percpu: relegate chunks unusable when failing small allocations

In certain cases, requestors of percpu memory may want specific
alignments. However, it is possible to end up in situations where the
contig_hint matches, but the alignment does not. This causes excess
scanning of chunks that will fail. To prevent this, if a small
allocation fails (< 32B), the chunk is moved to the empty list. Once an
allocation is freed from that chunk, it is placed back into rotation.
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>

mm/percpu.c

@@ -94,6 +94,8 @@
 
 /* the slots are sorted by free bytes left, 1-31 bytes share the same slot */
 #define PCPU_SLOT_BASE_SHIFT		5
+/* chunks in slots below this are subject to being sidelined on failed alloc */
+#define PCPU_SLOT_FAIL_THRESHOLD	3
 
 #define PCPU_EMPTY_POP_PAGES_LOW	2
 #define PCPU_EMPTY_POP_PAGES_HIGH	4
@@ -488,6 +490,22 @@ static void pcpu_mem_free(void *ptr)
 	kvfree(ptr);
 }
 
+static void __pcpu_chunk_move(struct pcpu_chunk *chunk, int slot,
+			      bool move_front)
+{
+	if (chunk != pcpu_reserved_chunk) {
+		if (move_front)
+			list_move(&chunk->list, &pcpu_slot[slot]);
+		else
+			list_move_tail(&chunk->list, &pcpu_slot[slot]);
+	}
+}
+
+static void pcpu_chunk_move(struct pcpu_chunk *chunk, int slot)
+{
+	__pcpu_chunk_move(chunk, slot, true);
+}
+
 /**
  * pcpu_chunk_relocate - put chunk in the appropriate chunk slot
  * @chunk: chunk of interest
@@ -505,12 +523,8 @@ static void pcpu_chunk_relocate(struct pcpu_chunk *chunk, int oslot)
 {
 	int nslot = pcpu_chunk_slot(chunk);
 
-	if (chunk != pcpu_reserved_chunk && oslot != nslot) {
-		if (oslot < nslot)
-			list_move(&chunk->list, &pcpu_slot[nslot]);
-		else
-			list_move_tail(&chunk->list, &pcpu_slot[nslot]);
-	}
+	if (oslot != nslot)
+		__pcpu_chunk_move(chunk, nslot, oslot < nslot);
 }
 
 /**
@@ -1395,7 +1409,7 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
 	bool is_atomic = (gfp & GFP_KERNEL) != GFP_KERNEL;
 	bool do_warn = !(gfp & __GFP_NOWARN);
 	static int warn_limit = 10;
-	struct pcpu_chunk *chunk;
+	struct pcpu_chunk *chunk, *next;
 	const char *err;
 	int slot, off, cpu, ret;
 	unsigned long flags;
@@ -1457,11 +1471,14 @@ static void __percpu *pcpu_alloc(size_t size, size_t align, bool reserved,
 restart:
 	/* search through normal chunks */
 	for (slot = pcpu_size_to_slot(size); slot < pcpu_nr_slots; slot++) {
-		list_for_each_entry(chunk, &pcpu_slot[slot], list) {
+		list_for_each_entry_safe(chunk, next, &pcpu_slot[slot], list) {
 			off = pcpu_find_block_fit(chunk, bits, bit_align,
 						  is_atomic);
-			if (off < 0)
+			if (off < 0) {
+				if (slot < PCPU_SLOT_FAIL_THRESHOLD)
+					pcpu_chunk_move(chunk, 0);
 				continue;
+			}
 
 			off = pcpu_alloc_area(chunk, bits, bit_align, off);
 			if (off >= 0)