sched: Fallback to naive approach if no runtime KIST support

It is possible that tor was compiled with KIST support but the running kernel
has no support for it. In that case, fallback to a naive approach and flag
that we have no kernel support.

At this commit, if the kernel support is disabled, there are no ways to come
back from it other than restarting tor with a kernel that supporst KIST.

Signed-off-by: David Goulet <dgoulet@torproject.org>

src/or/scheduler_kist.c

@@ -24,6 +24,11 @@
  * Data structures and supporting functions
  *****************************************************************************/
 
+/* Indicate if we don't have the kernel support. This can happen if the kernel
+ * changed and it doesn't recognized the values passed to the syscalls needed
+ * by KIST. In that case, fallback to the naive approach. */
+static unsigned int kist_no_kernel_support = 0;
+
 /* Socket_table hash table stuff. The socket_table keeps track of per-socket
  * limit information imposed by kist and used by kist. */
 
@@ -174,9 +179,34 @@ update_socket_info_impl, (socket_table_ent_t *ent))
   struct tcp_info tcp;
   socklen_t tcp_info_len = sizeof(tcp);
 
+  if (kist_no_kernel_support) {
+    goto fallback;
+  }
+
   /* Gather information */
-  getsockopt(sock, SOL_TCP, TCP_INFO, (void *)&(tcp), &tcp_info_len);
-  ioctl(sock, SIOCOUTQNSD, &(ent->notsent));
+  if (getsockopt(sock, SOL_TCP, TCP_INFO, (void *)&(tcp), &tcp_info_len) < 0) {
+    if (errno == EINVAL) {
+      /* Oops, this option is not provided by the kernel, we'll have to
+       * disable KIST entirely. This can happen if tor was built on a machine
+       * with the support previously or if the kernel was updated and lost the
+       * support. */
+      log_notice(LD_SCHED, "Looks like our kernel doesn't have the support "
+                           "for KIST anymore. Fallback to the naive approach. "
+                           "Set KISTSchedRunInterval=-1 to disable KIST.");
+      kist_no_kernel_support = 1;
+    }
+    goto fallback;
+  }
+  if (ioctl(sock, SIOCOUTQNSD, &(ent->notsent)) < 0) {
+    if (errno == EINVAL) {
+      log_notice(LD_SCHED, "Looks like our kernel doesn't have the support "
+                           "for KIST anymore. Fallback to the naive approach. "
+                           "Set KISTSchedRunInterval=-1 to disable KIST.");
+      /* Same reason as the above. */
+      kist_no_kernel_support = 1;
+    }
+    goto fallback;
+  }
   ent->cwnd = tcp.tcpi_snd_cwnd;
   ent->unacked = tcp.tcpi_unacked;
   ent->mss = tcp.tcpi_snd_mss;
@@ -193,6 +223,10 @@ update_socket_info_impl, (socket_table_ent_t *ent))
   }
   ent->limit = tcp_space + extra_space;
   return;
+
+ fallback:
+  ent->cwnd = ent->unacked = ent->mss = ent->notsent = 0;
+  ent->limit = INT_MAX;
 }
 
 /* Given a socket that isn't in the table, add it.