migration/postcopy: Report fault latencies in blocktime

Blocktime so far only cares about the time one vcpu (or the whole system)
got blocked.  It would be also be helpful if it can also report the latency
of page requests, which could be very sensitive during postcopy.

Blocktime itself is sometimes not very important, especially when one
thinks about KVM async PF support, which means vCPUs are literally almost
not blocked at all because the guest OS is smart enough to switch to
another task when a remote fault is needed.

However, latency is still sensitive and important because even if the guest
vCPU is running on threads that do not need a remote fault, the workload
that accesses some missing page is still affected.

Add two entries to the report, showing how long it takes to resolve a
remote fault.  Mention in the QAPI doc that this is not the real average
fault latency, but only the ones that was requested for a remote fault.

Unwrap get_vcpu_blocktime_list() so we don't need to walk the list twice,
meanwhile add the entry checks in qtests for all postcopy tests.

Cc: Markus Armbruster <armbru@redhat.com>
Cc: Dr. David Alan Gilbert <dave@treblig.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Link: https://lore.kernel.org/r/20250613141217.474825-9-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

1 files changed, 46 insertions, 21 deletions

diff --git a/migration/migration-hmp-cmds.c b/migration/migration-hmp-cmds.c
index 867e017b32..8b3846dab5 100644
--- a/migration/migration-hmp-cmds.c
+++ b/migration/migration-hmp-cmds.c
@@ -52,6 +52,51 @@ static void migration_global_dump(Monitor *mon)
                    ms->clear_bitmap_shift);
 }
 
+static void migration_dump_blocktime(Monitor *mon, MigrationInfo *info)
+{
+    if (info->has_postcopy_blocktime) {
+        monitor_printf(mon, "Postcopy Blocktime (ms): %" PRIu32 "\n",
+                       info->postcopy_blocktime);
+    }
+
+    if (info->has_postcopy_vcpu_blocktime) {
+        uint32List *item = info->postcopy_vcpu_blocktime;
+        const char *sep = "";
+        int count = 0;
+
+        monitor_printf(mon, "Postcopy vCPU Blocktime (ms):\n [");
+
+        while (item) {
+            monitor_printf(mon, "%s%"PRIu32, sep, item->value);
+            item = item->next;
+            /* Each line 10 vcpu results, newline if there's more */
+            sep = ((++count % 10 == 0) && item) ? ",\n  " : ", ";
+        }
+        monitor_printf(mon, "]\n");
+    }
+
+    if (info->has_postcopy_latency) {
+        monitor_printf(mon, "Postcopy Latency (ns): %" PRIu64 "\n",
+                       info->postcopy_latency);
+    }
+
+    if (info->has_postcopy_vcpu_latency) {
+        uint64List *item = info->postcopy_vcpu_latency;
+        const char *sep = "";
+        int count = 0;
+
+        monitor_printf(mon, "Postcopy vCPU Latencies (ns):\n [");
+
+        while (item) {
+            monitor_printf(mon, "%s%"PRIu64, sep, item->value);
+            item = item->next;
+            /* Each line 10 vcpu results, newline if there's more */
+            sep = ((++count % 10 == 0) && item) ? ",\n  " : ", ";
+        }
+        monitor_printf(mon, "]\n");
+    }
+}
+
 void hmp_info_migrate(Monitor *mon, const QDict *qdict)
 {
     bool show_all = qdict_get_try_bool(qdict, "all", false);
@@ -202,27 +247,7 @@ void hmp_info_migrate(Monitor *mon, const QDict *qdict)
                        info->dirty_limit_ring_full_time);
     }
 
-    if (info->has_postcopy_blocktime) {
-        monitor_printf(mon, "Postcopy Blocktime (ms): %" PRIu32 "\n",
-                       info->postcopy_blocktime);
-    }
-
-    if (info->has_postcopy_vcpu_blocktime) {
-        uint32List *item = info->postcopy_vcpu_blocktime;
-        const char *sep = "";
-        int count = 0;
-
-        monitor_printf(mon, "Postcopy vCPU Blocktime (ms):\n [");
-
-        while (item) {
-            monitor_printf(mon, "%s%"PRIu32, sep, item->value);
-            item = item->next;
-            /* Each line 10 vcpu results, newline if there's more */
-            sep = ((++count % 10 == 0) && item) ? ",\n  " : ", ";
-        }
-        monitor_printf(mon, "]\n");
-    }
-
+    migration_dump_blocktime(mon, info);
 out:
     qapi_free_MigrationInfo(info);
 }