On MP machines, when a CPU executes mi_switch() and doesn't have any
thread on its runqueue it will try to steal one from another CPU's
runqueue. If it fails to steal a thread from another runqueue it
will pick its own Idle thread.
To decide which thread should be picked, the scheduler evaluate the
"cost" of moving all runnable threads to the current CPU and pick the
best one. This is done by calling sched_proc_to_cpu_cost(). However
this function doesn't really makes sense in this context because the
destination CPU is always the same. So all variables to determine
the cost of moving a thread are constant except the priority of the
given thread.
So I'd like to commit the diff below which makes clear what the
scheduler is currently doing: pick the runnable thread with higher
priority (lowest `p_priority' number).
This change is also necessary to improve the placement of threads
being awaken. Because the real function of sched_proc_to_cpu_cost()
is to select a *CPU* where a thread is going to be run. Since I don't
want to change the algorithm used to steal threads, let's stop using
sched_proc_to_cpu_cost() there.
Ok?
Index: kern/kern_sched.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_sched.c,v
retrieving revision 1.54
diff -u -p -r1.54 kern_sched.c
--- kern/kern_sched.c 17 Nov 2018 23:10:08 -0000 1.54
+++ kern/kern_sched.c 26 Jan 2019 16:30:27 -0000
@@ -494,8 +494,7 @@ sched_steal_proc(struct cpu_info *self)
if (p->p_flag & P_CPUPEG)
continue;
- cost = sched_proc_to_cpu_cost(self, p);
-
+ cost = p->p_priority;
if (best == NULL || cost < bestcost) {
best = p;
bestcost = cost;
