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The TUNABLE_SET interface took a primitive C type argument, which resulted in inconsistent type conversions internally due to incorrect dereferencing of types, especialy on 32-bit architectures. This change simplifies the TUNABLE setting logic along with the interfaces. Now all numeric tunable values are stored as signed numbers in tunable_num_t, which is intmax_t. All calls to set tunables cast the input value to its primitive type and then to tunable_num_t for storage. This relies on gcc-specific (although I suspect other compilers woul also do the same) unsigned to signed integer conversion semantics, i.e. the bit pattern is conserved. The reverse conversion is guaranteed by the standard.
155 lines
5.1 KiB
Plaintext
155 lines
5.1 KiB
Plaintext
TUNABLE FRAMEWORK
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=================
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Tunables is a feature in the GNU C Library that allows application authors and
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distribution maintainers to alter the runtime library behaviour to match their
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workload.
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The tunable framework allows modules within glibc to register variables that
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may be tweaked through an environment variable. It aims to enforce a strict
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namespace rule to bring consistency to naming of these tunable environment
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variables across the project. This document is a guide for glibc developers to
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add tunables to the framework.
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ADDING A NEW TUNABLE
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--------------------
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The TOP_NAMESPACE macro is defined by default as 'glibc'. If distributions
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intend to add their own tunables, they should do so in a different top
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namespace by overriding the TOP_NAMESPACE macro for that tunable. Downstream
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implementations are discouraged from using the 'glibc' top namespace for
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tunables they don't already have consensus to push upstream.
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There are three steps to adding a tunable:
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1. Add a tunable to the list and fully specify its properties:
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For each tunable you want to add, make an entry in elf/dl-tunables.list. The
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format of the file is as follows:
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TOP_NAMESPACE {
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NAMESPACE1 {
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TUNABLE1 {
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# tunable attributes, one per line
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}
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# A tunable with default attributes, i.e. string variable.
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TUNABLE2
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TUNABLE3 {
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# its attributes
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}
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}
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NAMESPACE2 {
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...
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}
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}
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The list of allowed attributes are:
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- type: Data type. Defaults to STRING. Allowed types are:
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INT_32, UINT_64, SIZE_T and STRING. Numeric types may
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be in octal or hexadecimal format too.
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- minval: Optional minimum acceptable value. For a string type
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this is the minimum length of the value.
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- maxval: Optional maximum acceptable value. For a string type
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this is the maximum length of the value.
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- default: Specify an optional default value for the tunable.
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- env_alias: An alias environment variable
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- security_level: Specify security level of the tunable for AT_SECURE
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binaries. Valid values are:
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SXID_ERASE: (default) Do not read and do not pass on to
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child processes.
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SXID_IGNORE: Do not read, but retain for non-AT_SECURE
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child processes.
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NONE: Read all the time.
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2. Use TUNABLE_GET/TUNABLE_SET/TUNABLE_SET_WITH_BOUNDS to get and set tunables.
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3. OPTIONAL: If tunables in a namespace are being used multiple times within a
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specific module, set the TUNABLE_NAMESPACE macro to reduce the amount of
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typing.
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GETTING AND SETTING TUNABLES
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----------------------------
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When the TUNABLE_NAMESPACE macro is defined, one may get tunables in that
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module using the TUNABLE_GET macro as follows:
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val = TUNABLE_GET (check, int32_t, TUNABLE_CALLBACK (check_callback))
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where 'check' is the tunable name, 'int32_t' is the C type of the tunable and
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'check_callback' is the function to call if the tunable got initialized to a
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non-default value. The macro returns the value as type 'int32_t'.
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The callback function should be defined as follows:
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void
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TUNABLE_CALLBACK (check_callback) (int32_t *valp)
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{
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...
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}
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where it can expect the tunable value to be passed in VALP.
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Tunables in the module can be updated using:
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TUNABLE_SET (check, val)
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where 'check' is the tunable name and 'val' is a value of same type.
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To get and set tunables in a different namespace from that module, use the full
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form of the macros as follows:
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val = TUNABLE_GET_FULL (glibc, cpu, hwcap_mask, uint64_t, NULL)
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TUNABLE_SET_FULL (glibc, cpu, hwcap_mask, val)
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where 'glibc' is the top namespace, 'cpu' is the tunable namespace and the
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remaining arguments are the same as the short form macros.
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The minimum and maximum values can updated together with the tunable value
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using:
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TUNABLE_SET_WITH_BOUNDS (check, val, min, max)
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where 'check' is the tunable name, 'val' is a value of same type, 'min' and
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'max' are the minimum and maximum values of the tunable.
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To set the minimum and maximum values of tunables in a different namespace
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from that module, use the full form of the macros as follows:
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val = TUNABLE_GET_FULL (glibc, cpu, hwcap_mask, uint64_t, NULL)
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TUNABLE_SET_WITH_BOUNDS_FULL (glibc, cpu, hwcap_mask, val, min, max)
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where 'glibc' is the top namespace, 'cpu' is the tunable namespace and the
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remaining arguments are the same as the short form macros.
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When TUNABLE_NAMESPACE is not defined in a module, TUNABLE_GET is equivalent to
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TUNABLE_GET_FULL, so you will need to provide full namespace information for
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both macros. Likewise for TUNABLE_SET, TUNABLE_SET_FULL,
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TUNABLE_SET_WITH_BOUNDS and TUNABLE_SET_WITH_BOUNDS_FULL.
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** IMPORTANT NOTE **
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The tunable list is set as read-only after the dynamic linker relocates itself,
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so setting tunable values must be limited only to tunables within the dynamic
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linker, that too before relocation.
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FUTURE WORK
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-----------
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The framework currently only allows a one-time initialization of variables
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through environment variables and in some cases, modification of variables via
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an API call. A future goals for this project include:
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- Setting system-wide and user-wide defaults for tunables through some
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mechanism like a configuration file.
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- Allow tweaking of some tunables at runtime
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