diff --git a/src/rt/rust_internal.h b/src/rt/rust_internal.h
index d962e894153..99f521656e8 100644
--- a/src/rt/rust_internal.h
+++ b/src/rt/rust_internal.h
@@ -67,6 +67,17 @@ struct frame_glue_fns;
 
 static size_t const TIME_SLICE_IN_MS = 10;
 
+// This helps our preemption scheme handle "running on valgrind".
+
+#if defined(__WIN32__)
+#define YIELD_C_THREAD_IF_ON_VALGRIND (void);
+#else
+#define YIELD_C_THREAD_IF_ON_VALGRIND  \
+    if (RUNNING_ON_VALGRIND) {         \
+        pthread_yield();               \
+    }
+#endif
+
 // Every reference counted object should derive from this base class.
 
 template <typename T>
diff --git a/src/rt/rust_timer.cpp b/src/rt/rust_timer.cpp
index bf4d18e836d..9427ec673ff 100644
--- a/src/rt/rust_timer.cpp
+++ b/src/rt/rust_timer.cpp
@@ -32,10 +32,9 @@ timer_loop(void *ptr) {
     rust_dom *dom = timer->dom;
     dom->log(rust_log::TIMER, "in timer 0x%" PRIxPTR, (uintptr_t)timer);
     size_t ms = TIME_SLICE_IN_MS;
-    if (!RUNNING_ON_VALGRIND)
-        ms = 1;
 
     while (!timer->exit_flag) {
+        YIELD_C_THREAD_IF_ON_VALGRIND;
 #if defined(__WIN32__)
         Sleep(ms);
 #else
@@ -66,8 +65,6 @@ rust_timer::rust_timer(rust_dom *dom) :
     pthread_attr_init(&attr);
     pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
     pthread_create(&thread, &attr, timer_loop, (void *)this);
-    if (RUNNING_ON_VALGRIND)
-        usleep(10000);
 #endif
 }
 
diff --git a/src/rt/rust_upcall.cpp b/src/rt/rust_upcall.cpp
index 1aaf89fba97..e931fc9b7f6 100644
--- a/src/rt/rust_upcall.cpp
+++ b/src/rt/rust_upcall.cpp
@@ -1,21 +1,24 @@
 #include "rust_internal.h"
+#include "valgrind.h"
 
 // Upcalls.
 
 #ifdef __GNUC__
 #define LOG_UPCALL_ENTRY(task)                              \
+    YIELD_C_THREAD_IF_ON_VALGRIND;                          \
     (task)->dom->get_log().reset_indent(0);                 \
     (task)->log(rust_log::UPCALL,                           \
                 "> UPCALL %s - task: 0x%" PRIxPTR           \
-                " retpc: x%" PRIxPTR,                      \
+                " retpc: x%" PRIxPTR,                       \
                 __FUNCTION__,                               \
                 (task), __builtin_return_address(0));       \
     (task)->dom->get_log().indent();
 #else
 #define LOG_UPCALL_ENTRY(task)                              \
+    YIELD_C_THREAD_IF_ON_VALGRIND;                          \
     (task)->dom->get_log().reset_indent(0);                 \
     (task)->log(rust_log::UPCALL,                           \
-                "> UPCALL task: x%" PRIxPTR (task));       \
+                "> UPCALL task: x%" PRIxPTR (task));        \
     (task)->dom->get_log().indent();
 #endif
 
diff --git a/src/test/run-pass/preempt.rs b/src/test/run-pass/preempt.rs
index 00fc29cae72..ee37bcf44bf 100644
--- a/src/test/run-pass/preempt.rs
+++ b/src/test/run-pass/preempt.rs
@@ -18,7 +18,7 @@ io fn main() {
   log "main waiting for alive signal";
   i <- alive;
   log "main got alive signal";
-  while (i < 1000) {
+  while (i < 50) {
     log "main iterated";
     i += 1;
   }