glibc/README.tunables
Siddhesh Poyarekar 67e58f3941 Add framework for tunables
The tunables framework allows us to uniformly manage and expose global
variables inside glibc as switches to users.  tunables/README has
instructions for glibc developers to add new tunables.

Tunables support can be enabled by passing the --enable-tunables
configure flag to the configure script.  This patch only adds a
framework and does not pose any limitations on how tunable values are
read from the user.  It also adds environment variables used in malloc
behaviour tweaking to the tunables framework as a PoC of the
compatibility interface.

	* manual/install.texi: Add --enable-tunables option.
	* INSTALL: Regenerate.
	* README.tunables: New file.
	* Makeconfig (CPPFLAGS): Define TOP_NAMESPACE.
	(before-compile): Generate dl-tunable-list.h early.
	* config.h.in: Add HAVE_TUNABLES.
	* config.make.in: Add have-tunables.
	* configure.ac: Add --enable-tunables option.
	* configure: Regenerate.
	* csu/init-first.c (__libc_init_first): Move
	__libc_init_secure earlier...
	* csu/init-first.c (LIBC_START_MAIN):... to here.
	Include dl-tunables.h, libc-internal.h.
	(LIBC_START_MAIN) [!SHARED]: Initialize tunables for static
	binaries.
	* elf/Makefile (dl-routines): Add dl-tunables.
	* elf/Versions (ld): Add __tunable_set_val to GLIBC_PRIVATE
	namespace.
	* elf/dl-support (_dl_nondynamic_init): Unset MALLOC_CHECK_
	only when !HAVE_TUNABLES.
	* elf/rtld.c (process_envvars): Likewise.
	* elf/dl-sysdep.c [HAVE_TUNABLES]: Include dl-tunables.h
	(_dl_sysdep_start): Call __tunables_init.
	* elf/dl-tunable-types.h: New file.
	* elf/dl-tunables.c: New file.
	* elf/dl-tunables.h: New file.
	* elf/dl-tunables.list: New file.
	* malloc/tst-malloc-usable-static.c: New test case.
	* malloc/Makefile (tests-static): Add it.
	* malloc/arena.c [HAVE_TUNABLES]: Include dl-tunables.h.
	Define TUNABLE_NAMESPACE.
	(DL_TUNABLE_CALLBACK (set_mallopt_check)): New function.
	(DL_TUNABLE_CALLBACK_FNDECL): New macro.  Use it to define
	callback functions.
	(ptmalloc_init): Set tunable values.
	* scripts/gen-tunables.awk: New file.
	* sysdeps/mach/hurd/dl-sysdep.c: Include dl-tunables.h.
	(_dl_sysdep_start): Call __tunables_init.
2016-12-31 23:49:24 +05:30

86 lines
2.9 KiB
Plaintext

TUNABLE FRAMEWORK
=================
Tunables is a feature in the GNU C Library that allows application authors and
distribution maintainers to alter the runtime library behaviour to match their
workload.
The tunable framework allows modules within glibc to register variables that
may be tweaked through an environment variable. It aims to enforce a strict
namespace rule to bring consistency to naming of these tunable environment
variables across the project. This document is a guide for glibc developers to
add tunables to the framework.
ADDING A NEW TUNABLE
--------------------
The TOP_NAMESPACE macro is defined by default as 'glibc'. If distributions
intend to add their own tunables, they should do so in a different top
namespace by overriding the TOP_NAMESPACE macro for that tunable. Downstream
implementations are discouraged from using the 'glibc' top namespace for
tunables they don't already have consensus to push upstream.
There are two steps to adding a tunable:
1. Add a tunable ID:
Modules that wish to use the tunables interface must define the
TUNABLE_NAMESPACE macro. Following this, for each tunable you want to
add, make an entry in elf/dl-tunables.list. The format of the file is as
follows:
TOP_NAMESPACE {
NAMESPACE1 {
TUNABLE1 {
# tunable attributes, one per line
}
# A tunable with default attributes, i.e. string variable.
TUNABLE2
TUNABLE3 {
# its attributes
}
}
NAMESPACE2 {
...
}
}
The list of allowed attributes are:
- type: Data type. Defaults to STRING. Allowed types are:
INT_32, SIZE_T and STRING.
- minval: Optional minimum acceptable value. For a string type
this is the minimum length of the value.
- maxval: Optional maximum acceptable value. For a string type
this is the maximum length of the value.
- env_alias: An alias environment variable
- is_secure: Specify whether the tunable should be read for setuid
binaries. True allows the tunable to be read for
setuid binaries while false disables it. Note that
even if this is set as true and the value is read, it
may not be used if it does not validate against the
acceptable values or is not considered safe by the
module.
2. Call either the TUNABLE_SET_VALUE and pass into it the tunable name and a
pointer to the variable that should be set with the tunable value.
If additional work needs to be done after setting the value, use the
TUNABLE_SET_VALUE_WITH_CALLBACK instead and additionally pass a pointer to
the function that should be called if the tunable value has been set.
FUTURE WORK
-----------
The framework currently only allows a one-time initialization of variables
through environment variables and in some cases, modification of variables via
an API call. A future goals for this project include:
- Setting system-wide and user-wide defaults for tunables through some
mechanism like a configuration file.
- Allow tweaking of some tunables at runtime