1. 15 5月, 2019 3 次提交
    • G
      lib/sort: avoid indirect calls to built-in swap · 8fb583c4
      George Spelvin 提交于
      Similar to what's being done in the net code, this takes advantage of
      the fact that most invocations use only a few common swap functions, and
      replaces indirect calls to them with (highly predictable) conditional
      branches.  (The downside, of course, is that if you *do* use a custom
      swap function, there are a few extra predicted branches on the code
      path.)
      
      This actually *shrinks* the x86-64 code, because it inlines the various
      swap functions inside do_swap, eliding function prologues & epilogues.
      
      x86-64 code size 767 -> 703 bytes (-64)
      
      Link: http://lkml.kernel.org/r/d10c5d4b393a1847f32f5b26f4bbaa2857140e1e.1552704200.git.lkml@sdf.orgSigned-off-by: NGeorge Spelvin <lkml@sdf.org>
      Acked-by: NAndrey Abramov <st5pub@yandex.ru>
      Acked-by: NRasmus Villemoes <linux@rasmusvillemoes.dk>
      Reviewed-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
      Cc: Daniel Wagner <daniel.wagner@siemens.com>
      Cc: Dave Chinner <dchinner@redhat.com>
      Cc: Don Mullis <don.mullis@gmail.com>
      Cc: Geert Uytterhoeven <geert@linux-m68k.org>
      Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
      8fb583c4
    • G
      lib/sort: use more efficient bottom-up heapsort variant · 22a241cc
      George Spelvin 提交于
      This uses fewer comparisons than the previous code (approaching half as
      many for large random inputs), but produces identical results; it
      actually performs the exact same series of swap operations.
      
      Specifically, it reduces the average number of compares from
        2*n*log2(n) - 3*n + o(n)
      to
          n*log2(n) + 0.37*n + o(n).
      
      This is still 1.63*n worse than glibc qsort() which manages n*log2(n) -
      1.26*n, but at least the leading coefficient is correct.
      
      Standard heapsort, when sifting down, performs two comparisons per
      level: one to find the greater child, and a second to see if the current
      node should be exchanged with that child.
      
      Bottom-up heapsort observes that it's better to postpone the second
      comparison and search for the leaf where -infinity would be sent to,
      then search back *up* for the current node's destination.
      
      Since sifting down usually proceeds to the leaf level (that's where half
      the nodes are), this does O(1) second comparisons rather than log2(n).
      That saves a lot of (expensive since Spectre) indirect function calls.
      
      The one time it's worse than the previous code is if there are large
      numbers of duplicate keys, when the top-down algorithm is O(n) and
      bottom-up is O(n log n).  For distinct keys, it's provably always
      better, doing 1.5*n*log2(n) + O(n) in the worst case.
      
      (The code is not significantly more complex.  This patch also merges the
      heap-building and -extracting sift-down loops, resulting in a net code
      size savings.)
      
      x86-64 code size 885 -> 767 bytes (-118)
      
      (I see the checkpatch complaint about "else if (n -= size)".  The
      alternative is significantly uglier.)
      
      Link: http://lkml.kernel.org/r/2de8348635a1a421a72620677898c7fd5bd4b19d.1552704200.git.lkml@sdf.orgSigned-off-by: NGeorge Spelvin <lkml@sdf.org>
      Acked-by: NAndrey Abramov <st5pub@yandex.ru>
      Acked-by: NRasmus Villemoes <linux@rasmusvillemoes.dk>
      Reviewed-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
      Cc: Daniel Wagner <daniel.wagner@siemens.com>
      Cc: Dave Chinner <dchinner@redhat.com>
      Cc: Don Mullis <don.mullis@gmail.com>
      Cc: Geert Uytterhoeven <geert@linux-m68k.org>
      Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
      22a241cc
    • G
      lib/sort: make swap functions more generic · 37d0ec34
      George Spelvin 提交于
      Patch series "lib/sort & lib/list_sort: faster and smaller", v2.
      
      Because CONFIG_RETPOLINE has made indirect calls much more expensive, I
      thought I'd try to reduce the number made by the library sort functions.
      
      The first three patches apply to lib/sort.c.
      
      Patch #1 is a simple optimization.  The built-in swap has special cases
      for aligned 4- and 8-byte objects.  But those are almost never used;
      most calls to sort() work on larger structures, which fall back to the
      byte-at-a-time loop.  This generalizes them to aligned *multiples* of 4
      and 8 bytes.  (If nothing else, it saves an awful lot of energy by not
      thrashing the store buffers as much.)
      
      Patch #2 grabs a juicy piece of low-hanging fruit.  I agree that nice
      simple solid heapsort is preferable to more complex algorithms (sorry,
      Andrey), but it's possible to implement heapsort with far fewer
      comparisons (50% asymptotically, 25-40% reduction for realistic sizes)
      than the way it's been done up to now.  And with some care, the code
      ends up smaller, as well.  This is the "big win" patch.
      
      Patch #3 adds the same sort of indirect call bypass that has been added
      to the net code of late.  The great majority of the callers use the
      builtin swap functions, so replace the indirect call to sort_func with a
      (highly preditable) series of if() statements.  Rather surprisingly,
      this decreased code size, as the swap functions were inlined and their
      prologue & epilogue code eliminated.
      
      lib/list_sort.c is a bit trickier, as merge sort is already close to
      optimal, and we don't want to introduce triumphs of theory over
      practicality like the Ford-Johnson merge-insertion sort.
      
      Patch #4, without changing the algorithm, chops 32% off the code size
      and removes the part[MAX_LIST_LENGTH+1] pointer array (and the
      corresponding upper limit on efficiently sortable input size).
      
      Patch #5 improves the algorithm.  The previous code is already optimal
      for power-of-two (or slightly smaller) size inputs, but when the input
      size is just over a power of 2, there's a very unbalanced final merge.
      
      There are, in the literature, several algorithms which solve this, but
      they all depend on the "breadth-first" merge order which was replaced by
      commit 835cc0c8 with a more cache-friendly "depth-first" order.
      Some hard thinking came up with a depth-first algorithm which defers
      merges as little as possible while avoiding bad merges.  This saves
      0.2*n compares, averaged over all sizes.
      
      The code size increase is minimal (64 bytes on x86-64, reducing the net
      savings to 26%), but the comments expanded significantly to document the
      clever algorithm.
      
      TESTING NOTES: I have some ugly user-space benchmarking code which I
      used for testing before moving this code into the kernel.  Shout if you
      want a copy.
      
      I'm running this code right now, with CONFIG_TEST_SORT and
      CONFIG_TEST_LIST_SORT, but I confess I haven't rebooted since the last
      round of minor edits to quell checkpatch.  I figure there will be at
      least one round of comments and final testing.
      
      This patch (of 5):
      
      Rather than having special-case swap functions for 4- and 8-byte
      objects, special-case aligned multiples of 4 or 8 bytes.  This speeds up
      most users of sort() by avoiding fallback to the byte copy loop.
      
      Despite what ca96ab85 ("lib/sort: Add 64 bit swap function") claims,
      very few users of sort() sort pointers (or pointer-sized objects); most
      sort structures containing at least two words.  (E.g.
      drivers/acpi/fan.c:acpi_fan_get_fps() sorts an array of 40-byte struct
      acpi_fan_fps.)
      
      The functions also got renamed to reflect the fact that they support
      multiple words.  In the great tradition of bikeshedding, the names were
      by far the most contentious issue during review of this patch series.
      
      x86-64 code size 872 -> 886 bytes (+14)
      
      With feedback from Andy Shevchenko, Rasmus Villemoes and Geert
      Uytterhoeven.
      
      Link: http://lkml.kernel.org/r/f24f932df3a7fa1973c1084154f1cea596bcf341.1552704200.git.lkml@sdf.orgSigned-off-by: NGeorge Spelvin <lkml@sdf.org>
      Acked-by: NAndrey Abramov <st5pub@yandex.ru>
      Acked-by: NRasmus Villemoes <linux@rasmusvillemoes.dk>
      Reviewed-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
      Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
      Cc: Geert Uytterhoeven <geert@linux-m68k.org>
      Cc: Daniel Wagner <daniel.wagner@siemens.com>
      Cc: Don Mullis <don.mullis@gmail.com>
      Cc: Dave Chinner <dchinner@redhat.com>
      Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
      37d0ec34
  2. 02 11月, 2017 1 次提交
    • G
      License cleanup: add SPDX GPL-2.0 license identifier to files with no license · b2441318
      Greg Kroah-Hartman 提交于
      Many source files in the tree are missing licensing information, which
      makes it harder for compliance tools to determine the correct license.
      
      By default all files without license information are under the default
      license of the kernel, which is GPL version 2.
      
      Update the files which contain no license information with the 'GPL-2.0'
      SPDX license identifier.  The SPDX identifier is a legally binding
      shorthand, which can be used instead of the full boiler plate text.
      
      This patch is based on work done by Thomas Gleixner and Kate Stewart and
      Philippe Ombredanne.
      
      How this work was done:
      
      Patches were generated and checked against linux-4.14-rc6 for a subset of
      the use cases:
       - file had no licensing information it it.
       - file was a */uapi/* one with no licensing information in it,
       - file was a */uapi/* one with existing licensing information,
      
      Further patches will be generated in subsequent months to fix up cases
      where non-standard license headers were used, and references to license
      had to be inferred by heuristics based on keywords.
      
      The analysis to determine which SPDX License Identifier to be applied to
      a file was done in a spreadsheet of side by side results from of the
      output of two independent scanners (ScanCode & Windriver) producing SPDX
      tag:value files created by Philippe Ombredanne.  Philippe prepared the
      base worksheet, and did an initial spot review of a few 1000 files.
      
      The 4.13 kernel was the starting point of the analysis with 60,537 files
      assessed.  Kate Stewart did a file by file comparison of the scanner
      results in the spreadsheet to determine which SPDX license identifier(s)
      to be applied to the file. She confirmed any determination that was not
      immediately clear with lawyers working with the Linux Foundation.
      
      Criteria used to select files for SPDX license identifier tagging was:
       - Files considered eligible had to be source code files.
       - Make and config files were included as candidates if they contained >5
         lines of source
       - File already had some variant of a license header in it (even if <5
         lines).
      
      All documentation files were explicitly excluded.
      
      The following heuristics were used to determine which SPDX license
      identifiers to apply.
      
       - when both scanners couldn't find any license traces, file was
         considered to have no license information in it, and the top level
         COPYING file license applied.
      
         For non */uapi/* files that summary was:
      
         SPDX license identifier                            # files
         ---------------------------------------------------|-------
         GPL-2.0                                              11139
      
         and resulted in the first patch in this series.
      
         If that file was a */uapi/* path one, it was "GPL-2.0 WITH
         Linux-syscall-note" otherwise it was "GPL-2.0".  Results of that was:
      
         SPDX license identifier                            # files
         ---------------------------------------------------|-------
         GPL-2.0 WITH Linux-syscall-note                        930
      
         and resulted in the second patch in this series.
      
       - if a file had some form of licensing information in it, and was one
         of the */uapi/* ones, it was denoted with the Linux-syscall-note if
         any GPL family license was found in the file or had no licensing in
         it (per prior point).  Results summary:
      
         SPDX license identifier                            # files
         ---------------------------------------------------|------
         GPL-2.0 WITH Linux-syscall-note                       270
         GPL-2.0+ WITH Linux-syscall-note                      169
         ((GPL-2.0 WITH Linux-syscall-note) OR BSD-2-Clause)    21
         ((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause)    17
         LGPL-2.1+ WITH Linux-syscall-note                      15
         GPL-1.0+ WITH Linux-syscall-note                       14
         ((GPL-2.0+ WITH Linux-syscall-note) OR BSD-3-Clause)    5
         LGPL-2.0+ WITH Linux-syscall-note                       4
         LGPL-2.1 WITH Linux-syscall-note                        3
         ((GPL-2.0 WITH Linux-syscall-note) OR MIT)              3
         ((GPL-2.0 WITH Linux-syscall-note) AND MIT)             1
      
         and that resulted in the third patch in this series.
      
       - when the two scanners agreed on the detected license(s), that became
         the concluded license(s).
      
       - when there was disagreement between the two scanners (one detected a
         license but the other didn't, or they both detected different
         licenses) a manual inspection of the file occurred.
      
       - In most cases a manual inspection of the information in the file
         resulted in a clear resolution of the license that should apply (and
         which scanner probably needed to revisit its heuristics).
      
       - When it was not immediately clear, the license identifier was
         confirmed with lawyers working with the Linux Foundation.
      
       - If there was any question as to the appropriate license identifier,
         the file was flagged for further research and to be revisited later
         in time.
      
      In total, over 70 hours of logged manual review was done on the
      spreadsheet to determine the SPDX license identifiers to apply to the
      source files by Kate, Philippe, Thomas and, in some cases, confirmation
      by lawyers working with the Linux Foundation.
      
      Kate also obtained a third independent scan of the 4.13 code base from
      FOSSology, and compared selected files where the other two scanners
      disagreed against that SPDX file, to see if there was new insights.  The
      Windriver scanner is based on an older version of FOSSology in part, so
      they are related.
      
      Thomas did random spot checks in about 500 files from the spreadsheets
      for the uapi headers and agreed with SPDX license identifier in the
      files he inspected. For the non-uapi files Thomas did random spot checks
      in about 15000 files.
      
      In initial set of patches against 4.14-rc6, 3 files were found to have
      copy/paste license identifier errors, and have been fixed to reflect the
      correct identifier.
      
      Additionally Philippe spent 10 hours this week doing a detailed manual
      inspection and review of the 12,461 patched files from the initial patch
      version early this week with:
       - a full scancode scan run, collecting the matched texts, detected
         license ids and scores
       - reviewing anything where there was a license detected (about 500+
         files) to ensure that the applied SPDX license was correct
       - reviewing anything where there was no detection but the patch license
         was not GPL-2.0 WITH Linux-syscall-note to ensure that the applied
         SPDX license was correct
      
      This produced a worksheet with 20 files needing minor correction.  This
      worksheet was then exported into 3 different .csv files for the
      different types of files to be modified.
      
      These .csv files were then reviewed by Greg.  Thomas wrote a script to
      parse the csv files and add the proper SPDX tag to the file, in the
      format that the file expected.  This script was further refined by Greg
      based on the output to detect more types of files automatically and to
      distinguish between header and source .c files (which need different
      comment types.)  Finally Greg ran the script using the .csv files to
      generate the patches.
      Reviewed-by: NKate Stewart <kstewart@linuxfoundation.org>
      Reviewed-by: NPhilippe Ombredanne <pombredanne@nexb.com>
      Reviewed-by: NThomas Gleixner <tglx@linutronix.de>
      Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
      b2441318
  3. 25 2月, 2017 1 次提交
  4. 26 6月, 2015 1 次提交
    • D
      lib/sort: Add 64 bit swap function · ca96ab85
      Daniel Wagner 提交于
      In case the call side is not providing a swap function, we either use a
      32 bit or a generic swap function.  When swapping around pointers on 64
      bit architectures falling back to use the generic swap function seems
      like an unnecessary waste.
      
      There at least 9 users ('sort' is of difficult to grep for) of sort()
      and all of them use the sort function without a customized swap
      function.  Furthermore, they are all using pointers to swap around:
      
      arch/x86/kernel/e820.c:sanitize_e820_map()
      arch/x86/mm/extable.c:sort_extable()
      drivers/acpi/fan.c:acpi_fan_get_fps()
      fs/btrfs/super.c:btrfs_descending_sort_devices()
      fs/xfs/libxfs/xfs_dir2_block.c:xfs_dir2_sf_to_block()
      kernel/range.c:clean_sort_range()
      mm/memcontrol.c:__mem_cgroup_usage_register_event()
      sound/pci/hda/hda_auto_parser.c:snd_hda_parse_pin_defcfg()
      sound/pci/hda/hda_auto_parser.c:sort_pins_by_sequence()
      
      Obviously, we could improve the swap for other sizes as well
      but this is overkill at this point.
      
      A simple test shows sorting a 400 element array (try to stay in one
      page) with either with u32_swap() or u64_swap() show that the theory
      actually works. This test was done on a x86_64 (Intel Xeon E5-4610)
      machine.
      
      - swap_32:
      
      NumSamples = 100; Min = 48.00; Max = 49.00
      Mean = 48.320000; Variance = 0.217600; SD = 0.466476; Median 48.000000
      each * represents a count of 1
         48.0000 -    48.1000 [    68]: ********************************************************************
         48.1000 -    48.2000 [     0]:
         48.2000 -    48.3000 [     0]:
         48.3000 -    48.4000 [     0]:
         48.4000 -    48.5000 [     0]:
         48.5000 -    48.6000 [     0]:
         48.6000 -    48.7000 [     0]:
         48.7000 -    48.8000 [     0]:
         48.8000 -    48.9000 [     0]:
         48.9000 -    49.0000 [    32]: ********************************
      
      - swap_64:
      
      NumSamples = 100; Min = 44.00; Max = 63.00
      Mean = 48.250000; Variance = 18.687500; SD = 4.322904; Median 47.000000
      each * represents a count of 1
         44.0000 -    45.9000 [    15]: ***************
         45.9000 -    47.8000 [    37]: *************************************
         47.8000 -    49.7000 [    39]: ***************************************
         49.7000 -    51.6000 [     0]:
         51.6000 -    53.5000 [     0]:
         53.5000 -    55.4000 [     0]:
         55.4000 -    57.3000 [     0]:
         57.3000 -    59.2000 [     1]: *
         59.2000 -    61.1000 [     3]: ***
         61.1000 -    63.0000 [     5]: *****
      
      - swap_72:
      
      NumSamples = 100; Min = 53.00; Max = 71.00
      Mean = 55.070000; Variance = 21.565100; SD = 4.643824; Median 53.000000
      each * represents a count of 1
         53.0000 -    54.8000 [    73]: *************************************************************************
         54.8000 -    56.6000 [     9]: *********
         56.6000 -    58.4000 [     9]: *********
         58.4000 -    60.2000 [     0]:
         60.2000 -    62.0000 [     0]:
         62.0000 -    63.8000 [     0]:
         63.8000 -    65.6000 [     0]:
         65.6000 -    67.4000 [     1]: *
         67.4000 -    69.2000 [     4]: ****
         69.2000 -    71.0000 [     4]: ****
      
      - test program:
      
      static int cmp_32(const void *a, const void *b)
      {
      	u32 l = *(u32 *)a;
      	u32 r = *(u32 *)b;
      
      	if (l < r)
      		return -1;
      	if (l > r)
      		return 1;
      	return 0;
      }
      
      static int cmp_64(const void *a, const void *b)
      {
      	u64 l = *(u64 *)a;
      	u64 r = *(u64 *)b;
      
      	if (l < r)
      		return -1;
      	if (l > r)
      		return 1;
      	return 0;
      }
      
      static int cmp_72(const void *a, const void *b)
      {
      	u32 l = get_unaligned((u32 *) a);
      	u32 r = get_unaligned((u32 *) b);
      
      	if (l < r)
      		return -1;
      	if (l > r)
      		return 1;
      	return 0;
      }
      
      static void init_array32(void *array)
      {
      	u32 *a = array;
      	int i;
      
      	a[0] = 3821;
      	for (i = 1; i < ARRAY_ELEMENTS; i++)
      		a[i] = next_pseudo_random32(a[i-1]);
      }
      
      static void init_array64(void *array)
      {
      	u64 *a = array;
      	int i;
      
      	a[0] = 3821;
      	for (i = 1; i < ARRAY_ELEMENTS; i++)
      		a[i] = next_pseudo_random32(a[i-1]);
      }
      
      static void init_array72(void *array)
      {
      	char *p;
      	u32 v;
      	int i;
      
      	v = 3821;
      	for (i = 0; i < ARRAY_ELEMENTS; i++) {
      		p = (char *)array + (i * 9);
      		put_unaligned(v, (u32*) p);
      		v = next_pseudo_random32(v);
      	}
      }
      
      static void sort_test(void (*init)(void *array),
      		      int (*cmp) (const void *, const void *),
      		      void *array, size_t size)
      {
      	ktime_t start, stop;
      	int i;
      
      	for (i = 0; i < 10000; i++) {
      		init(array);
      
      		local_irq_disable();
      		start = ktime_get();
      
      		sort(array, ARRAY_ELEMENTS, size, cmp, NULL);
      
      		stop = ktime_get();
      		local_irq_enable();
      
      		if (i > 10000 - 101)
      		  pr_info("%lld\n",  ktime_to_us(ktime_sub(stop, start)));
      	}
      }
      
      static void *create_array(size_t size)
      {
      	void *array;
      
      	array = kmalloc(ARRAY_ELEMENTS * size, GFP_KERNEL);
      	if (!array)
      		return NULL;
      
      	return array;
      }
      
      static int perform_test(size_t size)
      {
      	void *array;
      
      	array = create_array(size);
      	if (!array)
      		return -ENOMEM;
      
      	pr_info("test element size %d bytes\n", (int)size);
      	switch (size) {
      	case 4:
      		sort_test(init_array32, cmp_32, array, size);
      		break;
      	case 8:
      		sort_test(init_array64, cmp_64, array, size);
      		break;
      	case 9:
      		sort_test(init_array72, cmp_72, array, size);
      		break;
      	}
      	kfree(array);
      
      	return 0;
      }
      
      static int __init sort_tests_init(void)
      {
      	int err;
      
      	err = perform_test(sizeof(u32));
      	if (err)
      		return err;
      
      	err = perform_test(sizeof(u64));
      	if (err)
      		return err;
      
      	err = perform_test(sizeof(u64)+1);
      	if (err)
      		return err;
      
      	return 0;
      }
      
      static void __exit sort_tests_exit(void)
      {
      }
      
      module_init(sort_tests_init);
      module_exit(sort_tests_exit);
      
      MODULE_LICENSE("GPL v2");
      MODULE_AUTHOR("Daniel Wagner");
      MODULE_DESCRIPTION("sort perfomance tests");
      Signed-off-by: NDaniel Wagner <daniel.wagner@bmw-carit.de>
      Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
      Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
      Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
      ca96ab85
  5. 13 2月, 2015 2 次提交
  6. 09 1月, 2009 1 次提交
  7. 17 10月, 2007 1 次提交
  8. 12 2月, 2007 1 次提交
  9. 03 10月, 2006 1 次提交
  10. 31 10月, 2005 1 次提交
    • T
      [PATCH] fix missing includes · 4e57b681
      Tim Schmielau 提交于
      I recently picked up my older work to remove unnecessary #includes of
      sched.h, starting from a patch by Dave Jones to not include sched.h
      from module.h. This reduces the number of indirect includes of sched.h
      by ~300. Another ~400 pointless direct includes can be removed after
      this disentangling (patch to follow later).
      However, quite a few indirect includes need to be fixed up for this.
      
      In order to feed the patches through -mm with as little disturbance as
      possible, I've split out the fixes I accumulated up to now (complete for
      i386 and x86_64, more archs to follow later) and post them before the real
      patch.  This way this large part of the patch is kept simple with only
      adding #includes, and all hunks are independent of each other.  So if any
      hunk rejects or gets in the way of other patches, just drop it.  My scripts
      will pick it up again in the next round.
      Signed-off-by: NTim Schmielau <tim@physik3.uni-rostock.de>
      Signed-off-by: NAndrew Morton <akpm@osdl.org>
      Signed-off-by: NLinus Torvalds <torvalds@osdl.org>
      4e57b681
  11. 11 9月, 2005 1 次提交
  12. 06 5月, 2005 1 次提交
  13. 17 4月, 2005 1 次提交
    • L
      Linux-2.6.12-rc2 · 1da177e4
      Linus Torvalds 提交于
      Initial git repository build. I'm not bothering with the full history,
      even though we have it. We can create a separate "historical" git
      archive of that later if we want to, and in the meantime it's about
      3.2GB when imported into git - space that would just make the early
      git days unnecessarily complicated, when we don't have a lot of good
      infrastructure for it.
      
      Let it rip!
      1da177e4