Merge tag 'docs-4.19' of git://git.lwn.net/linux
Pull documentation update from Jonathan Corbet: "This was a moderately busy cycle for docs, with the usual collection of small fixes and updates. We also have new ktime_get_*() docs from Arnd, some kernel-doc fixes, a new set of Italian translations (non so se vale la pena, ma non fa male - speriamo bene), and some extensive early memory-management documentation improvements from Mike Rapoport" * tag 'docs-4.19' of git://git.lwn.net/linux: (52 commits) Documentation: corrections to console/console.txt Documentation: add ioctl number entry for v4l2-subdev.h Remove gendered language from management style documentation scripts/kernel-doc: Escape all literal braces in regexes docs/mm: add description of boot time memory management docs/mm: memblock: add overview documentation docs/mm: memblock: add kernel-doc description for memblock types docs/mm: memblock: add kernel-doc comments for memblock_add[_node] docs/mm: memblock: update kernel-doc comments mm/memblock: add a name for memblock flags enumeration docs/mm: bootmem: add overview documentation docs/mm: bootmem: add kernel-doc description of 'struct bootmem_data' docs/mm: bootmem: fix kernel-doc warnings docs/mm: nobootmem: fixup kernel-doc comments mm/bootmem: drop duplicated kernel-doc comments Documentation: vm.txt: Adding 'nr_hugepages_mempolicy' parameter description. doc:it_IT: translation for kernel-hacking docs: Fix the reference labels in Locking.rst doc: tracing: Fix a typo of trace_stat mm: Introduce new type vm_fault_t ...
This commit is contained in:
Linus Torvalds
92
Documentation/core-api/boot-time-mm.rst
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Documentation/core-api/boot-time-mm.rst
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===========================
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Boot time memory management
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===========================
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Early system initialization cannot use "normal" memory management
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simply because it is not set up yet. But there is still need to
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allocate memory for various data structures, for instance for the
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physical page allocator. To address this, a specialized allocator
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called the :ref:`Boot Memory Allocator <bootmem>`, or bootmem, was
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introduced. Several years later PowerPC developers added a "Logical
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Memory Blocks" allocator, which was later adopted by other
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architectures and renamed to :ref:`memblock <memblock>`. There is also
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a compatibility layer called `nobootmem` that translates bootmem
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allocation interfaces to memblock calls.
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The selection of the early allocator is done using
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``CONFIG_NO_BOOTMEM`` and ``CONFIG_HAVE_MEMBLOCK`` kernel
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configuration options. These options are enabled or disabled
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statically by the architectures' Kconfig files.
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* Architectures that rely only on bootmem select
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``CONFIG_NO_BOOTMEM=n && CONFIG_HAVE_MEMBLOCK=n``.
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* The users of memblock with the nobootmem compatibility layer set
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``CONFIG_NO_BOOTMEM=y && CONFIG_HAVE_MEMBLOCK=y``.
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* And for those that use both memblock and bootmem the configuration
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includes ``CONFIG_NO_BOOTMEM=n && CONFIG_HAVE_MEMBLOCK=y``.
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Whichever allocator is used, it is the responsibility of the
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architecture specific initialization to set it up in
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:c:func:`setup_arch` and tear it down in :c:func:`mem_init` functions.
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Once the early memory management is available it offers a variety of
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functions and macros for memory allocations. The allocation request
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may be directed to the first (and probably the only) node or to a
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particular node in a NUMA system. There are API variants that panic
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when an allocation fails and those that don't. And more recent and
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advanced memblock even allows controlling its own behaviour.
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.. _bootmem:
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Bootmem
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=======
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(mostly stolen from Mel Gorman's "Understanding the Linux Virtual
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Memory Manager" `book`_)
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.. _book: https://www.kernel.org/doc/gorman/
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.. kernel-doc:: mm/bootmem.c
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:doc: bootmem overview
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.. _memblock:
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Memblock
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========
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.. kernel-doc:: mm/memblock.c
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:doc: memblock overview
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Functions and structures
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========================
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Common API
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----------
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The functions that are described in this section are available
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regardless of what early memory manager is enabled.
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.. kernel-doc:: mm/nobootmem.c
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Bootmem specific API
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--------------------
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These interfaces available only with bootmem, i.e when ``CONFIG_NO_BOOTMEM=n``
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.. kernel-doc:: include/linux/bootmem.h
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.. kernel-doc:: mm/bootmem.c
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:nodocs:
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Memblock specific API
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---------------------
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Here is the description of memblock data structures, functions and
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macros. Some of them are actually internal, but since they are
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documented it would be silly to omit them. Besides, reading the
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descriptions for the internal functions can help to understand what
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really happens under the hood.
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.. kernel-doc:: include/linux/memblock.h
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.. kernel-doc:: mm/memblock.c
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:nodocs:
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@@ -76,4 +76,6 @@ Functions and structures
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========================
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.. kernel-doc:: include/linux/idr.h
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:functions:
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.. kernel-doc:: lib/idr.c
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:functions:
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@@ -28,6 +28,8 @@ Core utilities
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printk-formats
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circular-buffers
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gfp_mask-from-fs-io
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timekeeping
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boot-time-mm
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Interfaces for kernel debugging
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===============================
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185
Documentation/core-api/timekeeping.rst
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Documentation/core-api/timekeeping.rst
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ktime accessors
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===============
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Device drivers can read the current time using ktime_get() and the many
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related functions declared in linux/timekeeping.h. As a rule of thumb,
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using an accessor with a shorter name is preferred over one with a longer
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name if both are equally fit for a particular use case.
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Basic ktime_t based interfaces
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------------------------------
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The recommended simplest form returns an opaque ktime_t, with variants
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that return time for different clock references:
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.. c:function:: ktime_t ktime_get( void )
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CLOCK_MONOTONIC
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Useful for reliable timestamps and measuring short time intervals
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accurately. Starts at system boot time but stops during suspend.
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.. c:function:: ktime_t ktime_get_boottime( void )
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CLOCK_BOOTTIME
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Like ktime_get(), but does not stop when suspended. This can be
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used e.g. for key expiration times that need to be synchronized
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with other machines across a suspend operation.
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.. c:function:: ktime_t ktime_get_real( void )
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CLOCK_REALTIME
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Returns the time in relative to the UNIX epoch starting in 1970
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using the Coordinated Universal Time (UTC), same as gettimeofday()
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user space. This is used for all timestamps that need to
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persist across a reboot, like inode times, but should be avoided
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for internal uses, since it can jump backwards due to a leap
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second update, NTP adjustment settimeofday() operation from user
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space.
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.. c:function:: ktime_t ktime_get_clocktai( void )
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CLOCK_TAI
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Like ktime_get_real(), but uses the International Atomic Time (TAI)
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reference instead of UTC to avoid jumping on leap second updates.
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This is rarely useful in the kernel.
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.. c:function:: ktime_t ktime_get_raw( void )
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CLOCK_MONOTONIC_RAW
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Like ktime_get(), but runs at the same rate as the hardware
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clocksource without (NTP) adjustments for clock drift. This is
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also rarely needed in the kernel.
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nanosecond, timespec64, and second output
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-----------------------------------------
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For all of the above, there are variants that return the time in a
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different format depending on what is required by the user:
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.. c:function:: u64 ktime_get_ns( void )
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u64 ktime_get_boottime_ns( void )
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u64 ktime_get_real_ns( void )
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u64 ktime_get_tai_ns( void )
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u64 ktime_get_raw_ns( void )
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Same as the plain ktime_get functions, but returning a u64 number
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of nanoseconds in the respective time reference, which may be
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more convenient for some callers.
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.. c:function:: void ktime_get_ts64( struct timespec64 * )
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void ktime_get_boottime_ts64( struct timespec64 * )
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void ktime_get_real_ts64( struct timespec64 * )
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void ktime_get_clocktai_ts64( struct timespec64 * )
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void ktime_get_raw_ts64( struct timespec64 * )
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Same above, but returns the time in a 'struct timespec64', split
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into seconds and nanoseconds. This can avoid an extra division
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when printing the time, or when passing it into an external
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interface that expects a 'timespec' or 'timeval' structure.
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.. c:function:: time64_t ktime_get_seconds( void )
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time64_t ktime_get_boottime_seconds( void )
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time64_t ktime_get_real_seconds( void )
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time64_t ktime_get_clocktai_seconds( void )
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time64_t ktime_get_raw_seconds( void )
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Return a coarse-grained version of the time as a scalar
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time64_t. This avoids accessing the clock hardware and rounds
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down the seconds to the full seconds of the last timer tick
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using the respective reference.
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Coarse and fast_ns access
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-------------------------
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Some additional variants exist for more specialized cases:
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.. c:function:: ktime_t ktime_get_coarse_boottime( void )
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ktime_t ktime_get_coarse_real( void )
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ktime_t ktime_get_coarse_clocktai( void )
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ktime_t ktime_get_coarse_raw( void )
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.. c:function:: void ktime_get_coarse_ts64( struct timespec64 * )
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void ktime_get_coarse_boottime_ts64( struct timespec64 * )
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void ktime_get_coarse_real_ts64( struct timespec64 * )
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void ktime_get_coarse_clocktai_ts64( struct timespec64 * )
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void ktime_get_coarse_raw_ts64( struct timespec64 * )
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These are quicker than the non-coarse versions, but less accurate,
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corresponding to CLOCK_MONONOTNIC_COARSE and CLOCK_REALTIME_COARSE
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in user space, along with the equivalent boottime/tai/raw
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timebase not available in user space.
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The time returned here corresponds to the last timer tick, which
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may be as much as 10ms in the past (for CONFIG_HZ=100), same as
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reading the 'jiffies' variable. These are only useful when called
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in a fast path and one still expects better than second accuracy,
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but can't easily use 'jiffies', e.g. for inode timestamps.
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Skipping the hardware clock access saves around 100 CPU cycles
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on most modern machines with a reliable cycle counter, but
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up to several microseconds on older hardware with an external
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clocksource.
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.. c:function:: u64 ktime_get_mono_fast_ns( void )
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u64 ktime_get_raw_fast_ns( void )
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u64 ktime_get_boot_fast_ns( void )
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u64 ktime_get_real_fast_ns( void )
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These variants are safe to call from any context, including from
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a non-maskable interrupt (NMI) during a timekeeper update, and
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while we are entering suspend with the clocksource powered down.
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This is useful in some tracing or debugging code as well as
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machine check reporting, but most drivers should never call them,
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since the time is allowed to jump under certain conditions.
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Deprecated time interfaces
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--------------------------
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Older kernels used some other interfaces that are now being phased out
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but may appear in third-party drivers being ported here. In particular,
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all interfaces returning a 'struct timeval' or 'struct timespec' have
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been replaced because the tv_sec member overflows in year 2038 on 32-bit
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architectures. These are the recommended replacements:
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.. c:function:: void ktime_get_ts( struct timespec * )
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Use ktime_get() or ktime_get_ts64() instead.
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.. c:function:: struct timeval do_gettimeofday( void )
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struct timespec getnstimeofday( void )
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struct timespec64 getnstimeofday64( void )
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void ktime_get_real_ts( struct timespec * )
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ktime_get_real_ts64() is a direct replacement, but consider using
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monotonic time (ktime_get_ts64()) and/or a ktime_t based interface
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(ktime_get()/ktime_get_real()).
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.. c:function:: struct timespec current_kernel_time( void )
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struct timespec64 current_kernel_time64( void )
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struct timespec get_monotonic_coarse( void )
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struct timespec64 get_monotonic_coarse64( void )
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These are replaced by ktime_get_coarse_real_ts64() and
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ktime_get_coarse_ts64(). However, A lot of code that wants
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coarse-grained times can use the simple 'jiffies' instead, while
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some drivers may actually want the higher resolution accessors
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these days.
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.. c:function:: struct timespec getrawmonotonic( void )
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struct timespec64 getrawmonotonic64( void )
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struct timespec timekeeping_clocktai( void )
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struct timespec64 timekeeping_clocktai64( void )
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struct timespec get_monotonic_boottime( void )
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struct timespec64 get_monotonic_boottime64( void )
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These are replaced by ktime_get_raw()/ktime_get_raw_ts64(),
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ktime_get_clocktai()/ktime_get_clocktai_ts64() as well
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as ktime_get_boottime()/ktime_get_boottime_ts64().
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However, if the particular choice of clock source is not
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important for the user, consider converting to
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ktime_get()/ktime_get_ts64() instead for consistency.
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