From d69d80467941aaccd41cef4dfe7ea3c5de34f146 Mon Sep 17 00:00:00 2001
From: Maria Matejka <mq@ucw.cz>
Date: Sun, 15 Dec 2024 21:04:22 +0100
Subject: [PATCH] Table prune inhibited during reconfiguration

When many changes are done during reconfiguration, the table may
start pruning old routes before everything is settled down, slowing
down not only the reconfiguration, but also the shutdown process.
---
 nest/route.h    |  2 ++
 nest/rt-table.c | 53 +++++++++++++++++++++++++++++++++++++++++++++----
 2 files changed, 51 insertions(+), 4 deletions(-)

diff --git a/nest/route.h b/nest/route.h
index 89502f7a..e512dc36 100644
--- a/nest/route.h
+++ b/nest/route.h
@@ -413,6 +413,8 @@ struct rtable_private {
 					 * delete as soon as use_count becomes 0 and remove
 					 * obstacle from this routing table.
 					 */
+
+  struct deferred_call *reconf_end;	/* Reconfiguration done callback */
   struct rt_export_request best_req;	/* Internal request from best route announcement cleanup */
   struct rt_uncork_callback nhu_uncork;	/* Helper event to schedule NHU on uncork */
   struct rt_uncork_callback hcu_uncork;	/* Helper event to schedule HCU on uncork */
diff --git a/nest/rt-table.c b/nest/rt-table.c
index fcbc6db8..9b0e86dd 100644
--- a/nest/rt-table.c
+++ b/nest/rt-table.c
@@ -2881,7 +2881,34 @@ rt_schedule_prune(struct rtable_private *tab)
 {
   /* state change 0->1, 2->3 */
   tab->prune_state |= 1;
-  ev_send_loop(tab->loop, tab->prune_event);
+  if (!tab->reconf_end)
+    ev_send_loop(tab->loop, tab->prune_event);
+
+  /* If reconfiguring, we explicitly activate the prune after done
+   * to stop expensive operations from happening too early. */
+}
+
+struct rt_reconf_finished_deferred_call {
+  struct deferred_call dc;
+  rtable *tab;
+};
+
+static void
+rt_reconf_finished(struct deferred_call *dc)
+{
+  /* Reconfiguration ended, let's reinstate prune events */
+  ASSERT_DIE(birdloop_inside(&main_birdloop));
+  RT_LOCKED(SKIP_BACK(struct rt_reconf_finished_deferred_call, dc, dc)->tab, tab)
+  {
+    rt_unlock_table(tab);
+    if (dc == tab->reconf_end)
+    {
+      tab->reconf_end = NULL;
+
+      if (tab->prune_state & 1)
+	ev_send_loop(tab->loop, tab->prune_event);
+    }
+  }
 }
 
 static void
@@ -3569,8 +3596,10 @@ rt_prune_table(void *_tab)
   lfjour_announce_now(&tab->export_all.journal);
   lfjour_announce_now(&tab->export_best.journal);
 
-  /* state change 2->0, 3->1 */
-  if (tab->prune_state &= 1)
+  /* state change 2->0, 3->1
+   * pausing expensive prune while reconfiguring to allow for
+   * the imports to settle */
+  if ((tab->prune_state &= 1) && !tab->reconf_end)
     ev_send_loop(tab->loop, tab->prune_event);
 
   struct f_trie *trie = atomic_load_explicit(&tab->trie, memory_order_relaxed);
@@ -4652,6 +4681,14 @@ rt_reconfigure(struct rtable_private *tab, struct rtable_config *new, struct rta
     ||  (new->digest_settle.max != tab->export_digest->settle.cf.max)))
     tab->export_digest->settle.cf = new->digest_settle;
 
+  rt_lock_table(tab); /* Unlocked in rt_reconf_finished() */
+  struct rt_reconf_finished_deferred_call rrfdc = {
+    .dc.hook = rt_reconf_finished,
+    .tab = RT_PUB(tab),
+  };
+
+  tab->reconf_end = defer_call(&rrfdc.dc, sizeof rrfdc);
+
   return 1;
 }
 
@@ -4702,7 +4739,15 @@ rt_commit(struct config *new, struct config *old)
 	  OBSREF_SET(tab->deleted, old);
 	  rt_check_cork_low(tab);
 	  rt_lock_table(tab);
-	  rt_unlock_table(tab);
+
+	  /* No actual table stopping before reconfiguring the rest.
+	   * Table unlocked in the deferred call. */
+	  struct rt_reconf_finished_deferred_call rrfdc = {
+	    .dc.hook = rt_reconf_finished,
+	    .tab = RT_PUB(tab),
+	  };
+
+	  tab->reconf_end = defer_call(&rrfdc.dc, sizeof rrfdc);
 	}
 
 	CALL(o->table->config->master.stop, o->table);