* [Qemu-devel] [PATCH 1/2] memory: RCU ram_list.dirty_memory[] for safe RAM hotplug
2016-01-25 13:33 [Qemu-devel] [PATCH 0/2] dataplane: RCU dirty memory bitmap for safe RAM hotplug Stefan Hajnoczi
@ 2016-01-25 13:33 ` Stefan Hajnoczi
2016-01-25 13:33 ` [Qemu-devel] [PATCH 2/2] virtio-blk: drop unused migration.h include Stefan Hajnoczi
2016-01-28 10:40 ` [Qemu-devel] [PATCH 0/2] dataplane: RCU dirty memory bitmap for safe RAM hotplug Stefan Hajnoczi
2 siblings, 0 replies; 6+ messages in thread
From: Stefan Hajnoczi @ 2016-01-25 13:33 UTC (permalink / raw)
To: qemu-devel; +Cc: Paolo Bonzini, Stefan Hajnoczi
Although accesses to ram_list.dirty_memory[] use atomics so multiple
threads can safely dirty the bitmap, the data structure is not fully
thread-safe yet.
This patch handles the RAM hotplug case where ram_list.dirty_memory[] is
grown. ram_list.dirty_memory[] is change from a regular bitmap to an
RCU array of pointers to fixed-size bitmap blocks. Threads can continue
accessing bitmap blocks while the array is being extended. See the
comments in the code for an in-depth explanation of struct
DirtyMemoryBlocks.
I have tested that live migration with virtio-blk dataplane works.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
Note this patch is based on Paolo's https://github.com/bonzini/qemu.git
dataplane branch.
---
exec.c | 73 +++++++++++++++----
include/exec/ram_addr.h | 189 ++++++++++++++++++++++++++++++++++++++++++------
migration/ram.c | 4 -
3 files changed, 223 insertions(+), 43 deletions(-)
diff --git a/exec.c b/exec.c
index 7f0ce42..25ecf79 100644
--- a/exec.c
+++ b/exec.c
@@ -945,8 +945,9 @@ bool cpu_physical_memory_test_and_clear_dirty(ram_addr_t start,
ram_addr_t length,
unsigned client)
{
+ DirtyMemoryBlocks *blocks;
unsigned long end, page;
- bool dirty;
+ bool dirty = false;
if (length == 0) {
return false;
@@ -954,8 +955,22 @@ bool cpu_physical_memory_test_and_clear_dirty(ram_addr_t start,
end = TARGET_PAGE_ALIGN(start + length) >> TARGET_PAGE_BITS;
page = start >> TARGET_PAGE_BITS;
- dirty = bitmap_test_and_clear_atomic(ram_list.dirty_memory[client],
- page, end - page);
+
+ rcu_read_lock();
+
+ blocks = atomic_rcu_read(&ram_list.dirty_memory[client]);
+
+ while (page < end) {
+ unsigned long idx = page / DIRTY_MEMORY_BLOCK_SIZE;
+ unsigned long offset = page % DIRTY_MEMORY_BLOCK_SIZE;
+ unsigned long num = MIN(end - page, DIRTY_MEMORY_BLOCK_SIZE - offset);
+
+ dirty |= bitmap_test_and_clear_atomic(blocks->blocks[idx],
+ offset, num);
+ page += num;
+ }
+
+ rcu_read_unlock();
if (dirty && tcg_enabled()) {
tlb_reset_dirty_range_all(start, length);
@@ -1469,6 +1484,45 @@ int qemu_ram_resize(ram_addr_t base, ram_addr_t newsize, Error **errp)
return 0;
}
+/* Called with ram_list.mutex held */
+static void dirty_memory_extend(ram_addr_t old_ram_size,
+ ram_addr_t new_ram_size)
+{
+ ram_addr_t old_num_blocks = DIV_ROUND_UP(old_ram_size,
+ DIRTY_MEMORY_BLOCK_SIZE);
+ ram_addr_t new_num_blocks = DIV_ROUND_UP(new_ram_size,
+ DIRTY_MEMORY_BLOCK_SIZE);
+ int i;
+
+ /* Only need to extend if block count increased */
+ if (new_num_blocks <= old_num_blocks) {
+ return;
+ }
+
+ for (i = 0; i < DIRTY_MEMORY_NUM; i++) {
+ DirtyMemoryBlocks *old_blocks;
+ DirtyMemoryBlocks *new_blocks;
+ int j;
+
+ old_blocks = atomic_rcu_read(&ram_list.dirty_memory[i]);
+ new_blocks = g_malloc(sizeof(*new_blocks) +
+ sizeof(new_blocks->blocks[0]) * new_num_blocks);
+
+ memcpy(new_blocks->blocks, old_blocks->blocks,
+ old_num_blocks * sizeof(old_blocks->blocks[0]));
+
+ for (j = old_num_blocks; j < new_num_blocks; j++) {
+ new_blocks->blocks[j] = bitmap_new(DIRTY_MEMORY_BLOCK_SIZE);
+ }
+
+ atomic_rcu_set(&ram_list.dirty_memory[i], new_blocks);
+
+ if (old_blocks) {
+ g_free_rcu(old_blocks, rcu);
+ }
+ }
+}
+
static ram_addr_t ram_block_add(RAMBlock *new_block, Error **errp)
{
RAMBlock *block;
@@ -1502,6 +1556,7 @@ static ram_addr_t ram_block_add(RAMBlock *new_block, Error **errp)
(new_block->offset + new_block->max_length) >> TARGET_PAGE_BITS);
if (new_ram_size > old_ram_size) {
migration_bitmap_extend(old_ram_size, new_ram_size);
+ dirty_memory_extend(old_ram_size, new_ram_size);
}
/* Keep the list sorted from biggest to smallest block. Unlike QTAILQ,
* QLIST (which has an RCU-friendly variant) does not have insertion at
@@ -1527,18 +1582,6 @@ static ram_addr_t ram_block_add(RAMBlock *new_block, Error **errp)
ram_list.version++;
qemu_mutex_unlock_ramlist();
- new_ram_size = last_ram_offset() >> TARGET_PAGE_BITS;
-
- if (new_ram_size > old_ram_size) {
- int i;
-
- /* ram_list.dirty_memory[] is protected by the iothread lock. */
- for (i = 0; i < DIRTY_MEMORY_NUM; i++) {
- ram_list.dirty_memory[i] =
- bitmap_zero_extend(ram_list.dirty_memory[i],
- old_ram_size, new_ram_size);
- }
- }
cpu_physical_memory_set_dirty_range(new_block->offset,
new_block->used_length,
DIRTY_CLIENTS_ALL);
diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
index ef1489d..534854c 100644
--- a/include/exec/ram_addr.h
+++ b/include/exec/ram_addr.h
@@ -45,13 +45,43 @@ static inline void *ramblock_ptr(RAMBlock *block, ram_addr_t offset)
return (char *)block->host + offset;
}
+/* The dirty memory bitmap is split into fixed-size blocks to allow growth
+ * under RCU. The bitmap for a block can be accessed as follows:
+ *
+ * rcu_read_lock();
+ *
+ * DirtyMemoryBlocks *blocks =
+ * atomic_rcu_read(&ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION]);
+ *
+ * ram_addr_t idx = (addr >> TARGET_PAGE_BITS) / DIRTY_MEMORY_BLOCK_SIZE;
+ * unsigned long *block = blocks.blocks[idx];
+ * ...access block bitmap...
+ *
+ * rcu_read_unlock();
+ *
+ * Remember to check for the end of the block when accessing a range of
+ * addresses. Move on to the next block if you reach the end.
+ *
+ * Organization into blocks allows dirty memory to grow (but not shrink) under
+ * RCU. When adding new RAMBlocks requires the dirty memory to grow, a new
+ * DirtyMemoryBlocks array is allocated with pointers to existing blocks kept
+ * the same. Other threads can safely access existing blocks while dirty
+ * memory is being grown. When no threads are using the old DirtyMemoryBlocks
+ * anymore it is freed by RCU (but the underlying blocks stay because they are
+ * pointed to from the new DirtyMemoryBlocks).
+ */
+#define DIRTY_MEMORY_BLOCK_SIZE ((ram_addr_t)256 * 1024 * 8)
+typedef struct {
+ struct rcu_head rcu;
+ unsigned long *blocks[];
+} DirtyMemoryBlocks;
+
typedef struct RAMList {
QemuMutex mutex;
- /* Protected by the iothread lock. */
- unsigned long *dirty_memory[DIRTY_MEMORY_NUM];
RAMBlock *mru_block;
/* RCU-enabled, writes protected by the ramlist lock. */
QLIST_HEAD(, RAMBlock) blocks;
+ DirtyMemoryBlocks *dirty_memory[DIRTY_MEMORY_NUM];
uint32_t version;
} RAMList;
extern RAMList ram_list;
@@ -85,30 +115,70 @@ static inline bool cpu_physical_memory_get_dirty(ram_addr_t start,
ram_addr_t length,
unsigned client)
{
- unsigned long end, page, next;
+ DirtyMemoryBlocks *blocks;
+ unsigned long end, page;
+ bool dirty = false;
assert(client < DIRTY_MEMORY_NUM);
end = TARGET_PAGE_ALIGN(start + length) >> TARGET_PAGE_BITS;
page = start >> TARGET_PAGE_BITS;
- next = find_next_bit(ram_list.dirty_memory[client], end, page);
- return next < end;
+ rcu_read_lock();
+
+ blocks = atomic_rcu_read(&ram_list.dirty_memory[client]);
+
+ while (page < end) {
+ unsigned long idx = page / DIRTY_MEMORY_BLOCK_SIZE;
+ unsigned long offset = page % DIRTY_MEMORY_BLOCK_SIZE;
+ unsigned long num = MIN(end - page, DIRTY_MEMORY_BLOCK_SIZE - offset);
+
+ if (find_next_bit(blocks->blocks[idx], offset, num) < num) {
+ dirty = true;
+ break;
+ }
+
+ page += num;
+ }
+
+ rcu_read_unlock();
+
+ return dirty;
}
static inline bool cpu_physical_memory_all_dirty(ram_addr_t start,
ram_addr_t length,
unsigned client)
{
- unsigned long end, page, next;
+ DirtyMemoryBlocks *blocks;
+ unsigned long end, page;
+ bool dirty = true;
assert(client < DIRTY_MEMORY_NUM);
end = TARGET_PAGE_ALIGN(start + length) >> TARGET_PAGE_BITS;
page = start >> TARGET_PAGE_BITS;
- next = find_next_zero_bit(ram_list.dirty_memory[client], end, page);
- return next >= end;
+ rcu_read_lock();
+
+ blocks = atomic_rcu_read(&ram_list.dirty_memory[client]);
+
+ while (page < end) {
+ unsigned long idx = page / DIRTY_MEMORY_BLOCK_SIZE;
+ unsigned long offset = page % DIRTY_MEMORY_BLOCK_SIZE;
+ unsigned long num = MIN(end - page, DIRTY_MEMORY_BLOCK_SIZE - offset);
+
+ if (find_next_zero_bit(blocks->blocks[idx], offset, num) < num) {
+ dirty = false;
+ break;
+ }
+
+ page += num;
+ }
+
+ rcu_read_unlock();
+
+ return dirty;
}
static inline bool cpu_physical_memory_get_dirty_flag(ram_addr_t addr,
@@ -150,28 +220,64 @@ static inline uint8_t cpu_physical_memory_range_includes_clean(ram_addr_t start,
static inline void cpu_physical_memory_set_dirty_flag(ram_addr_t addr,
unsigned client)
{
+ unsigned long page, idx, offset;
+ DirtyMemoryBlocks *blocks;
+
assert(client < DIRTY_MEMORY_NUM);
- set_bit_atomic(addr >> TARGET_PAGE_BITS, ram_list.dirty_memory[client]);
+
+ page = addr >> TARGET_PAGE_BITS;
+ idx = page / DIRTY_MEMORY_BLOCK_SIZE;
+ offset = page % DIRTY_MEMORY_BLOCK_SIZE;
+
+ rcu_read_lock();
+
+ blocks = atomic_rcu_read(&ram_list.dirty_memory[client]);
+
+ set_bit_atomic(offset, blocks->blocks[idx]);
+
+ rcu_read_unlock();
}
static inline void cpu_physical_memory_set_dirty_range(ram_addr_t start,
ram_addr_t length,
uint8_t mask)
{
+ DirtyMemoryBlocks *blocks[DIRTY_MEMORY_NUM];
unsigned long end, page;
- unsigned long **d = ram_list.dirty_memory;
+ int i;
end = TARGET_PAGE_ALIGN(start + length) >> TARGET_PAGE_BITS;
page = start >> TARGET_PAGE_BITS;
- if (likely(mask & (1 << DIRTY_MEMORY_MIGRATION))) {
- bitmap_set_atomic(d[DIRTY_MEMORY_MIGRATION], page, end - page);
+
+ rcu_read_lock();
+
+ for (i = 0; i < DIRTY_MEMORY_NUM; i++) {
+ blocks[i] = atomic_rcu_read(&ram_list.dirty_memory[i]);
}
- if (unlikely(mask & (1 << DIRTY_MEMORY_VGA))) {
- bitmap_set_atomic(d[DIRTY_MEMORY_VGA], page, end - page);
- }
- if (unlikely(mask & (1 << DIRTY_MEMORY_CODE))) {
- bitmap_set_atomic(d[DIRTY_MEMORY_CODE], page, end - page);
+
+ while (page < end) {
+ unsigned long idx = page / DIRTY_MEMORY_BLOCK_SIZE;
+ unsigned long offset = page % DIRTY_MEMORY_BLOCK_SIZE;
+ unsigned long num = MIN(end - page, DIRTY_MEMORY_BLOCK_SIZE - offset);
+
+ if (likely(mask & (1 << DIRTY_MEMORY_MIGRATION))) {
+ bitmap_set_atomic(blocks[DIRTY_MEMORY_MIGRATION]->blocks[idx],
+ offset, num);
+ }
+ if (unlikely(mask & (1 << DIRTY_MEMORY_VGA))) {
+ bitmap_set_atomic(blocks[DIRTY_MEMORY_VGA]->blocks[idx],
+ offset, num);
+ }
+ if (unlikely(mask & (1 << DIRTY_MEMORY_CODE))) {
+ bitmap_set_atomic(blocks[DIRTY_MEMORY_CODE]->blocks[idx],
+ offset, num);
+ }
+
+ page += num;
}
+
+ rcu_read_unlock();
+
xen_modified_memory(start, length);
}
@@ -191,21 +297,41 @@ static inline void cpu_physical_memory_set_dirty_lebitmap(unsigned long *bitmap,
/* start address is aligned at the start of a word? */
if ((((page * BITS_PER_LONG) << TARGET_PAGE_BITS) == start) &&
(hpratio == 1)) {
+ unsigned long **blocks[DIRTY_MEMORY_NUM];
+ unsigned long idx;
+ unsigned long offset;
long k;
long nr = BITS_TO_LONGS(pages);
+ idx = (start >> TARGET_PAGE_BITS) / DIRTY_MEMORY_BLOCK_SIZE;
+ offset = BIT_WORD((start >> TARGET_PAGE_BITS) %
+ DIRTY_MEMORY_BLOCK_SIZE);
+
+ rcu_read_lock();
+
+ for (i = 0; i < DIRTY_MEMORY_NUM; i++) {
+ blocks[i] = atomic_rcu_read(&ram_list.dirty_memory[i])->blocks;
+ }
+
for (k = 0; k < nr; k++) {
if (bitmap[k]) {
unsigned long temp = leul_to_cpu(bitmap[k]);
- unsigned long **d = ram_list.dirty_memory;
- atomic_or(&d[DIRTY_MEMORY_MIGRATION][page + k], temp);
- atomic_or(&d[DIRTY_MEMORY_VGA][page + k], temp);
+ atomic_or(&blocks[DIRTY_MEMORY_MIGRATION][idx][offset], temp);
+ atomic_or(&blocks[DIRTY_MEMORY_VGA][idx][offset], temp);
if (tcg_enabled()) {
- atomic_or(&d[DIRTY_MEMORY_CODE][page + k], temp);
+ atomic_or(&blocks[DIRTY_MEMORY_CODE][idx][offset], temp);
}
}
+
+ if (++offset >= BITS_TO_LONGS(DIRTY_MEMORY_BLOCK_SIZE)) {
+ offset = 0;
+ idx++;
+ }
}
+
+ rcu_read_unlock();
+
xen_modified_memory(start, pages << TARGET_PAGE_BITS);
} else {
uint8_t clients = tcg_enabled() ? DIRTY_CLIENTS_ALL : DIRTY_CLIENTS_NOCODE;
@@ -257,18 +383,33 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(unsigned long *dest,
if (((page * BITS_PER_LONG) << TARGET_PAGE_BITS) == start) {
int k;
int nr = BITS_TO_LONGS(length >> TARGET_PAGE_BITS);
- unsigned long *src = ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION];
+ unsigned long * const *src;
+ unsigned long idx = (page * BITS_PER_LONG) / DIRTY_MEMORY_BLOCK_SIZE;
+ unsigned long offset = BIT_WORD((page * BITS_PER_LONG) %
+ DIRTY_MEMORY_BLOCK_SIZE);
+
+ rcu_read_lock();
+
+ src = atomic_rcu_read(
+ &ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION])->blocks;
for (k = page; k < page + nr; k++) {
- if (src[k]) {
- unsigned long bits = atomic_xchg(&src[k], 0);
+ if (src[idx][offset]) {
+ unsigned long bits = atomic_xchg(&src[idx][offset], 0);
unsigned long new_dirty;
new_dirty = ~dest[k];
dest[k] |= bits;
new_dirty &= bits;
num_dirty += ctpopl(new_dirty);
}
+
+ if (++offset >= BITS_TO_LONGS(DIRTY_MEMORY_BLOCK_SIZE)) {
+ offset = 0;
+ idx++;
+ }
}
+
+ rcu_read_unlock();
} else {
for (addr = 0; addr < length; addr += TARGET_PAGE_SIZE) {
if (cpu_physical_memory_test_and_clear_dirty(
diff --git a/migration/ram.c b/migration/ram.c
index 4e606ab..fdf42df 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -609,7 +609,6 @@ static void migration_bitmap_sync_init(void)
iterations_prev = 0;
}
-/* Called with iothread lock held, to protect ram_list.dirty_memory[] */
static void migration_bitmap_sync(void)
{
RAMBlock *block;
@@ -1916,8 +1915,6 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
acct_clear();
}
- /* iothread lock needed for ram_list.dirty_memory[] */
- qemu_mutex_lock_iothread();
qemu_mutex_lock_ramlist();
rcu_read_lock();
bytes_transferred = 0;
@@ -1942,7 +1939,6 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
memory_global_dirty_log_start();
migration_bitmap_sync();
qemu_mutex_unlock_ramlist();
- qemu_mutex_unlock_iothread();
qemu_put_be64(f, ram_bytes_total() | RAM_SAVE_FLAG_MEM_SIZE);
--
2.5.0
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