xv6

commit 1c0d873592bb6d44ab2d9e8042fdd4feabbc5e47
parent 2c00491ad7eaab5b0b2eecb0c5f84ccc7be71688
Author: Brian Swetland <swetland@frotz.net>
Date:   Sat, 28 Dec 2013 06:19:17 -0800

scheduler: halt and wait for irq if nothing was scheduleable

h/t tankenmate@hackernews who did this a bit more verbosely
https://news.ycombinator.com/item?id=6974384

Diffstat:
M
kernel/proc.c
 | 
7
++++++-

1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/kernel/proc.c b/kernel/proc.c
@@ -257,12 +257,17 @@ wait(void)
 void
 scheduler(void)
 {
-  struct proc *p;
+  struct proc *p = 0;
 
   for(;;){
     // Enable interrupts on this processor.
     sti();
 
+    // no runnable processes? (did we hit the end of the table last time?)
+    // if so, wait for irq before trying again.
+    if (p == &ptable.proc[NPROC])
+      hlt();
+
     // Loop over process table looking for process to run.
     acquire(&ptable.lock);
     for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){