[PATCH] migration: savevm_state_insert_handler: constant-time element insertion

[Scott Cheloha]

Registering a SaveStateEntry object via savevm_state_insert_handler()
is an O(n) operation because the list is a priority queue maintained by
walking the list from head to tail to find a suitable insertion point.

This adds considerable overhead for VMs with many such objects.  For
instance, ppc64 machines with large maxmem (8T+) spend ~10% or more of
their CPU time in savevm_state_insert_handler() before attempting to
boot a kernel.

If we track the head for each priority's subqueue we can insert new
elements in constant time.

This commit also introduces a new function, savevm_state_remove_handler(),
which abstracts the logic for replacing the head of an element's subqueue
when removing it.

Signed-off-by: Scott Cheloha <chel...@linux.vnet.ibm.com>
---
 migration/savevm.c | 35 ++++++++++++++++++++++++++++++-----
 1 file changed, 30 insertions(+), 5 deletions(-)

diff --git a/migration/savevm.c b/migration/savevm.c
index 8d95e261f6..f7a2d36bba 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -250,6 +250,7 @@ typedef struct SaveStateEntry {
 
 typedef struct SaveState {
     QTAILQ_HEAD(, SaveStateEntry) handlers;
+    SaveStateEntry *handler_pri_head[MIG_PRI_MAX + 1];
     int global_section_id;
     uint32_t len;
     const char *name;
@@ -261,6 +262,7 @@ typedef struct SaveState {
 
 static SaveState savevm_state = {
     .handlers = QTAILQ_HEAD_INITIALIZER(savevm_state.handlers),
+    .handler_pri_head = { [MIG_PRI_DEFAULT ... MIG_PRI_MAX] = NULL },
     .global_section_id = 0,
 };
 
@@ -709,20 +711,43 @@ static void savevm_state_handler_insert(SaveStateEntry 
*nse)
 {
     MigrationPriority priority = save_state_priority(nse);
     SaveStateEntry *se;
+    int i;
 
     assert(priority <= MIG_PRI_MAX);
 
-    QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
-        if (save_state_priority(se) < priority) {
+    for (i = priority - 1; i >= 0; i--) {
+        se = savevm_state.handler_pri_head[i];
+        if (se != NULL) {
+            assert(save_state_priority(se) < priority);
             break;
         }
     }
 
-    if (se) {
+    if (i >= 0) {
         QTAILQ_INSERT_BEFORE(se, nse, entry);
     } else {
         QTAILQ_INSERT_TAIL(&savevm_state.handlers, nse, entry);
     }
+
+    if (savevm_state.handler_pri_head[priority] == NULL) {
+        savevm_state.handler_pri_head[priority] = nse;
+    }
+}
+
+static void savevm_state_handler_remove(SaveStateEntry *se)
+{
+    SaveStateEntry *next;
+    MigrationPriority priority = save_state_priority(se);
+
+    if (se == savevm_state.handler_pri_head[priority]) {
+        next = QTAILQ_NEXT(se, entry);
+        if (next != NULL && save_state_priority(next) == priority) {
+            savevm_state.handler_pri_head[priority] = next;
+        } else {
+            savevm_state.handler_pri_head[priority] = NULL;
+        }
+    }
+    QTAILQ_REMOVE(&savevm_state.handlers, se, entry);
 }
 
 /* TODO: Individual devices generally have very little idea about the rest
@@ -777,7 +802,7 @@ void unregister_savevm(DeviceState *dev, const char *idstr, 
void *opaque)
 
     QTAILQ_FOREACH_SAFE(se, &savevm_state.handlers, entry, new_se) {
         if (strcmp(se->idstr, id) == 0 && se->opaque == opaque) {
-            QTAILQ_REMOVE(&savevm_state.handlers, se, entry);
+            savevm_state_handler_remove(se);
             g_free(se->compat);
             g_free(se);
         }
@@ -841,7 +866,7 @@ void vmstate_unregister(DeviceState *dev, const 
VMStateDescription *vmsd,
 
     QTAILQ_FOREACH_SAFE(se, &savevm_state.handlers, entry, new_se) {
         if (se->vmsd == vmsd && se->opaque == opaque) {
-            QTAILQ_REMOVE(&savevm_state.handlers, se, entry);
+            savevm_state_handler_remove(se);
             g_free(se->compat);
             g_free(se);
         }
-- 
2.23.0