diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index dd6282404c..ba4fc8f2ee 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,11 @@
+2016-05-24  Pedro Alves  <palves@redhat.com>
+
+	PR gdb/19828
+	* linux-nat.c (linux_resume_one_lwp_throw): Clear the LWP's core
+	field.
+	(linux_nat_update_thread_list): Don't fetch the core if already
+	known.
+
 2016-05-24  Pedro Alves  <palves@redhat.com>
 
 	PR gdb/19828
diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index 19fe12f80a..dbadf2900e 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -1432,6 +1432,7 @@ linux_resume_one_lwp_throw (struct lwp_info *lp, int step,
      status.  Note that we must not throw after this is cleared,
      otherwise handle_zombie_lwp_error would get confused.  */
   lp->stopped = 0;
+  lp->core = -1;
   lp->stop_reason = TARGET_STOPPED_BY_NO_REASON;
   registers_changed_ptid (lp->ptid);
 }
@@ -3785,7 +3786,13 @@ linux_nat_update_thread_list (struct target_ops *ops)
   /* Update the processor core that each lwp/thread was last seen
      running on.  */
   ALL_LWPS (lwp)
-    lwp->core = linux_common_core_of_thread (lwp->ptid);
+    {
+      /* Avoid accessing /proc if the thread hasn't run since we last
+	 time we fetched the thread's core.  Accessing /proc becomes
+	 noticeably expensive when we have thousands of LWPs.  */
+      if (lwp->core == -1)
+	lwp->core = linux_common_core_of_thread (lwp->ptid);
+    }
 }
 
 static char *