README.tunables

   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
  23 There are three steps to adding a tunable:
  24
  25 1. Add a tunable to the list and fully specify its properties:
  26
  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:
  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:
  49                         INT_32, UINT_64, SIZE_T and STRING.  Numeric types may
  50                         be in octal or hexadecimal format too.
  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
  58 - default:              Specify an optional default value for the tunable.
  59
  60 - env_alias:            An alias environment variable
  61
  62 - security_level:       Specify security level of the tunable.  Valid values:
  63
  64                         SXID_ERASE: (default) Don't read for AT_SECURE binaries and
  65                                     removed so that child processes can't read it.
  66                         SXID_IGNORE: Don't read for AT_SECURE binaries, but retained for
  67                                      non-AT_SECURE subprocesses.
  68                         NONE: Read all the time.
  69
  70 2. Use TUNABLE_GET/TUNABLE_SET to get and set tunables.
  71
  72 3. OPTIONAL: If tunables in a namespace are being used multiple times within a
  73    specific module, set the TUNABLE_NAMESPACE macro to reduce the amount of
  74    typing.
  75
  76 GETTING AND SETTING TUNABLES
  77 ----------------------------
  78
  79 When the TUNABLE_NAMESPACE macro is defined, one may get tunables in that
  80 module using the TUNABLE_GET macro as follows:
  81
  82   val = TUNABLE_GET (check, int32_t, TUNABLE_CALLBACK (check_callback))
  83
  84 where 'check' is the tunable name, 'int32_t' is the C type of the tunable and
  85 'check_callback' is the function to call if the tunable got initialized to a
  86 non-default value.  The macro returns the value as type 'int32_t'.
  87
  88 The callback function should be defined as follows:
  89
  90   void
  91   TUNABLE_CALLBACK (check_callback) (int32_t *valp)
  92   {
  93   ...
  94   }
  95
  96 where it can expect the tunable value to be passed in VALP.
  97
  98 Tunables in the module can be updated using:
  99
 100   TUNABLE_SET (check, int32_t, val)
 101
 102 where 'check' is the tunable name, 'int32_t' is the C type of the tunable and
 103 'val' is a value of same type.
 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
 108   val = TUNABLE_GET_FULL (glibc, tune, hwcap_mask, uint64_t, NULL)
 109
 110   TUNABLE_SET_FULL (glibc, tune, hwcap_mask, uint64_t, val)
 111
 112 where 'glibc' is the top namespace, 'tune' is the tunable namespace and the
 113 remaining arguments are the same as the short form macros.
 114
 115 When TUNABLE_NAMESPACE is not defined in a module, TUNABLE_GET is equivalent to
 116 TUNABLE_GET_FULL, so you will need to provide full namespace information for
 117 both macros.  Likewise for TUNABLE_SET and TUNABLE_SET_FULL.
 118
 119 ** IMPORTANT NOTE **
 120
 121 The tunable list is set as read-only after the dynamic linker relocates itself,
 122 so setting tunable values must be limited only to tunables within the dynamic
 123 linker, that too before relocation.
 124
 125 FUTURE WORK
 126 -----------
 127
 128 The framework currently only allows a one-time initialization of variables
 129 through environment variables and in some cases, modification of variables via
 130 an API call.  A future goals for this project include:
 131
 132 - Setting system-wide and user-wide defaults for tunables through some
 133   mechanism like a configuration file.
 134
 135 - Allow tweaking of some tunables at runtime