hs_pow: update_suggested_effort fix and cleanup

This is trying to be an AIMD event-driven algorithm, but we ended up with
two different add paths with diverging behavior. This fix makes the AIMD
events more explicit, and it fixes an earlier behavior where the effort
could be decreased (by the add/recalculate branch) even when the pqueue
was not emptying at all. With this patch we shouldn't drop down to an
effort of zero as long as even low-effort attacks are flooding the
pqueue.

Signed-off-by: Micah Elizabeth Scott <beth@torproject.org>

diff --git a/src/feature/hs/hs_service.c b/src/feature/hs/hs_service.c
index 3b93ee67bade601fae709b551804a1e520a9627b..1a40e34cf770b3f07d8fba6cfeec42a6508b0522 100644 (file)
--- a/src/feature/hs/hs_service.c
+++ b/src/feature/hs/hs_service.c
@@ -2686,26 +2686,39 @@ update_suggested_effort(hs_service_t *service, time_t now)
 
   /* Calculate the new suggested effort, using an additive-increase
    * multiplicative-decrease estimation scheme. */
+  enum {
+    NONE,
+    INCREASE,
+    DECREASE
+  } aimd_event = NONE;
+
   if (pow_state->max_trimmed_effort > pow_state->suggested_effort) {
-    /* If we trimmed a request above our suggested effort, re-estimate the
-     * effort */
-    pow_state->suggested_effort = (uint32_t)(pow_state->total_effort /
-                                             pow_state->rend_handled);
+    /* Increase when we notice that high-effort requests are trimmed */
+    aimd_event = INCREASE;
   } else if (pow_state->had_queue) {
-    /* If we had a queue during this period, and the current top of queue
-     * is at or above the suggested effort, we should re-estimate the effort
-     * and increase it at least a minimal amount. Otherwise, it can stay the
-     * same (no change to effort). */
     if (smartlist_len(pow_state->rend_request_pqueue) > 0 &&
         top_of_rend_pqueue_is_worthwhile(pow_state)) {
-      pow_state->suggested_effort = MAX(pow_state->suggested_effort + 1,
-                                        (uint32_t)(pow_state->total_effort /
-                                                   pow_state->rend_handled));
+      /* Increase when the top of queue is high-effort */
+      aimd_event = INCREASE;
     }
-  } else {
-    /* If we were able to keep the queue drained the entire update period,
-     * multiplicative decrease the pow by 2/3. */
-    pow_state->suggested_effort = 2*pow_state->suggested_effort/3;
+  } else if (smartlist_len(pow_state->rend_request_pqueue) == 0) {
+    /* Dec when the queue is empty now and had_queue wasn't set this period */
+    aimd_event = DECREASE;
+  }
+
+  switch (aimd_event) {
+    case INCREASE:
+      if (pow_state->suggested_effort < UINT32_MAX) {
+        pow_state->suggested_effort = MAX(pow_state->suggested_effort + 1,
+                                          (uint32_t)(pow_state->total_effort /
+                                                     pow_state->rend_handled));
+      }
+      break;
+    case DECREASE:
+      pow_state->suggested_effort = 2*pow_state->suggested_effort/3;
+      break;
+    case NONE:
+      break;
   }
 
   hs_metrics_pow_suggested_effort(service, pow_state->suggested_effort);