migration/cpr: use hashtable for cpr fds

Use a GHashTable to store cpr fds to reduce the time
consumption of `cpr_find_fd` in scenarios with a large
number of fds. The time complexity for `cpr_find_fd` is
reduced from O(N) to O(1).
Keep cpr fds lookups in a GHashTable during normal runtime
while preserving the existing QLIST migration ABI. Build a
temporary QLIST from the hash table in pre_save and rebuild
the hash table from the loaded QLIST in post_load.

To demonstrate the performance improvement, we tested the total time
consumed by `cpr_find_fd` (called N times for N fds) under our real-world
business scenarios with different numbers of file descriptors. The results
are measured in nanoseconds:

| Number of FDs | Total time with QLIST (ns) | Total time with GHashTable (ns) |
|---------------|----------------------------|---------------------------------|
| 540           | 936,753                    | 393,358                         |
| 2,870         | 24,102,342                 | 2,212,113                       |
| 7,530         | 152,715,916                | 5,474,310                       |

As shown in the data, the lookup time grows exponentially with the QLIST
as the number of fds increases. With the GHashTable, the time consumption
remains linear (O(1) per lookup), significantly reducing the downtime during
the CPR process.

Signed-off-by: hongmianquan <hongmianquan@bytedance.com>
Link: https://lore.kernel.org/r/20260519134315.27997-1-hongmianquan@bytedance.com
Signed-off-by: Peter Xu <peterx@redhat.com>

diff --git a/migration/cpr.c b/migration/cpr.c
index 05266dfcfd33526d1bcf01c2e6d12521d171f239..bca43e9bf3555f14ec3b3dd617daa63d16321284 100644 (file)
--- a/migration/cpr.c
+++ b/migration/cpr.c
@@ -24,6 +24,7 @@
 /* cpr state container for all information to be saved. */
 
 CprState cpr_state;
+static GHashTable *cpr_fds_hash;
 
 /****************************************************************************/
 
@@ -48,6 +49,84 @@ static const VMStateDescription vmstate_cpr_fd = {
     }
 };
 
+static guint cpr_fd_hash(gconstpointer v)
+{
+    const CprFd *elem = v;
+
+    return g_str_hash(elem->name) ^ elem->id;
+}
+
+static gboolean cpr_fd_equal(gconstpointer a, gconstpointer b)
+{
+    const CprFd *elem_a = a;
+    const CprFd *elem_b = b;
+
+    return !strcmp(elem_a->name, elem_b->name) && elem_a->id == elem_b->id;
+}
+
+static void cpr_fd_destroy(gpointer data)
+{
+    CprFd *elem = data;
+
+    g_free(elem->name);
+    g_free(elem);
+}
+
+static GHashTable *get_cpr_fds_hash(void)
+{
+    if (!cpr_fds_hash) {
+        cpr_fds_hash = g_hash_table_new_full(cpr_fd_hash, cpr_fd_equal,
+                                             cpr_fd_destroy, NULL);
+    }
+
+    return cpr_fds_hash;
+}
+
+static void cpr_fd_hash_insert(CprFd *elem)
+{
+    /* Use the same CprFd as key and value. */
+    g_hash_table_insert(get_cpr_fds_hash(), elem, elem);
+}
+
+static int cpr_fd_pre_save(void *opaque)
+{
+    CprState *state = (CprState *)opaque;
+    GHashTableIter iter;
+    CprFd *elem;
+
+    QLIST_INIT(&state->fds);
+
+    g_hash_table_iter_init(&iter, get_cpr_fds_hash());
+    while (g_hash_table_iter_next(&iter, (gpointer *)&elem, NULL)) {
+        QLIST_INSERT_HEAD(&state->fds, elem, next);
+    }
+
+    return 0;
+}
+
+static int cpr_fd_post_load(void *opaque, int version_id)
+{
+    CprState *state = (CprState *)opaque;
+    CprFd *elem;
+
+    while ((elem = QLIST_FIRST(&state->fds))) {
+        QLIST_REMOVE(elem, next);
+
+        /*
+         * Preserve legacy QLIST lookup semantics if duplicate keys exist in
+         * the incoming stream: the first matching entry wins.
+         */
+        if (g_hash_table_contains(get_cpr_fds_hash(), elem)) {
+            cpr_fd_destroy(elem);
+            continue;
+        }
+
+        cpr_fd_hash_insert(elem);
+    }
+
+    return 0;
+}
+
 void cpr_save_fd(const char *name, int id, int fd)
 {
     CprFd *elem = g_new0(CprFd, 1);
@@ -57,37 +136,34 @@ void cpr_save_fd(const char *name, int id, int fd)
     elem->namelen = strlen(name) + 1;
     elem->id = id;
     elem->fd = fd;
-    QLIST_INSERT_HEAD(&cpr_state.fds, elem, next);
+    cpr_fd_hash_insert(elem);
 }
 
-static CprFd *find_fd(CprFdList *head, const char *name, int id)
+static CprFd *find_fd(const char *name, int id)
 {
-    CprFd *elem;
+    CprFd key = {
+        .name = (char *)name,
+        .id = id,
+    };
 
-    QLIST_FOREACH(elem, head, next) {
-        if (!strcmp(elem->name, name) && elem->id == id) {
-            return elem;
-        }
-    }
-    return NULL;
+    return g_hash_table_lookup(get_cpr_fds_hash(), &key);
 }
 
 void cpr_delete_fd(const char *name, int id)
 {
-    CprFd *elem = find_fd(&cpr_state.fds, name, id);
+    CprFd key = {
+        .name = (char *)name,
+        .id = id,
+    };
 
-    if (elem) {
-        QLIST_REMOVE(elem, next);
-        g_free(elem->name);
-        g_free(elem);
-    }
+    g_hash_table_remove(get_cpr_fds_hash(), &key);
 
     trace_cpr_delete_fd(name, id);
 }
 
 int cpr_find_fd(const char *name, int id)
 {
-    CprFd *elem = find_fd(&cpr_state.fds, name, id);
+    CprFd *elem = find_fd(name, id);
     int fd = elem ? elem->fd : -1;
 
     trace_cpr_find_fd(name, id, fd);
@@ -96,7 +172,7 @@ int cpr_find_fd(const char *name, int id)
 
 void cpr_resave_fd(const char *name, int id, int fd)
 {
-    CprFd *elem = find_fd(&cpr_state.fds, name, id);
+    CprFd *elem = find_fd(name, id);
     int old_fd = elem ? elem->fd : -1;
 
     if (old_fd < 0) {
@@ -125,9 +201,11 @@ int cpr_open_fd(const char *path, int flags, const char *name, int id,
 
 bool cpr_walk_fd(cpr_walk_fd_cb cb)
 {
+    GHashTableIter iter;
     CprFd *elem;
 
-    QLIST_FOREACH(elem, &cpr_state.fds, next) {
+    g_hash_table_iter_init(&iter, get_cpr_fds_hash());
+    while (g_hash_table_iter_next(&iter, (gpointer *)&elem, NULL)) {
         g_assert(elem->fd >= 0);
         if (!cb(elem->fd)) {
             return false;
@@ -141,6 +219,8 @@ static const VMStateDescription vmstate_cpr_state = {
     .name = CPR_STATE,
     .version_id = 1,
     .minimum_version_id = 1,
+    .pre_save = cpr_fd_pre_save,
+    .post_load = cpr_fd_post_load,
     .fields = (VMStateField[]) {
         VMSTATE_QLIST_V(fds, CprState, 1, vmstate_cpr_fd, CprFd, next),
         VMSTATE_END_OF_LIST()