hwcap_mask, uint64_t, NULL) | |
127 | ||
61117bfa | 128 | TUNABLE_SET_WITH_BOUNDS_FULL (glibc, cpu, hwcap_mask, val, min, max) |
dfb8e514 L |
129 | |
130 | where 'glibc' is the top namespace, 'cpu' is the tunable namespace and the | |
131 | remaining arguments are the same as the short form macros. | |
132 | ||
44330b6d SP |
133 | When TUNABLE_NAMESPACE is not defined in a module, TUNABLE_GET is equivalent to |
134 | TUNABLE_GET_FULL, so you will need to provide full namespace information for | |
dfb8e514 L |
135 | both macros. Likewise for TUNABLE_SET, TUNABLE_SET_FULL, |
136 | TUNABLE_SET_WITH_BOUNDS and TUNABLE_SET_WITH_BOUNDS_FULL. | |
44330b6d SP |
137 | |
138 | ** IMPORTANT NOTE ** | |
139 | ||
140 | The tunable list is set as read-only after the dynamic linker relocates itself, | |
141 | so setting tunable values must be limited only to tunables within the dynamic | |
142 | linker, that too before relocation. | |
67e58f39 SP |
143 | |
144 | FUTURE WORK | |
145 | ----------- | |
146 | ||
147 | The framework currently only allows a one-time initialization of variables | |
148 | through environment variables and in some cases, modification of variables via | |
149 | an API call. A future goals for this project include: | |
150 | ||
151 | - Setting system-wide and user-wide defaults for tunables through some | |
152 | mechanism like a configuration file. | |
153 | ||
154 | - Allow tweaking of some tunables at runtime |