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lib/list_sort: simplify and remove MAX_LIST_LENGTH_BITS 

Rather than a fixed-size array of pending sorted runs, use the ->prev
links to keep track of things.  This reduces stack usage, eliminates
some ugly overflow handling, and reduces the code size.

Also:
* merge() no longer needs to handle NULL inputs, so simplify.
* The same applies to merge_and_restore_back_links(), which is renamed
  to the less ponderous merge_final().  (It's a static helper function,
  so we don't need a super-descriptive name; comments will do.)
* Document the actual return value requirements on the (*cmp)()
  function; some callers are already using this feature.

x86-64 code size 1086 -> 739 bytes (-347)

(Yes, I see checkpatch complaining about no space after comma in
"__attribute__((nonnull(2,3,4,5)))".  Checkpatch is wrong.)

Feedback from Rasmus Villemoes, Andy Shevchenko and Geert Uytterhoeven.

[akpm@linux-foundation.org: remove __pure usage due to mysterious warning]
Link: http://lkml.kernel.org/r/f63c410e0ff76009c9b58e01027e751ff7fdb749.1552704200.git.lkml@sdf.org
Signed-off-by: George Spelvin <lkml@sdf.org>
Acked-by: Andrey Abramov <st5pub@yandex.ru>
Acked-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Daniel Wagner <daniel.wagner@siemens.com>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Don Mullis <don.mullis@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
George Spelvin 2019-05-14 15:42:58 -07:00 committed by Linus Torvalds
parent 8fb583c425
commit 043b3f7b63
2 changed files with 103 additions and 61 deletions
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1
include/linux/list_sort.h
View File

@ -6,6 +6,7 @@
struct list_head;
__attribute__((nonnull(2,3)))
void list_sort(void *priv, struct list_head *head,
int (*cmp)(void *priv, struct list_head *a,
struct list_head *b));

161
lib/list_sort.c
View File

@ -7,33 +7,41 @@
#include <linux/list_sort.h>
#include <linux/list.h>
#define MAX_LIST_LENGTH_BITS 20
typedef int __attribute__((nonnull(2,3))) (*cmp_func)(void *,
struct list_head const *, struct list_head const *);
/*
* Returns a list organized in an intermediate format suited
* to chaining of merge() calls: null-terminated, no reserved or
* sentinel head node, "prev" links not maintained.
*/
static struct list_head *merge(void *priv,
int (*cmp)(void *priv, struct list_head *a,
struct list_head *b),
__attribute__((nonnull(2,3,4)))
static struct list_head *merge(void *priv, cmp_func cmp,
struct list_head *a, struct list_head *b)
{
struct list_head head, *tail = &head;
struct list_head *head, **tail = &head;
while (a && b) {
for (;;) {
/* if equal, take 'a' -- important for sort stability */
if ((*cmp)(priv, a, b) <= 0) {
tail->next = a;
if (cmp(priv, a, b) <= 0) {
*tail = a;
tail = &a->next;
a = a->next;
if (!a) {
*tail = b;
break;
}
} else {
tail->next = b;
*tail = b;
tail = &b->next;
b = b->next;
if (!b) {
*tail = a;
break;
}
tail = tail->next;
}
tail->next = a?:b;
return head.next;
}
return head;
}
/*
@ -43,44 +51,52 @@ static struct list_head *merge(void *priv,
* prev-link restoration pass, or maintaining the prev links
* throughout.
*/
static void merge_and_restore_back_links(void *priv,
int (*cmp)(void *priv, struct list_head *a,
struct list_head *b),
struct list_head *head,
__attribute__((nonnull(2,3,4,5)))
static void merge_final(void *priv, cmp_func cmp, struct list_head *head,
struct list_head *a, struct list_head *b)
{
struct list_head *tail = head;
u8 count = 0;
while (a && b) {
for (;;) {
/* if equal, take 'a' -- important for sort stability */
if ((*cmp)(priv, a, b) <= 0) {
if (cmp(priv, a, b) <= 0) {
tail->next = a;
a->prev = tail;
tail = a;
a = a->next;
if (!a)
break;
} else {
tail->next = b;
b->prev = tail;
tail = b;
b = b->next;
if (!b) {
b = a;
break;
}
}
tail = tail->next;
}
tail->next = a ? : b;
/* Finish linking remainder of list b on to tail */
tail->next = b;
do {
/*
* In worst cases this loop may run many iterations.
* If the merge is highly unbalanced (e.g. the input is
* already sorted), this loop may run many iterations.
* Continue callbacks to the client even though no
* element comparison is needed, so the client's cmp()
* routine can invoke cond_resched() periodically.
*/
if (unlikely(!(++count)))
(*cmp)(priv, tail->next, tail->next);
tail->next->prev = tail;
tail = tail->next;
} while (tail->next);
if (unlikely(!++count))
cmp(priv, b, b);
b->prev = tail;
tail = b;
b = b->next;
} while (b);
/* And the final links to make a circular doubly-linked list */
tail->next = head;
head->prev = tail;
}
@ -91,55 +107,80 @@ static void merge_and_restore_back_links(void *priv,
* @head: the list to sort
* @cmp: the elements comparison function
*
* This function implements "merge sort", which has O(nlog(n))
* complexity.
* This function implements a bottom-up merge sort, which has O(nlog(n))
* complexity. We use depth-first order to take advantage of cacheing.
* (E.g. when we get to the fourth element, we immediately merge the
* first two 2-element lists.)
*
* The comparison function @cmp must return a negative value if @a
* should sort before @b, and a positive value if @a should sort after
* @b. If @a and @b are equivalent, and their original relative
* ordering is to be preserved, @cmp must return 0.
* The comparison funtion @cmp must return > 0 if @a should sort after
* @b ("@a > @b" if you want an ascending sort), and <= 0 if @a should
* sort before @b *or* their original order should be preserved. It is
* always called with the element that came first in the input in @a,
* and list_sort is a stable sort, so it is not necessary to distinguish
* the @a < @b and @a == @b cases.
*
* This is compatible with two styles of @cmp function:
* - The traditional style which returns <0 / =0 / >0, or
* - Returning a boolean 0/1.
* The latter offers a chance to save a few cycles in the comparison
* (which is used by e.g. plug_ctx_cmp() in block/blk-mq.c).
*
* A good way to write a multi-word comparison is
* if (a->high != b->high)
* return a->high > b->high;
* if (a->middle != b->middle)
* return a->middle > b->middle;
* return a->low > b->low;
*/
__attribute__((nonnull(2,3)))
void list_sort(void *priv, struct list_head *head,
int (*cmp)(void *priv, struct list_head *a,
struct list_head *b))
{
struct list_head *part[MAX_LIST_LENGTH_BITS+1]; /* sorted partial lists
-- last slot is a sentinel */
int lev; /* index into part[] */
int max_lev = 0;
struct list_head *list;
struct list_head *list = head->next, *pending = NULL;
size_t count = 0; /* Count of pending */
if (list_empty(head))
if (list == head->prev) /* Zero or one elements */
return;
memset(part, 0, sizeof(part));
/* Convert to a null-terminated singly-linked list. */
head->prev->next = NULL;
list = head->next;
while (list) {
/*
* Data structure invariants:
* - All lists are singly linked and null-terminated; prev
* pointers are not maintained.
* - pending is a prev-linked "list of lists" of sorted
* sublists awaiting further merging.
* - Each of the sorted sublists is power-of-two in size,
* corresponding to bits set in "count".
* - Sublists are sorted by size and age, smallest & newest at front.
*/
do {
size_t bits;
struct list_head *cur = list;
/* Extract the head of "list" as a single-element list "cur" */
list = list->next;
cur->next = NULL;
for (lev = 0; part[lev]; lev++) {
cur = merge(priv, cmp, part[lev], cur);
part[lev] = NULL;
}
if (lev > max_lev) {
if (unlikely(lev >= ARRAY_SIZE(part)-1)) {
printk_once(KERN_DEBUG "list too long for efficiency\n");
lev--;
}
max_lev = lev;
}
part[lev] = cur;
/* Do merges corresponding to set lsbits in count */
for (bits = count; bits & 1; bits >>= 1) {
cur = merge(priv, (cmp_func)cmp, pending, cur);
pending = pending->prev; /* Untouched by merge() */
}
/* And place the result at the head of "pending" */
cur->prev = pending;
pending = cur;
count++;
} while (list->next);
for (lev = 0; lev < max_lev; lev++)
if (part[lev])
list = merge(priv, cmp, part[lev], list);
merge_and_restore_back_links(priv, cmp, head, part[max_lev], list);
/* Now merge together last element with all pending lists */
while (pending->prev) {
list = merge(priv, (cmp_func)cmp, pending, list);
pending = pending->prev;
}
/* The final merge, rebuilding prev links */
merge_final(priv, (cmp_func)cmp, head, pending, list);
}
EXPORT_SYMBOL(list_sort);