> Date: Sat, 26 Jan 2019 14:46:41 -0200
> From: Martin Pieuchot <m...@openbsd.org>
>
> 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?
Hmm. The idea has always been that sched_proc_to_cpu_cost() should
also take into account the CPU the process last ran on. The idea
being that moving processes between cores of the same physical
processor may very well be cheaper that moving between different
physical processors since a bigger part of the cache hierarchy is
shared. See the comment above the function.
Now coming up with a good cost model for this isn't trivial,
especially since we don't really have the right topology information
available in the kernel. But I'd still sat that calling
sched_proc_to_cpu_cost() is the right function to call here.
> 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;
>
>
