More mtrace un-bit-rotting

Previously, sched always paused call tracing but if it then returned directly because there was nothing else to schedule, call tracing didn't get resumed. We could simply resume tracing in this case, but this fix instead moves the pause *after* the next proc is picked, so that we don't get lots of spurious pause/resume pairs.

More mtrace un-bit-rotting
69b12bc9 · Austin Clements · 523ac2a9 · 69b12bc9
--- a/kernel/sched.cc
+++ b/kernel/sched.cc
@@ -55,11 +55,6 @@ sched(void)
    panic("sched interruptible");
  intena = mycpu()->intena;
  myproc()->curcycles += rdtsc() - myproc()->tsc;
-  if (myproc()->get_state() == ZOMBIE)
-    mtstop(myproc());
-  else
-    mtpause(myproc());
-  mtign();

  struct proc *next = schednext();
  if (next == nullptr) {
@@ -80,6 +75,12 @@ sched(void)
  mycpu()->proc = next;
  mycpu()->prev = prev;

+  if (prev->get_state() == ZOMBIE)
+    mtstop(prev);
+  else
+    mtpause(prev);
+  mtign();
+
  switchvm(next);
  next->set_state(RUNNING);
  next->tsc = rdtsc();