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percpu: store offsets instead of lengths in ->map[]
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Current code keeps +-length for each area in chunk->map[].  It has
several unpleasant consequences:
	* even if we know that first 50 areas are all in use, allocation
still needs to go through all those areas just to sum their sizes, just
to get the offset of free one.
	* freeing needs to find the array entry refering to the area
in question; again, the need to sum the sizes until we reach the offset
we are interested in.  Note that offsets are monotonous, so simple
binary search would do here.

	New data representation: array of <offset,in-use flag> pairs.
Each pair is represented by one int - we use offset|1 for <offset, in use>
and offset for <offset, free> (we make sure that all offsets are even).
In the end we put a sentry entry - <total size, in use>.  The first
entry is <0, flag>; it would be possible to store together the flag
for Nth area and offset for N+1st, but that leads to much hairier code.

In other words, where the old variant would have
	4, -8, -4, 4, -12, 100
(4 bytes free, 8 in use, 4 in use, 4 free, 12 in use, 100 free) we store
	<0,0>, <4,1>, <12,1>, <16,0>, <20,1>, <32,0>, <132,1>
i.e.
	0, 5, 13, 16, 21, 32, 133

This commit switches to new data representation and takes care of a couple
of low-hanging fruits in free_pcpu_area() - one is the switch to binary
search, another is not doing two memmove() when one would do.  Speeding
the alloc side up (by keeping track of how many areas in the beginning are
known to be all in use) also becomes possible - that'll be done in the next
commit.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Tejun Heo <tj@kernel.org>
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Al Viro authored and Tejun Heo committed Mar 7, 2014
1 parent 706c16f commit 723ad1d
Showing 1 changed file with 81 additions and 55 deletions.
136 changes: 81 additions & 55 deletions mm/percpu.c
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Original file line number Diff line number Diff line change
Expand Up @@ -102,7 +102,7 @@ struct pcpu_chunk {
int free_size; /* free bytes in the chunk */
int contig_hint; /* max contiguous size hint */
void *base_addr; /* base address of this chunk */
int map_used; /* # of map entries used */
int map_used; /* # of map entries used before the sentry */
int map_alloc; /* # of map entries allocated */
int *map; /* allocation map */
void *data; /* chunk data */
Expand Down Expand Up @@ -356,11 +356,11 @@ static int pcpu_need_to_extend(struct pcpu_chunk *chunk)
{
int new_alloc;

if (chunk->map_alloc >= chunk->map_used + 2)
if (chunk->map_alloc >= chunk->map_used + 3)
return 0;

new_alloc = PCPU_DFL_MAP_ALLOC;
while (new_alloc < chunk->map_used + 2)
while (new_alloc < chunk->map_used + 3)
new_alloc *= 2;

return new_alloc;
Expand Down Expand Up @@ -441,19 +441,22 @@ static int pcpu_alloc_area(struct pcpu_chunk *chunk, int size, int align)
int oslot = pcpu_chunk_slot(chunk);
int max_contig = 0;
int i, off;
int *p;

for (i = 0, off = 0; i < chunk->map_used; off += abs(chunk->map[i++])) {
bool is_last = i + 1 == chunk->map_used;
for (i = 0, p = chunk->map; i < chunk->map_used; i++, p++) {
int head, tail;
int this_size;

off = *p;
if (off & 1)
continue;

/* extra for alignment requirement */
head = ALIGN(off, align) - off;
BUG_ON(i == 0 && head != 0);

if (chunk->map[i] < 0)
continue;
if (chunk->map[i] < head + size) {
max_contig = max(chunk->map[i], max_contig);
this_size = (p[1] & ~1) - off;
if (this_size < head + size) {
max_contig = max(this_size, max_contig);
continue;
}

Expand All @@ -463,55 +466,50 @@ static int pcpu_alloc_area(struct pcpu_chunk *chunk, int size, int align)
* than sizeof(int), which is very small but isn't too
* uncommon for percpu allocations.
*/
if (head && (head < sizeof(int) || chunk->map[i - 1] > 0)) {
if (chunk->map[i - 1] > 0)
chunk->map[i - 1] += head;
else {
chunk->map[i - 1] -= head;
if (head && (head < sizeof(int) || !(p[-1] & 1))) {
if (p[-1] & 1)
chunk->free_size -= head;
}
chunk->map[i] -= head;
off += head;
*p = off += head;
this_size -= head;
head = 0;
}

/* if tail is small, just keep it around */
tail = chunk->map[i] - head - size;
if (tail < sizeof(int))
tail = this_size - head - size;
if (tail < sizeof(int)) {
tail = 0;
size = this_size - head;
}

/* split if warranted */
if (head || tail) {
int nr_extra = !!head + !!tail;

/* insert new subblocks */
memmove(&chunk->map[i + nr_extra], &chunk->map[i],
memmove(p + nr_extra + 1, p + 1,
sizeof(chunk->map[0]) * (chunk->map_used - i));
chunk->map_used += nr_extra;

if (head) {
chunk->map[i + 1] = chunk->map[i] - head;
chunk->map[i] = head;
off += head;
i++;
*++p = off += head;
++i;
max_contig = max(head, max_contig);
}
if (tail) {
chunk->map[i] -= tail;
chunk->map[i + 1] = tail;
p[1] = off + size;
max_contig = max(tail, max_contig);
}
}

/* update hint and mark allocated */
if (is_last)
if (i + 1 == chunk->map_used)
chunk->contig_hint = max_contig; /* fully scanned */
else
chunk->contig_hint = max(chunk->contig_hint,
max_contig);

chunk->free_size -= chunk->map[i];
chunk->map[i] = -chunk->map[i];
chunk->free_size -= size;
*p |= 1;

pcpu_chunk_relocate(chunk, oslot);
return off;
Expand Down Expand Up @@ -539,34 +537,47 @@ static int pcpu_alloc_area(struct pcpu_chunk *chunk, int size, int align)
static void pcpu_free_area(struct pcpu_chunk *chunk, int freeme)
{
int oslot = pcpu_chunk_slot(chunk);
int i, off;

for (i = 0, off = 0; i < chunk->map_used; off += abs(chunk->map[i++]))
if (off == freeme)
break;
int off = 0;
unsigned i, j;
int to_free = 0;
int *p;

freeme |= 1; /* we are searching for <given offset, in use> pair */

i = 0;
j = chunk->map_used;
while (i != j) {
unsigned k = (i + j) / 2;
off = chunk->map[k];
if (off < freeme)
i = k + 1;
else if (off > freeme)
j = k;
else
i = j = k;
}
BUG_ON(off != freeme);
BUG_ON(chunk->map[i] > 0);

chunk->map[i] = -chunk->map[i];
chunk->free_size += chunk->map[i];
p = chunk->map + i;
*p = off &= ~1;
chunk->free_size += (p[1] & ~1) - off;

/* merge with next? */
if (!(p[1] & 1))
to_free++;
/* merge with previous? */
if (i > 0 && chunk->map[i - 1] >= 0) {
chunk->map[i - 1] += chunk->map[i];
chunk->map_used--;
memmove(&chunk->map[i], &chunk->map[i + 1],
(chunk->map_used - i) * sizeof(chunk->map[0]));
if (i > 0 && !(p[-1] & 1)) {
to_free++;
i--;
p--;
}
/* merge with next? */
if (i + 1 < chunk->map_used && chunk->map[i + 1] >= 0) {
chunk->map[i] += chunk->map[i + 1];
chunk->map_used--;
memmove(&chunk->map[i + 1], &chunk->map[i + 2],
(chunk->map_used - (i + 1)) * sizeof(chunk->map[0]));
if (to_free) {
chunk->map_used -= to_free;
memmove(p + 1, p + 1 + to_free,
(chunk->map_used - i) * sizeof(chunk->map[0]));
}

chunk->contig_hint = max(chunk->map[i], chunk->contig_hint);
chunk->contig_hint = max(chunk->map[i + 1] - chunk->map[i] - 1, chunk->contig_hint);
pcpu_chunk_relocate(chunk, oslot);
}

Expand All @@ -586,7 +597,9 @@ static struct pcpu_chunk *pcpu_alloc_chunk(void)
}

chunk->map_alloc = PCPU_DFL_MAP_ALLOC;
chunk->map[chunk->map_used++] = pcpu_unit_size;
chunk->map[0] = 0;
chunk->map[1] = pcpu_unit_size | 1;
chunk->map_used = 1;

INIT_LIST_HEAD(&chunk->list);
chunk->free_size = pcpu_unit_size;
Expand Down Expand Up @@ -682,6 +695,13 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved)
unsigned long flags;
void __percpu *ptr;

/*
* We want the lowest bit of offset available for in-use/free
* indicator.
*/
if (unlikely(align < 2))
align = 2;

if (unlikely(!size || size > PCPU_MIN_UNIT_SIZE || align > PAGE_SIZE)) {
WARN(true, "illegal size (%zu) or align (%zu) for "
"percpu allocation\n", size, align);
Expand Down Expand Up @@ -1312,9 +1332,13 @@ int __init pcpu_setup_first_chunk(const struct pcpu_alloc_info *ai,
}
schunk->contig_hint = schunk->free_size;

schunk->map[schunk->map_used++] = -ai->static_size;
schunk->map[0] = 1;
schunk->map[1] = ai->static_size;
schunk->map_used = 1;
if (schunk->free_size)
schunk->map[schunk->map_used++] = schunk->free_size;
schunk->map[++schunk->map_used] = 1 | (ai->static_size + schunk->free_size);
else
schunk->map[1] |= 1;

/* init dynamic chunk if necessary */
if (dyn_size) {
Expand All @@ -1327,8 +1351,10 @@ int __init pcpu_setup_first_chunk(const struct pcpu_alloc_info *ai,
bitmap_fill(dchunk->populated, pcpu_unit_pages);

dchunk->contig_hint = dchunk->free_size = dyn_size;
dchunk->map[dchunk->map_used++] = -pcpu_reserved_chunk_limit;
dchunk->map[dchunk->map_used++] = dchunk->free_size;
dchunk->map[0] = 1;
dchunk->map[1] = pcpu_reserved_chunk_limit;
dchunk->map[2] = (pcpu_reserved_chunk_limit + dchunk->free_size) | 1;
dchunk->map_used = 2;
}

/* link the first chunk in */
Expand Down

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