Re: [PATCH 1/5][RFC][CFT] percpu fixes, part 1

[Al Viro <viro@ZenIV.linux.org.uk>]

There's a missing piece in 2/3 (percpu: store offsets instead of lengths in
->map[]) - we need to round size up to multiple of 2 in pcpu_alloc().
Updated patch follows; if you want an incremental instead of replacement -
please, yell.

commit 0b0fc67ae4b574ec35c37b508b84329b99d07bf7
Author: Al Viro <viro@zeniv.linux.org.uk>
Date:   Thu Mar 6 21:13:18 2014 -0500

    percpu: store offsets instead of lengths in ->map[]
    
    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>

diff --git a/mm/percpu.c b/mm/percpu.c
index 592f289..648ccbf 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -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 */
@@ -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;
@@ -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;
 		}
 
@@ -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;
@@ -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);
 }
 
@@ -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;
@@ -682,6 +695,16 @@ 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 & 1))
+		size++;
+
 	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);
@@ -1312,9 +1335,13 @@ 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) {
@@ -1327,8 +1354,10 @@ 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 */