[PATCH] migration/rdma: add x-rdma-chunk-shift parameter

[Samuel Zhang <guoqing.zhang@amd.com>]

The default 1MB RDMA chunk size causes slow live migration because
each chunk triggers a write_flush (ibv_post_send). For 8GB RAM,
1MB chunks produce ~15000 flushes vs ~3700 with 1GB chunks.

Add x-rdma-chunk-shift parameter to configure the RDMA chunk size
(2^N bytes) for faster migration.
Usage: -global migration.x-rdma-chunk-shift=30

Performance with RDMA live migration of 8GB RAM VM:

| x-rdma-chunk-shift | chunk size | time (s) | throughput (Mbps) |
|--------------------|------------|----------|-------------------|
| 20 (default)       | 1 MB       | 37.915   | 1,007             |
| 25                 | 32 MB      | 17.880   | 2,260             |
| 30                 | 1 GB       |  4.368   | 17,529            |

Signed-off-by: Samuel Zhang <guoqing.zhang@amd.com>
---
 migration/options.c | 13 +++++++++++++
 migration/options.h |  1 +
 migration/rdma.c    | 37 ++++++++++++++++++++++---------------
 qapi/migration.json |  9 ++++++++-
 4 files changed, 44 insertions(+), 16 deletions(-)

diff --git a/migration/options.c b/migration/options.c
index f33b297929..1503ae35a2 100644
--- a/migration/options.c
+++ b/migration/options.c
@@ -90,6 +90,7 @@ const PropertyInfo qdev_prop_StrOrNull;
 
 #define DEFAULT_MIGRATE_VCPU_DIRTY_LIMIT_PERIOD     1000    /* milliseconds */
 #define DEFAULT_MIGRATE_VCPU_DIRTY_LIMIT            1       /* MB/s */
+#define DEFAULT_MIGRATE_X_RDMA_CHUNK_SHIFT          20      /* 1MB */
 
 const Property migration_properties[] = {
     DEFINE_PROP_BOOL("store-global-state", MigrationState,
@@ -183,6 +184,9 @@ const Property migration_properties[] = {
     DEFINE_PROP_ZERO_PAGE_DETECTION("zero-page-detection", MigrationState,
                        parameters.zero_page_detection,
                        ZERO_PAGE_DETECTION_MULTIFD),
+    DEFINE_PROP_UINT8("x-rdma-chunk-shift", MigrationState,
+                      parameters.x_rdma_chunk_shift,
+                      DEFAULT_MIGRATE_X_RDMA_CHUNK_SHIFT),
 
     /* Migration capabilities */
     DEFINE_PROP_MIG_CAP("x-xbzrle", MIGRATION_CAPABILITY_XBZRLE),
@@ -993,6 +997,15 @@ ZeroPageDetection migrate_zero_page_detection(void)
     return s->parameters.zero_page_detection;
 }
 
+uint8_t migrate_rdma_chunk_shift(void)
+{
+    MigrationState *s = migrate_get_current();
+    uint8_t chunk_shift = s->parameters.x_rdma_chunk_shift;
+
+    assert(20 <= chunk_shift && chunk_shift <= 30);
+    return chunk_shift;
+}
+
 /* parameters helpers */
 
 AnnounceParameters *migrate_announce_params(void)
diff --git a/migration/options.h b/migration/options.h
index b502871097..3f214465a3 100644
--- a/migration/options.h
+++ b/migration/options.h
@@ -87,6 +87,7 @@ const char *migrate_tls_creds(void);
 const char *migrate_tls_hostname(void);
 uint64_t migrate_xbzrle_cache_size(void);
 ZeroPageDetection migrate_zero_page_detection(void);
+uint8_t migrate_rdma_chunk_shift(void);
 
 /* parameters helpers */
 
diff --git a/migration/rdma.c b/migration/rdma.c
index 55ab85650a..d914a7cd3b 100644
--- a/migration/rdma.c
+++ b/migration/rdma.c
@@ -44,11 +44,18 @@
 
 #define RDMA_RESOLVE_TIMEOUT_MS 10000
 
-/* Do not merge data if larger than this. */
-#define RDMA_MERGE_MAX (2 * 1024 * 1024)
-#define RDMA_SIGNALED_SEND_MAX (RDMA_MERGE_MAX / 4096)
+#define RDMA_SIGNALED_SEND_MAX 512
+
+static inline uint64_t rdma_chunk_size(void)
+{
+    return 1UL << migrate_rdma_chunk_shift();
+}
 
-#define RDMA_REG_CHUNK_SHIFT 20 /* 1 MB */
+/* Do not merge data if larger than this. */
+static inline uint64_t rdma_merge_max(void)
+{
+    return rdma_chunk_size() * 2;
+}
 
 /*
  * This is only for non-live state being migrated.
@@ -527,21 +534,21 @@ static int qemu_rdma_exchange_send(RDMAContext *rdma, RDMAControlHeader *head,
 static inline uint64_t ram_chunk_index(const uint8_t *start,
                                        const uint8_t *host)
 {
-    return ((uintptr_t) host - (uintptr_t) start) >> RDMA_REG_CHUNK_SHIFT;
+    return ((uintptr_t) host - (uintptr_t) start) >> migrate_rdma_chunk_shift();
 }
 
 static inline uint8_t *ram_chunk_start(const RDMALocalBlock *rdma_ram_block,
                                        uint64_t i)
 {
     return (uint8_t *)(uintptr_t)(rdma_ram_block->local_host_addr +
-                                  (i << RDMA_REG_CHUNK_SHIFT));
+                                  (i << migrate_rdma_chunk_shift()));
 }
 
 static inline uint8_t *ram_chunk_end(const RDMALocalBlock *rdma_ram_block,
                                      uint64_t i)
 {
     uint8_t *result = ram_chunk_start(rdma_ram_block, i) +
-                                         (1UL << RDMA_REG_CHUNK_SHIFT);
+                                         rdma_chunk_size();
 
     if (result > (rdma_ram_block->local_host_addr + rdma_ram_block->length)) {
         result = rdma_ram_block->local_host_addr + rdma_ram_block->length;
@@ -1841,6 +1848,7 @@ static int qemu_rdma_write_one(RDMAContext *rdma,
     struct ibv_send_wr *bad_wr;
     int reg_result_idx, ret, count = 0;
     uint64_t chunk, chunks;
+    uint64_t chunk_size = rdma_chunk_size();
     uint8_t *chunk_start, *chunk_end;
     RDMALocalBlock *block = &(rdma->local_ram_blocks.block[current_index]);
     RDMARegister reg;
@@ -1861,22 +1869,21 @@ retry:
     chunk_start = ram_chunk_start(block, chunk);
 
     if (block->is_ram_block) {
-        chunks = length / (1UL << RDMA_REG_CHUNK_SHIFT);
+        chunks = length / chunk_size;
 
-        if (chunks && ((length % (1UL << RDMA_REG_CHUNK_SHIFT)) == 0)) {
+        if (chunks && ((length % chunk_size) == 0)) {
             chunks--;
         }
     } else {
-        chunks = block->length / (1UL << RDMA_REG_CHUNK_SHIFT);
+        chunks = block->length / chunk_size;
 
-        if (chunks && ((block->length % (1UL << RDMA_REG_CHUNK_SHIFT)) == 0)) {
+        if (chunks && ((block->length % chunk_size) == 0)) {
             chunks--;
         }
     }
 
     trace_qemu_rdma_write_one_top(chunks + 1,
-                                  (chunks + 1) *
-                                  (1UL << RDMA_REG_CHUNK_SHIFT) / 1024 / 1024);
+                                  (chunks + 1) * chunk_size / 1024 / 1024);
 
     chunk_end = ram_chunk_end(block, chunk + chunks);
 
@@ -2176,7 +2183,7 @@ static int qemu_rdma_write(RDMAContext *rdma,
     rdma->current_length += len;
 
     /* flush it if buffer is too large */
-    if (rdma->current_length >= RDMA_MERGE_MAX) {
+    if (rdma->current_length >= rdma_merge_max()) {
         return qemu_rdma_write_flush(rdma, errp);
     }
 
@@ -3522,7 +3529,7 @@ int rdma_registration_handle(QEMUFile *f)
                 } else {
                     chunk = reg->key.chunk;
                     host_addr = block->local_host_addr +
-                        (reg->key.chunk * (1UL << RDMA_REG_CHUNK_SHIFT));
+                        (reg->key.chunk * rdma_chunk_size());
                     /* Check for particularly bad chunk value */
                     if (host_addr < (void *)block->local_host_addr) {
                         error_report("rdma: bad chunk for block %s"
diff --git a/qapi/migration.json b/qapi/migration.json
index 7134d4ce47..0521bf3d69 100644
--- a/qapi/migration.json
+++ b/qapi/migration.json
@@ -1007,9 +1007,14 @@
 #     is @cpr-exec.  The first list element is the program's filename,
 #     the remainder its arguments.  (Since 10.2)
 #
+# @x-rdma-chunk-shift: RDMA memory registration chunk shift.
+#     The chunk size is 2^N bytes where N is the value.
+#     Defaults to 20 (1 MiB).  Only takes effect for RDMA migration.
+#     (Since 10.2)
+#
 # Features:
 #
-# @unstable: Members @x-checkpoint-delay and
+# @unstable: Members @x-rdma-chunk-shift, @x-checkpoint-delay and
 #     @x-vcpu-dirty-limit-period are experimental.
 #
 # Since: 2.4
@@ -1045,6 +1050,8 @@
             '*vcpu-dirty-limit': 'uint64',
             '*mode': 'MigMode',
             '*zero-page-detection': 'ZeroPageDetection',
+            '*x-rdma-chunk-shift': { 'type': 'uint8',
+                                     'features': [ 'unstable' ] },
             '*direct-io': 'bool',
             '*cpr-exec-command': [ 'str' ]} }
 
-- 
2.43.7