swiotlb: reduce area lock contention for non-primary IO TLB pools

If multiple areas and multiple IO TLB pools exist, first iterate the
current CPU specific area in all pools. Then move to the next area index.

This is best illustrated by a diagram:

        area 0 |  area 1 | ... | area M |
pool 0    A         B              C
pool 1    D         E
...
pool N    F         G              H

Currently, each pool is searched before moving on to the next pool,
i.e. the search order is A, B ... C, D, E ... F, G ... H. With this patch,
each area is searched in all pools before moving on to the next area,
i.e. the search order is A, D ... F, B, E ... G ... C ... H.

Note that preemption is not disabled, and raw_smp_processor_id() may not
return a stable result, but it is called only once to determine the initial
area index. The search will iterate over all areas eventually, even if the
current task is preempted.

Next, some pools may have less (but not more) areas than default_nareas.
Skip such pools if the distance from the initial area index is greater than
pool->nareas. This logic ensures that for every pool the search starts in
the initial CPU's own area and never tries any area twice.

To verify performance impact, I booted the kernel with a minimum pool
size ("swiotlb=512,4,force"), so multiple pools get allocated, and I ran
these benchmarks:

- small: single-threaded I/O of 4 KiB blocks,
- big: single-threaded I/O of 64 KiB blocks,
- 4way: 4-way parallel I/O of 4 KiB blocks.

The "var" column in the tables below is the coefficient of variance over 5
runs of the test, the "diff" column is the relative difference against base
in read-write I/O bandwidth (MiB/s).

Tested on an x86 VM against a QEMU virtio SATA driver backed by a RAM-based
block device on the host:

	base	   patched
	var	var	diff
small	0.69%	0.62%	+25.4%
big	2.14%	2.27%	+25.7%
4way	2.65%	1.70%	+23.6%

Tested on a Raspberry Pi against a class-10 A1 microSD card:

	base	   patched
	var	var	diff
small	0.53%	1.96%	-0.3%
big	0.02%	0.57%	+0.8%
4way	6.17%	0.40%	+0.3%

These results confirm that there is significant performance boost in the
software IO TLB slot allocation itself. Where performance is dominated by
actual hardware, there is no measurable change.

Signed-off-by: Petr Tesarik <petr.tesarik1@huawei-partners.com>
Reviewed-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Signed-off-by: Christoph Hellwig <hch@lst.de>

Author: Petr Tesarik

kernel/dma/swiotlb.c

@@ -957,7 +957,7 @@ static void dec_used(struct io_tlb_mem *mem, unsigned int nslots)
 #endif /* CONFIG_DEBUG_FS */
 
 /**
- * swiotlb_area_find_slots() - search for slots in one IO TLB memory area
+ * swiotlb_search_pool_area() - search one memory area in one pool
  * @dev:	Device which maps the buffer.
  * @pool:	Memory pool to be searched.
  * @area_index:	Index of the IO TLB memory area to be searched.
@@ -972,7 +972,7 @@ static void dec_used(struct io_tlb_mem *mem, unsigned int nslots)
  *
  * Return: Index of the first allocated slot, or -1 on error.
  */
-static int swiotlb_area_find_slots(struct device *dev, struct io_tlb_pool *pool,
+static int swiotlb_search_pool_area(struct device *dev, struct io_tlb_pool *pool,
 		int area_index, phys_addr_t orig_addr, size_t alloc_size,
 		unsigned int alloc_align_mask)
 {
@@ -1066,41 +1066,50 @@ static int swiotlb_area_find_slots(struct device *dev, struct io_tlb_pool *pool,
 	return slot_index;
 }
 
+#ifdef CONFIG_SWIOTLB_DYNAMIC
+
 /**
- * swiotlb_pool_find_slots() - search for slots in one memory pool
+ * swiotlb_search_area() - search one memory area in all pools
  * @dev:	Device which maps the buffer.
- * @pool:	Memory pool to be searched.
+ * @start_cpu:	Start CPU number.
+ * @cpu_offset:	Offset from @start_cpu.
  * @orig_addr:	Original (non-bounced) IO buffer address.
  * @alloc_size: Total requested size of the bounce buffer,
  *		including initial alignment padding.
  * @alloc_align_mask:	Required alignment of the allocated buffer.
+ * @retpool:	Used memory pool, updated on return.
  *
- * Search through one memory pool to find a sequence of slots that match the
+ * Search one memory area in all pools for a sequence of slots that match the
  * allocation constraints.
  *
  * Return: Index of the first allocated slot, or -1 on error.
  */
-static int swiotlb_pool_find_slots(struct device *dev, struct io_tlb_pool *pool,
-		phys_addr_t orig_addr, size_t alloc_size,
-		unsigned int alloc_align_mask)
+static int swiotlb_search_area(struct device *dev, int start_cpu,
+		int cpu_offset, phys_addr_t orig_addr, size_t alloc_size,
+		unsigned int alloc_align_mask, struct io_tlb_pool **retpool)
 {
-	int start = raw_smp_processor_id() & (pool->nareas - 1);
-	int i = start, index;
-
-	do {
-		index = swiotlb_area_find_slots(dev, pool, i, orig_addr,
-						alloc_size, alloc_align_mask);
-		if (index >= 0)
-			return index;
-		if (++i >= pool->nareas)
-			i = 0;
-	} while (i != start);
+	struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
+	struct io_tlb_pool *pool;
+	int area_index;
+	int index = -1;
 
-	return -1;
+	rcu_read_lock();
+	list_for_each_entry_rcu(pool, &mem->pools, node) {
+		if (cpu_offset >= pool->nareas)
+			continue;
+		area_index = (start_cpu + cpu_offset) & (pool->nareas - 1);
+		index = swiotlb_search_pool_area(dev, pool, area_index,
+						 orig_addr, alloc_size,
+						 alloc_align_mask);
+		if (index >= 0) {
+			*retpool = pool;
+			break;
+		}
+	}
+	rcu_read_unlock();
+	return index;
 }
 
-#ifdef CONFIG_SWIOTLB_DYNAMIC
-
 /**
  * swiotlb_find_slots() - search for slots in the whole swiotlb
  * @dev:	Device which maps the buffer.
@@ -1124,18 +1133,17 @@ static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
 	unsigned long nslabs;
 	unsigned long flags;
 	u64 phys_limit;
+	int cpu, i;
 	int index;
 
-	rcu_read_lock();
-	list_for_each_entry_rcu(pool, &mem->pools, node) {
-		index = swiotlb_pool_find_slots(dev, pool, orig_addr,
-						alloc_size, alloc_align_mask);
-		if (index >= 0) {
-			rcu_read_unlock();
+	cpu = raw_smp_processor_id();
+	for (i = 0; i < default_nareas; ++i) {
+		index = swiotlb_search_area(dev, cpu, i, orig_addr, alloc_size,
+					    alloc_align_mask, &pool);
+		if (index >= 0)
 			goto found;
-		}
 	}
-	rcu_read_unlock();
+
 	if (!mem->can_grow)
 		return -1;
 
@@ -1148,8 +1156,8 @@ static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
 	if (!pool)
 		return -1;
 
-	index = swiotlb_pool_find_slots(dev, pool, orig_addr,
-					alloc_size, alloc_align_mask);
+	index = swiotlb_search_pool_area(dev, pool, 0, orig_addr,
+					 alloc_size, alloc_align_mask);
 	if (index < 0) {
 		swiotlb_dyn_free(&pool->rcu);
 		return -1;
@@ -1192,9 +1200,21 @@ static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
 		size_t alloc_size, unsigned int alloc_align_mask,
 		struct io_tlb_pool **retpool)
 {
-	*retpool = &dev->dma_io_tlb_mem->defpool;
-	return swiotlb_pool_find_slots(dev, *retpool,
-				       orig_addr, alloc_size, alloc_align_mask);
+	struct io_tlb_pool *pool;
+	int start, i;
+	int index;
+
+	*retpool = pool = &dev->dma_io_tlb_mem->defpool;
+	i = start = raw_smp_processor_id() & (pool->nareas - 1);
+	do {
+		index = swiotlb_search_pool_area(dev, pool, i, orig_addr,
+						 alloc_size, alloc_align_mask);
+		if (index >= 0)
+			return index;
+		if (++i >= pool->nareas)
+			i = 0;
+	} while (i != start);
+	return -1;
 }
 
 #endif /* CONFIG_SWIOTLB_DYNAMIC */