From: Tim Leslie <tleslie@protonmail.com>
Subject: Re: Move fork CPU selection into scheduler
To: "tech@openbsd.org" <tech@openbsd.org>
Date: Tue, 04 Nov 2025 17:17:24 +0000

On Monday, October 13th, 2025 at 5:53 PM, Tim Leslie <tleslie@protonmail.com> wrote:

> Tech,
> 
> The following patch consolidates fork-time CPU selection behavior entirely within the scheduler by integrating the fork_thread_start functionality into sched_choosecpu_fork.

Anyone?

> diff --git a/sys/kern/kern_fork.c b/sys/kern/kern_fork.c
> --- a/sys/kern/kern_fork.c
> +++ b/sys/kern/kern_fork.c
> @@ -333,18 +333,6 @@ fork_check_maxthread(uid_t uid)
> return 0;
> }
> 
> -static inline void
> -fork_thread_start(struct proc *p, struct proc *parent, int flags)
> -{
> - struct cpu_info *ci;
> -
> - SCHED_LOCK();
> - ci = sched_choosecpu_fork(parent, flags);
> - TRACEPOINT(sched, fork, p->p_tid + THREAD_PID_OFFSET,
> 
> - p->p_p->ps_pid, CPU_INFO_UNIT(ci));
> 
> - setrunqueue(ci, p, p->p_usrpri);
> 
> - SCHED_UNLOCK();
> -}
> 
> int
> fork1(struct proc *curp, int flags, void (*func)(void *), void *arg,
> @@ -494,7 +482,7 @@ fork1(struct proc *curp, int flags, void (*func)(void *), void arg,
> / for consistency mark idle procs as pegged */
> atomic_setbits_int(&p->p_flag, P_CPUPEG);
> 
> } else
> - fork_thread_start(p, curp, flags);
> + sched_choosecpu_fork(p, curp, flags);
> 
> free(newptstat, M_SUBPROC, sizeof(*newptstat));
> 
> @@ -617,7 +605,7 @@ thread_fork(struct proc *curp, void *stack, void *tcb, pid_t tidptr,
> psignal(curp, SIGSEGV);
> }
> 
> - fork_thread_start(p, curp, 0);
> + sched_choosecpu_fork(p, curp, 0);
> 
> /
> * Update stats now that we know the fork was successful.
> diff --git a/sys/kern/kern_sched.c b/sys/kern/kern_sched.c
> --- a/sys/kern/kern_sched.c
> +++ b/sys/kern/kern_sched.c
> @@ -371,56 +371,51 @@ again:
> return (p);
> }
> 
> -struct cpu_info *
> -sched_choosecpu_fork(struct proc *parent, int flags)
> +void
> +sched_choosecpu_fork(struct proc *p, struct proc *parent, int flags)
> {
> + struct cpu_info *choice = NULL, *ci;
> +
> + SCHED_LOCK();
> #ifdef MULTIPROCESSOR
> - struct cpu_info *choice = NULL;
> int run, best_run = INT_MAX;
> - struct cpu_info ci;
> struct cpuset set;
> 
> -#if 0
> - /
> - * XXX
> - * Don't do this until we have a painless way to move the cpu in exec.
> - * Preferably when nuking the old pmap and getting a new one on a
> - * new cpu.
> - /
> - /
> - * PPWAIT forks are simple. We know that the parent will not
> - * run until we exec and choose another cpu, so we just steal its
> - * cpu.
> - */
> - if (flags & FORK_PPWAIT)
> - return (parent->p_cpu);
> 
> -#endif
> -
> - /*
> - * Look at all cpus that are currently idle and have nothing queued.
> - * If there are none, pick the one with least queued procs first,
> - * then the one with lowest load average.
> - */
> - cpuset_complement(&set, &sched_queued_cpus, &sched_idle_cpus);
> - cpuset_intersection(&set, &set, &sched_all_cpus);
> - if (cpuset_first(&set) == NULL)
> - cpuset_copy(&set, &sched_all_cpus);
> -
> - while ((ci = cpuset_first(&set)) != NULL) {
> - cpuset_del(&set, ci);
> -
> - run = ci->ci_schedstate.spc_nrun;
> 
> + /* Fast path: any idle CPUs at all? /
> + if (cpuset_first(&sched_idle_cpus) != NULL) {
> + / Intersect idle with unqueued and available set /
> + cpuset_complement(&set, &sched_queued_cpus, &sched_idle_cpus);
> + cpuset_intersection(&set, &set, &sched_all_cpus);
> 
> - if (choice == NULL || run < best_run) {
> + / Take the first idle-and-unqueued CPU if any /
> + if ((ci = cpuset_first(&set)) != NULL)
> choice = ci;
> - best_run = run;
> + }
> +
> + if (choice == NULL) {
> + / Fallback: scan all CPUs for least spc_nrun */
> + cpuset_copy(&set, &sched_all_cpus);
> + while ((ci = cpuset_first(&set)) != NULL) {
> + cpuset_del(&set, ci);
> + run = ci->ci_schedstate.spc_nrun;
> 
> + if (choice == NULL || run < best_run) {
> + choice = ci;
> + best_run = run;
> + }
> }
> }
> 
> + /* current CPU if nothing else was suitable */
> if (choice != NULL)
> - return (choice);
> + choice = (curcpu());
> +#else
> + choice = curcpu();
> #endif
> - return (curcpu());
> +
> + TRACEPOINT(sched, fork, p->p_tid + THREAD_PID_OFFSET,
> 
> + p->p_p->ps_pid, CPU_INFO_UNIT(choice));
> 
> + setrunqueue(choice, p, p->p_usrpri);
> 
> + SCHED_UNLOCK();
> }
> 
> struct cpu_info *
> diff --git a/sys/sys/sched.h b/sys/sys/sched.h
> --- a/sys/sys/sched.h
> +++ b/sys/sys/sched.h
> @@ -168,7 +168,7 @@ void mi_switch(void);
> void cpu_switchto(struct proc *, struct proc *);
> struct proc *sched_chooseproc(void);
> struct cpu_info *sched_choosecpu(struct proc *);
> -struct cpu_info *sched_choosecpu_fork(struct proc *parent, int);
> +void sched_choosecpu_fork(struct proc *p, struct proc *parent, int flags);
> void cpu_idle_enter(void);
> void cpu_idle_cycle(void);
> void cpu_idle_leave(void);