[RFC PATCH V4 1/1] swiotlb: Split up single swiotlb lock

From: Tianyu Lan
Date: Fri Jun 17 2022 - 10:48:01 EST

From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>

Traditionally swiotlb was not performance critical because it was only
used for slow devices. But in some setups, like TDX/SEV confidential
guests, all IO has to go through swiotlb. Currently swiotlb only has a
single lock. Under high IO load with multiple CPUs this can lead to
significat lock contention on the swiotlb lock.

This patch splits the swiotlb bounce buffer pool into individual areas
which have their own lock. Each CPU tries to allocate in its own area
first. Only if that fails does it search other areas. On freeing the
allocation is freed into the area where the memory was originally
allocated from.

Area number can be set via swiotlb_adjust_nareas() and swiotlb kernel
parameter.

This idea from Andi Kleen patch(https://github.com/intel/tdx/commit/4529b578
4c141782c72ec9bd9a92df2b68cb7d45).

Based-on-idea-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
---
.../admin-guide/kernel-parameters.txt | 4 +-
include/linux/swiotlb.h | 27 +++
kernel/dma/swiotlb.c | 202 ++++++++++++++----
3 files changed, 194 insertions(+), 39 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 8090130b544b..5d46271982d5 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5869,8 +5869,10 @@
it if 0 is given (See Documentation/admin-guide/cgroup-v1/memory.rst)

swiotlb= [ARM,IA-64,PPC,MIPS,X86]
- Format: { <int> | force | noforce }
+ Format: { <int> [,<int>] | force | noforce }
<int> -- Number of I/O TLB slabs
+ <int> -- Second integer after comma. Number of swiotlb
+ areas with their own lock. Must be power of 2.
force -- force using of bounce buffers even if they
wouldn't be automatically used by the kernel
noforce -- Never use bounce buffers (for debugging)
diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
index 7ed35dd3de6e..7157428cf3ac 100644
--- a/include/linux/swiotlb.h
+++ b/include/linux/swiotlb.h
@@ -62,6 +62,22 @@ dma_addr_t swiotlb_map(struct device *dev, phys_addr_t phys,
#ifdef CONFIG_SWIOTLB
extern enum swiotlb_force swiotlb_force;

+/**
+ * struct io_tlb_area - IO TLB memory area descriptor
+ *
+ * This is a single area with a single lock.
+ *
+ * @used: The number of used IO TLB block.
+ * @index: The slot index to start searching in this area for next round.
+ * @lock: The lock to protect the above data structures in the map and
+ * unmap calls.
+ */
+struct io_tlb_area {
+ unsigned long used;
+ unsigned int index;
+ spinlock_t lock;
+};
+
/**
* struct io_tlb_mem - IO TLB Memory Pool Descriptor
*
@@ -89,6 +105,8 @@ extern enum swiotlb_force swiotlb_force;
* @late_alloc: %true if allocated using the page allocator
* @force_bounce: %true if swiotlb bouncing is forced
* @for_alloc: %true if the pool is used for memory allocation
+ * @nareas: The area number in the pool.
+ * @area_nslabs: The slot number in the area.
*/
struct io_tlb_mem {
phys_addr_t start;
@@ -102,6 +120,9 @@ struct io_tlb_mem {
bool late_alloc;
bool force_bounce;
bool for_alloc;
+ unsigned int nareas;
+ unsigned int area_nslabs;
+ struct io_tlb_area *areas;
struct io_tlb_slot {
phys_addr_t orig_addr;
size_t alloc_size;
@@ -130,6 +151,7 @@ unsigned int swiotlb_max_segment(void);
size_t swiotlb_max_mapping_size(struct device *dev);
bool is_swiotlb_active(struct device *dev);
void __init swiotlb_adjust_size(unsigned long size);
+void __init swiotlb_adjust_nareas(unsigned int nareas);
#else
static inline void swiotlb_init(bool addressing_limited, unsigned int flags)
{
@@ -162,6 +184,11 @@ static inline bool is_swiotlb_active(struct device *dev)
static inline void swiotlb_adjust_size(unsigned long size)
{
}
+
+static inline void swiotlb_adjust_nareas(unsigned int nareas)
+{
+}
+
#endif /* CONFIG_SWIOTLB */

extern void swiotlb_print_info(void);
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index cb50f8d38360..139d08068912 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -62,6 +62,8 @@

#define INVALID_PHYS_ADDR (~(phys_addr_t)0)

+#define DEFAULT_NUM_AREAS 1
+
static bool swiotlb_force_bounce;
static bool swiotlb_force_disable;

@@ -70,6 +72,7 @@ struct io_tlb_mem io_tlb_default_mem;
phys_addr_t swiotlb_unencrypted_base;

static unsigned long default_nslabs = IO_TLB_DEFAULT_SIZE >> IO_TLB_SHIFT;
+static unsigned long default_nareas = DEFAULT_NUM_AREAS;

static int __init
setup_io_tlb_npages(char *str)
@@ -79,6 +82,10 @@ setup_io_tlb_npages(char *str)
default_nslabs =
ALIGN(simple_strtoul(str, &str, 0), IO_TLB_SEGSIZE);
}
+ if (*str == ',')
+ ++str;
+ if (isdigit(*str))
+ swiotlb_adjust_nareas(simple_strtoul(str, &str, 0));
if (*str == ',')
++str;
if (!strcmp(str, "force"))
@@ -103,6 +110,27 @@ unsigned long swiotlb_size_or_default(void)
return default_nslabs << IO_TLB_SHIFT;
}

+void __init swiotlb_adjust_nareas(unsigned int nareas)
+{
+ if (!is_power_of_2(nareas)) {
+ pr_err("swiotlb: Invalid areas parameter %d.\n", nareas);
+ return;
+ }
+
+ default_nareas = nareas;
+
+ pr_info("area num %d.\n", nareas);
+ /* Round up number of slabs to the next power of 2.
+ * The last area is going be smaller than the rest if
+ * default_nslabs is not power of two.
+ */
+ if (nareas > DEFAULT_NUM_AREAS) {
+ default_nslabs = roundup_pow_of_two(default_nslabs);
+ pr_info("SWIOTLB bounce buffer size roundup to %luMB",
+ (default_nslabs << IO_TLB_SHIFT) >> 20);
+ }
+}
+
void __init swiotlb_adjust_size(unsigned long size)
{
/*
@@ -112,8 +140,18 @@ void __init swiotlb_adjust_size(unsigned long size)
*/
if (default_nslabs != IO_TLB_DEFAULT_SIZE >> IO_TLB_SHIFT)
return;
+
+ /* Round up number of slabs to the next power of 2.
+ * The last area is going be smaller than the rest if default_nslabs is
+ * not power of two.
+ */
size = ALIGN(size, IO_TLB_SIZE);
default_nslabs = ALIGN(size >> IO_TLB_SHIFT, IO_TLB_SEGSIZE);
+ if (default_nareas != DEFAULT_NUM_AREAS) {
+ default_nslabs = roundup_pow_of_two(default_nslabs);
+ size = default_nslabs << IO_TLB_SHIFT;
+ }
+
pr_info("SWIOTLB bounce buffer size adjusted to %luMB", size >> 20);
}

@@ -192,7 +230,8 @@ void __init swiotlb_update_mem_attributes(void)
}

static void swiotlb_init_io_tlb_mem(struct io_tlb_mem *mem, phys_addr_t start,
- unsigned long nslabs, unsigned int flags, bool late_alloc)
+ unsigned long nslabs, unsigned int flags,
+ bool late_alloc, unsigned int nareas)
{
void *vaddr = phys_to_virt(start);
unsigned long bytes = nslabs << IO_TLB_SHIFT, i;
@@ -202,10 +241,17 @@ static void swiotlb_init_io_tlb_mem(struct io_tlb_mem *mem, phys_addr_t start,
mem->end = mem->start + bytes;
mem->index = 0;
mem->late_alloc = late_alloc;
+ mem->nareas = nareas;
+ mem->area_nslabs = nslabs / mem->nareas;

mem->force_bounce = swiotlb_force_bounce || (flags & SWIOTLB_FORCE);

spin_lock_init(&mem->lock);
+ for (i = 0; i < mem->nareas; i++) {
+ spin_lock_init(&mem->areas[i].lock);
+ mem->areas[i].index = 0;
+ }
+
for (i = 0; i < mem->nslabs; i++) {
mem->slots[i].list = IO_TLB_SEGSIZE - io_tlb_offset(i);
mem->slots[i].orig_addr = INVALID_PHYS_ADDR;
@@ -274,7 +320,13 @@ void __init swiotlb_init_remap(bool addressing_limit, unsigned int flags,
panic("%s: Failed to allocate %zu bytes align=0x%lx\n",
__func__, alloc_size, PAGE_SIZE);

- swiotlb_init_io_tlb_mem(mem, __pa(tlb), nslabs, flags, false);
+ mem->areas = memblock_alloc(sizeof(struct io_tlb_area) *
+ default_nareas, SMP_CACHE_BYTES);
+ if (!mem->areas)
+ panic("%s: Failed to allocate mem->areas.\n", __func__);
+
+ swiotlb_init_io_tlb_mem(mem, __pa(tlb), nslabs, flags,
+ false, default_nareas);

if (flags & SWIOTLB_VERBOSE)
swiotlb_print_info();
@@ -296,7 +348,7 @@ int swiotlb_init_late(size_t size, gfp_t gfp_mask,
struct io_tlb_mem *mem = &io_tlb_default_mem;
unsigned long nslabs = ALIGN(size >> IO_TLB_SHIFT, IO_TLB_SEGSIZE);
unsigned char *vstart = NULL;
- unsigned int order;
+ unsigned int order, area_order;
bool retried = false;
int rc = 0;

@@ -337,19 +389,31 @@ int swiotlb_init_late(size_t size, gfp_t gfp_mask,
(PAGE_SIZE << order) >> 20);
}

+ area_order = get_order(array_size(sizeof(*mem->areas),
+ default_nareas));
+ mem->areas = (struct io_tlb_area *)
+ __get_free_pages(GFP_KERNEL | __GFP_ZERO, area_order);
+ if (!mem->areas)
+ goto error_area;
+
mem->slots = (void *)__get_free_pages(GFP_KERNEL | __GFP_ZERO,
get_order(array_size(sizeof(*mem->slots), nslabs)));
- if (!mem->slots) {
- free_pages((unsigned long)vstart, order);
- return -ENOMEM;
- }
+ if (!mem->slots)
+ goto error_slots;

set_memory_decrypted((unsigned long)vstart,
(nslabs << IO_TLB_SHIFT) >> PAGE_SHIFT);
- swiotlb_init_io_tlb_mem(mem, virt_to_phys(vstart), nslabs, 0, true);
+ swiotlb_init_io_tlb_mem(mem, virt_to_phys(vstart), nslabs, 0, true,
+ default_nareas);

swiotlb_print_info();
return 0;
+
+error_slots:
+ free_pages((unsigned long)mem->areas, area_order);
+error_area:
+ free_pages((unsigned long)vstart, order);
+ return -ENOMEM;
}

void __init swiotlb_exit(void)
@@ -357,6 +421,7 @@ void __init swiotlb_exit(void)
struct io_tlb_mem *mem = &io_tlb_default_mem;
unsigned long tbl_vaddr;
size_t tbl_size, slots_size;
+ unsigned int area_order;

if (swiotlb_force_bounce)
return;
@@ -371,9 +436,14 @@ void __init swiotlb_exit(void)

set_memory_encrypted(tbl_vaddr, tbl_size >> PAGE_SHIFT);
if (mem->late_alloc) {
+ area_order = get_order(array_size(sizeof(*mem->areas),
+ mem->nareas));
+ free_pages((unsigned long)mem->areas, area_order);
free_pages(tbl_vaddr, get_order(tbl_size));
free_pages((unsigned long)mem->slots, get_order(slots_size));
} else {
+ memblock_free_late(__pa(mem->areas),
+ mem->nareas * sizeof(struct io_tlb_area));
memblock_free_late(mem->start, tbl_size);
memblock_free_late(__pa(mem->slots), slots_size);
}
@@ -476,9 +546,9 @@ static inline unsigned long get_max_slots(unsigned long boundary_mask)
return nr_slots(boundary_mask + 1);
}

-static unsigned int wrap_index(struct io_tlb_mem *mem, unsigned int index)
+static unsigned int wrap_area_index(struct io_tlb_mem *mem, unsigned int index)
{
- if (index >= mem->nslabs)
+ if (index >= mem->area_nslabs)
return 0;
return index;
}
@@ -487,10 +557,13 @@ static unsigned int wrap_index(struct io_tlb_mem *mem, unsigned int index)
* Find a suitable number of IO TLB entries size that will fit this request and
* allocate a buffer from that IO TLB pool.
*/
-static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
- size_t alloc_size, unsigned int alloc_align_mask)
+static int swiotlb_do_find_slots(struct io_tlb_mem *mem,
+ struct io_tlb_area *area,
+ int area_index,
+ struct device *dev, phys_addr_t orig_addr,
+ size_t alloc_size,
+ unsigned int alloc_align_mask)
{
- struct io_tlb_mem *mem = dev->dma_io_tlb_mem;
unsigned long boundary_mask = dma_get_seg_boundary(dev);
dma_addr_t tbl_dma_addr =
phys_to_dma_unencrypted(dev, mem->start) & boundary_mask;
@@ -501,8 +574,11 @@ static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
unsigned int index, wrap, count = 0, i;
unsigned int offset = swiotlb_align_offset(dev, orig_addr);
unsigned long flags;
+ unsigned int slot_base;
+ unsigned int slot_index;

BUG_ON(!nslots);
+ BUG_ON(area_index >= mem->nareas);

/*
* For mappings with an alignment requirement don't bother looping to
@@ -514,16 +590,20 @@ static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
stride = max(stride, stride << (PAGE_SHIFT - IO_TLB_SHIFT));
stride = max(stride, (alloc_align_mask >> IO_TLB_SHIFT) + 1);

- spin_lock_irqsave(&mem->lock, flags);
- if (unlikely(nslots > mem->nslabs - mem->used))
+ spin_lock_irqsave(&area->lock, flags);
+ if (unlikely(nslots > mem->area_nslabs - area->used))
goto not_found;

- index = wrap = wrap_index(mem, ALIGN(mem->index, stride));
+ slot_base = area_index * mem->area_nslabs;
+ index = wrap = wrap_area_index(mem, ALIGN(area->index, stride));
+
do {
+ slot_index = slot_base + index;
+
if (orig_addr &&
- (slot_addr(tbl_dma_addr, index) & iotlb_align_mask) !=
- (orig_addr & iotlb_align_mask)) {
- index = wrap_index(mem, index + 1);
+ (slot_addr(tbl_dma_addr, slot_index) &
+ iotlb_align_mask) != (orig_addr & iotlb_align_mask)) {
+ index = wrap_area_index(mem, index + 1);
continue;
}

@@ -532,26 +612,26 @@ static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
* contiguous buffers, we allocate the buffers from that slot
* and mark the entries as '0' indicating unavailable.
*/
- if (!iommu_is_span_boundary(index, nslots,
+ if (!iommu_is_span_boundary(slot_index, nslots,
nr_slots(tbl_dma_addr),
max_slots)) {
- if (mem->slots[index].list >= nslots)
+ if (mem->slots[slot_index].list >= nslots)
goto found;
}
- index = wrap_index(mem, index + stride);
+ index = wrap_area_index(mem, index + stride);
} while (index != wrap);

not_found:
- spin_unlock_irqrestore(&mem->lock, flags);
+ spin_unlock_irqrestore(&area->lock, flags);
return -1;

found:
- for (i = index; i < index + nslots; i++) {
+ for (i = slot_index; i < slot_index + nslots; i++) {
mem->slots[i].list = 0;
mem->slots[i].alloc_size =
- alloc_size - (offset + ((i - index) << IO_TLB_SHIFT));
+ alloc_size - (offset + ((i - slot_index) << IO_TLB_SHIFT));
}
- for (i = index - 1;
+ for (i = slot_index - 1;
io_tlb_offset(i) != IO_TLB_SEGSIZE - 1 &&
mem->slots[i].list; i--)
mem->slots[i].list = ++count;
@@ -559,14 +639,43 @@ static int swiotlb_find_slots(struct device *dev, phys_addr_t orig_addr,
/*
* Update the indices to avoid searching in the next round.
*/
- if (index + nslots < mem->nslabs)
- mem->index = index + nslots;
+ if (index + nslots < mem->area_nslabs)
+ area->index = index + nslots;
else
- mem->index = 0;
- mem->used += nslots;
+ area->index = 0;
+ area->used += nslots;
+ spin_unlock_irqrestore(&area->lock, flags);
+ return slot_index;
+}

- spin_unlock_irqrestore(&mem->lock, flags);
- return index;
+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_mem *mem = dev->dma_io_tlb_mem;
+ int start = raw_smp_processor_id() & ((1U << __fls(mem->nareas)) - 1);
+ int i = start, index;
+
+ do {
+ index = swiotlb_do_find_slots(mem, mem->areas + i, i,
+ dev, orig_addr, alloc_size,
+ alloc_align_mask);
+ if (index >= 0)
+ return index;
+ if (++i >= mem->nareas)
+ i = 0;
+ } while (i != start);
+
+ return -1;
+}
+
+static unsigned long mem_used(struct io_tlb_mem *mem)
+{
+ int i;
+ unsigned long used = 0;
+
+ for (i = 0; i < mem->nareas; i++)
+ used += mem->areas[i].used;
+ return used;
}

phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
@@ -598,7 +707,7 @@ phys_addr_t swiotlb_tbl_map_single(struct device *dev, phys_addr_t orig_addr,
if (!(attrs & DMA_ATTR_NO_WARN))
dev_warn_ratelimited(dev,
"swiotlb buffer is full (sz: %zd bytes), total %lu (slots), used %lu (slots)\n",
- alloc_size, mem->nslabs, mem->used);
+ alloc_size, mem->nslabs, mem_used(mem));
return (phys_addr_t)DMA_MAPPING_ERROR;
}

@@ -628,6 +737,8 @@ static void swiotlb_release_slots(struct device *dev, phys_addr_t tlb_addr)
unsigned int offset = swiotlb_align_offset(dev, tlb_addr);
int index = (tlb_addr - offset - mem->start) >> IO_TLB_SHIFT;
int nslots = nr_slots(mem->slots[index].alloc_size + offset);
+ int aindex = index / mem->area_nslabs;
+ struct io_tlb_area *area = &mem->areas[aindex];
int count, i;

/*
@@ -636,7 +747,9 @@ static void swiotlb_release_slots(struct device *dev, phys_addr_t tlb_addr)
* While returning the entries to the free list, we merge the entries
* with slots below and above the pool being returned.
*/
- spin_lock_irqsave(&mem->lock, flags);
+ BUG_ON(aindex >= mem->nareas);
+
+ spin_lock_irqsave(&area->lock, flags);
if (index + nslots < ALIGN(index + 1, IO_TLB_SEGSIZE))
count = mem->slots[index + nslots].list;
else
@@ -660,8 +773,8 @@ static void swiotlb_release_slots(struct device *dev, phys_addr_t tlb_addr)
io_tlb_offset(i) != IO_TLB_SEGSIZE - 1 && mem->slots[i].list;
i--)
mem->slots[i].list = ++count;
- mem->used -= nslots;
- spin_unlock_irqrestore(&mem->lock, flags);
+ area->used -= nslots;
+ spin_unlock_irqrestore(&area->lock, flags);
}

/*
@@ -759,12 +872,14 @@ EXPORT_SYMBOL_GPL(is_swiotlb_active);
static void swiotlb_create_debugfs_files(struct io_tlb_mem *mem,
const char *dirname)
{
+ unsigned long used = mem_used(mem);
+
mem->debugfs = debugfs_create_dir(dirname, io_tlb_default_mem.debugfs);
if (!mem->nslabs)
return;

debugfs_create_ulong("io_tlb_nslabs", 0400, mem->debugfs, &mem->nslabs);
- debugfs_create_ulong("io_tlb_used", 0400, mem->debugfs, &mem->used);
+ debugfs_create_ulong("io_tlb_used", 0400, mem->debugfs, &used);
}

static int __init __maybe_unused swiotlb_create_default_debugfs(void)
@@ -815,6 +930,9 @@ static int rmem_swiotlb_device_init(struct reserved_mem *rmem,
struct io_tlb_mem *mem = rmem->priv;
unsigned long nslabs = rmem->size >> IO_TLB_SHIFT;

+ /* Set Per-device io tlb area to one */
+ unsigned int nareas = 1;
+
/*
* Since multiple devices can share the same pool, the private data,
* io_tlb_mem struct, will be initialized by the first device attached
@@ -831,10 +949,18 @@ static int rmem_swiotlb_device_init(struct reserved_mem *rmem,
return -ENOMEM;
}

+ mem->areas = kcalloc(nareas, sizeof(*mem->areas),
+ GFP_KERNEL);
+ if (!mem->areas) {
+ kfree(mem);
+ kfree(mem->slots);
+ return -ENOMEM;
+ }
+
set_memory_decrypted((unsigned long)phys_to_virt(rmem->base),
rmem->size >> PAGE_SHIFT);
swiotlb_init_io_tlb_mem(mem, rmem->base, nslabs, SWIOTLB_FORCE,
- false);
+ false, nareas);
mem->for_alloc = true;

rmem->priv = mem;
--
2.25.1