Do not retake the SCHED_LOCK when going out of mi_switch(). No code really depends on that and it is one extra mutex operation for no good reason.

Switch spc_curpriority to be volatile. It is only modified by the curcpu()
but read by other CPUs. Mark it like this in sched.h.
OK mpi@

sys/kern/kern_sched.c

···
-/*	$OpenBSD: kern_sched.c,v 1.107 2025/05/28 03:27:44 jsg Exp $	*/
+/*	$OpenBSD: kern_sched.c,v 1.108 2025/05/31 06:58:27 claudio Exp $	*/
 /*
  * Copyright (c) 2007, 2008 Artur Grabowski <[e-mail]>
  *
···
 	p->p_cpu = ci;
 	atomic_setbits_int(&p->p_flag, P_CPUPEG);
 	mi_switch();
-	SCHED_UNLOCK();
 
 	KASSERT(ci == curcpu());
 	KASSERT(curproc == spc->spc_idleproc);
···
 			SCHED_LOCK();
 			p->p_stat = SSLEEP;
 			mi_switch();
-			SCHED_UNLOCK();
 
 			while ((dead = TAILQ_FIRST(&spc->spc_deadproc))) {
 				TAILQ_REMOVE(&spc->spc_deadproc, dead, p_runq);
···
 	setrunqueue(ci, p, p->p_usrpri);
 	p->p_ru.ru_nvcsw++;
 	mi_switch();
-	SCHED_UNLOCK();
 }
 
 void

sys/kern/kern_sig.c

···
-/*	$OpenBSD: kern_sig.c,v 1.364 2025/03/10 09:28:56 claudio Exp $	*/
+/*	$OpenBSD: kern_sig.c,v 1.365 2025/05/31 06:58:27 claudio Exp $	*/
 /*	$NetBSD: kern_sig.c,v 1.54 1996/04/22 01:38:32 christos Exp $	*/
 
 /*
···
 	if (p->p_stat == SSTOP) {
 		p->p_ru.ru_nvcsw++;
 		mi_switch();
+	} else {
+		KASSERT(p->p_stat == SONPROC);
+		SCHED_UNLOCK();
 	}
-	KASSERT(p->p_stat == SONPROC);
-
-	SCHED_UNLOCK();
 	mtx_enter(&pr->ps_mtx);
 }
 

sys/kern/kern_synch.c

···
-/*	$OpenBSD: kern_synch.c,v 1.223 2025/05/01 06:58:21 dlg Exp $	*/
+/*	$OpenBSD: kern_synch.c,v 1.224 2025/05/31 06:58:27 claudio Exp $	*/
 /*	$NetBSD: kern_synch.c,v 1.37 1996/04/22 01:38:37 christos Exp $	*/
 
 /*
···
 	} else {
 		KASSERT(p->p_stat == SONPROC || p->p_stat == SSLEEP);
 		p->p_stat = SONPROC;
+		SCHED_UNLOCK();
 	}
 
 #ifdef DIAGNOSTIC
···
 #endif
 
 	p->p_cpu->ci_schedstate.spc_curpriority = p->p_usrpri;
-	SCHED_UNLOCK();
 
 	/*
 	 * Even though this belongs to the signal handling part of sleep,
···
 	setrunqueue(p->p_cpu, p, newprio);
 	p->p_ru.ru_nvcsw++;
 	mi_switch();
-	SCHED_UNLOCK();
 
 	return (0);
 }

sys/kern/sched_bsd.c

···
-/*	$OpenBSD: sched_bsd.c,v 1.99 2025/03/10 09:28:56 claudio Exp $	*/
+/*	$OpenBSD: sched_bsd.c,v 1.100 2025/05/31 06:58:27 claudio Exp $	*/
 /*	$NetBSD: kern_synch.c,v 1.37 1996/04/22 01:38:37 christos Exp $	*/
 
 /*-
···
 	setrunqueue(p->p_cpu, p, p->p_usrpri);
 	p->p_ru.ru_nvcsw++;
 	mi_switch();
-	SCHED_UNLOCK();
 }
 
 /*
···
 	setrunqueue(p->p_cpu, p, p->p_usrpri);
 	p->p_ru.ru_nivcsw++;
 	mi_switch();
-	SCHED_UNLOCK();
 }
 
 void
···
 	if (hold_count)
 		__mp_acquire_count(&kernel_lock, hold_count);
 #endif
-	SCHED_LOCK();
 }
 
 /*

sys/sys/sched.h

···
-/*	$OpenBSD: sched.h,v 1.74 2025/05/16 13:40:30 mpi Exp $	*/
+/*	$OpenBSD: sched.h,v 1.75 2025/05/31 06:58:27 claudio Exp $	*/
 /* $NetBSD: sched.h,v 1.2 1999/02/28 18:14:58 ross Exp $ */
 
 /*-
···
 
 /*
  * Per-CPU scheduler state.
+ *	o	owned (modified only) by this CPU
  */
 struct schedstate_percpu {
 	struct proc *spc_idleproc;	/* idle proc for this cpu */
···
 	volatile int spc_schedflags;	/* flags; see below */
 	u_int spc_schedticks;		/* ticks for schedclock() */
 	u_int64_t spc_cp_time[CPUSTATES]; /* CPU state statistics */
-	u_char spc_curpriority;		/* usrpri of curproc */
 
 	struct clockintr spc_itimer;	/* [o] itimer_update handle */
 	struct clockintr spc_profclock;	/* [o] profclock handle */
···
 	u_char spc_smrexpedite;		/* if set, dispatch smr entries
 					 * without delay */
 	u_char spc_smrgp;		/* this CPU's view of grace period */
+	volatile u_char spc_curpriority; /* [o] usrpri of curproc */
 };
 
 /* spc_flags */