[PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion

[Scott Cheloha <cheloha@linux.vnet.ibm.com>]

savevm_state's SaveStateEntry TAILQ is a priority queue.  Priority
sorting is maintained by searching from head to tail for a suitable
insertion spot.  Insertion is thus an O(n) operation.

If we instead keep track of the head of each priority's subqueue
within that larger queue we can reduce this operation to O(1) time.

savevm_state_handler_remove() becomes slightly more complex to
accomodate these gains: we need to replace the head of a priority's
subqueue when removing it.

With O(1) insertion, booting VMs with many SaveStateEntry objects is
more plausible.  For example, a ppc64 VM with maxmem=8T has 40000 such
objects to insert.

Signed-off-by: Scott Cheloha <cheloha@linux.vnet.ibm.com>
---
 migration/savevm.c | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/migration/savevm.c b/migration/savevm.c
index b2e3b7222a..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,
 };
 
@@ -709,24 +711,42 @@ 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);
 }
 
-- 
2.23.0