migration: check for rate_limit_max for RATE_LIMIT_DISABLED

In migration rate limiting atomic operations are used
to read the rate limit variables and transferred bytes and
they are expensive. Check first if rate_limit_max is equal
to RATE_LIMIT_DISABLED and return false immediately if so.

Note that with this patch we will also will stop flushing
by not calling qemu_fflush() from migration_transferred_bytes()
if the migration rate is not exceeded.
This should be fine since migration thread calls in the loop
migration_update_counters from migration_rate_limit() that
calls the migration_transferred_bytes() and flushes there.

Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20231011184358.97349-2-elena.ufimtseva@oracle.com>

1 files changed, 5 insertions, 4 deletions

diff --git a/migration/migration-stats.c b/migration/migration-stats.c
index 84e11e6dd8..4cc989d975 100644
--- a/migration/migration-stats.c
+++ b/migration/migration-stats.c
@@ -24,14 +24,15 @@ bool migration_rate_exceeded(QEMUFile *f)
         return true;
     }
 
+    uint64_t rate_limit_max = migration_rate_get();
+    if (rate_limit_max == RATE_LIMIT_DISABLED) {
+        return false;
+    }
+
     uint64_t rate_limit_start = stat64_get(&mig_stats.rate_limit_start);
     uint64_t rate_limit_current = migration_transferred_bytes(f);
     uint64_t rate_limit_used = rate_limit_current - rate_limit_start;
-    uint64_t rate_limit_max = stat64_get(&mig_stats.rate_limit_max);
 
-    if (rate_limit_max == RATE_LIMIT_DISABLED) {
-        return false;
-    }
     if (rate_limit_max > 0 && rate_limit_used > rate_limit_max) {
         return true;
     }