[PATCH 7/8] migration/multifd: cache migrate_multifd_channels() in send/recv hot paths

[Bin Guo <guobin@linux.alibaba.com>]

multifd_send() and multifd_recv() are on the per-page-batch hot path
of live migration.  Both functions call migrate_multifd_channels()
multiple times (3-4 calls each) for modulo arithmetic in the
round-robin channel selection loop.

Each call goes through migrate_get_current() -> dereference
MigrationState -> read parameters.multifd_channels.  While each
individual call is cheap, these functions execute for every page
batch during the entire migration, easily millions of times.

Cache the return value in a local variable at function entry.  The
channel count is fixed for the duration of a migration and cannot
change mid-flight.

For multifd_send(): 3 calls reduced to 1.
For multifd_recv(): 4 calls reduced to 1.

Signed-off-by: Bin Guo <guobin@linux.alibaba.com>
---
 migration/multifd.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/migration/multifd.c b/migration/multifd.c
index 67ee9bdf5e..cc2fa90204 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -362,13 +362,15 @@ bool multifd_send(MultiFDSendData **send_data)
     /* We wait here, until at least one channel is ready */
     qemu_sem_wait(&multifd_send_state->channels_ready);
 
+    int thread_count = migrate_multifd_channels();
+
     /*
      * next_channel can remain from a previous migration that was
      * using more channels, so ensure it doesn't overflow if the
      * limit is lower now.
      */
-    next_channel %= migrate_multifd_channels();
-    for (i = next_channel;; i = (i + 1) % migrate_multifd_channels()) {
+    next_channel %= thread_count;
+    for (i = next_channel;; i = (i + 1) % thread_count) {
         if (multifd_send_should_exit()) {
             return false;
         }
@@ -378,7 +380,7 @@ bool multifd_send(MultiFDSendData **send_data)
          * sender thread can clear it.
          */
         if (qatomic_read(&p->pending_job) == false) {
-            next_channel = (i + 1) % migrate_multifd_channels();
+            next_channel = (i + 1) % thread_count;
             break;
         }
     }
@@ -998,6 +1000,7 @@ bool multifd_recv(void)
     int i;
     static int next_recv_channel;
     MultiFDRecvParams *p = NULL;
+    int thread_count = migrate_multifd_channels();
     MultiFDRecvData *data = multifd_recv_state->data;
 
     /*
@@ -1005,8 +1008,8 @@ bool multifd_recv(void)
      * using more channels, so ensure it doesn't overflow if the
      * limit is lower now.
      */
-    next_recv_channel %= migrate_multifd_channels();
-    for (i = next_recv_channel;; i = (i + 1) % migrate_multifd_channels()) {
+    next_recv_channel %= thread_count;
+    for (i = next_recv_channel;; i = (i + 1) % thread_count) {
         if (multifd_recv_should_exit()) {
             return false;
         }
@@ -1014,7 +1017,7 @@ bool multifd_recv(void)
         p = &multifd_recv_state->params[i];
 
         if (qatomic_read(&p->pending_job) == false) {
-            next_recv_channel = (i + 1) % migrate_multifd_channels();
+            next_recv_channel = (i + 1) % thread_count;
             break;
         }
     }
-- 
2.50.1 (Apple Git-155)