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diff --git a/FreeBSD/share/man/man9/callout.9 b/FreeBSD/share/man/man9/callout.9
--- a/FreeBSD/share/man/man9/callout.9
+++ b/FreeBSD/share/man/man9/callout.9
@@ -27,7 +27,7 @@
.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
.\" POSSIBILITY OF SUCH DAMAGE.
.\"
-.Dd September 18, 2026
+.Dd November 9, 2026
.Dt CALLOUT 9
.Os
.Sh NAME
@@ -455,8 +455,15 @@
The callout subsystem provides a softclock thread for each CPU in the system.
Callouts are assigned to a single CPU and are executed by the softclock thread
for that CPU.
-Initially,
+By default,
callouts are assigned to CPU 0.
+If the
+.Va kern.callout_roundrobin
+sysctl is set to a non-zero value,
+callouts initialized after SMP startup are distributed among CPUs
+using round-robin CPU selection.
+This sysctl is also a loader tunable and defaults to zero.
+Changing it does not reassign existing callouts.
The
.Fn callout_reset_on ,
.Fn callout_reset_sbt_on ,
diff --git a/FreeBSD/sys/kern/kern_timeout.c b/FreeBSD/sys/kern/kern_timeout.c
--- a/FreeBSD/sys/kern/kern_timeout.c
+++ b/FreeBSD/sys/kern/kern_timeout.c
@@ -67,6 +67,7 @@
#include <machine/_inttypes.h>
#endif
+#include <machine/atomic.h>
#ifdef SMP
#include <machine/cpu.h>
#endif
@@ -110,6 +111,12 @@
SYSCTL_INT(_kern, OID_AUTO, ncallout, CTLFLAG_RDTUN | CTLFLAG_NOFETCH, &ncallout, 0,
"Number of entries in callwheel and size of timeout() preallocation");
+static bool __read_mostly callout_roundrobin;
+SYSCTL_BOOL(_kern, OID_AUTO, callout_roundrobin, CTLFLAG_RWTUN,
+ &callout_roundrobin, 0,
+ "Distribute initial callout CPU assignments after SMP startup");
+static u_int callout_next_cpu;
+
#ifdef RSS
static int pin_default_swi = 1;
static int pin_pcpu_swi = 1;
@@ -1348,6 +1355,21 @@
return (cancelled);
}
+static int
+callout_init_cpu(void)
+{
+ u_int cpu;
+
+ /* Boot callouts must not wait for AP startup. */
+ if (!callout_roundrobin || !smp_started)
+ return (cc_default_cpu);
+
+ cpu = atomic_fetchadd_int(&callout_next_cpu, 1) % (mp_maxid + 1);
+ if (CPU_ABSENT(cpu))
+ cpu = CPU_NEXT(cpu);
+ return (cpu);
+}
+
void
callout_init(struct callout *c, int mpsafe)
{
@@ -1359,7 +1381,7 @@
c->c_lock = &Giant.lock_object;
c->c_iflags = 0;
}
- c->c_cpu = cc_default_cpu;
+ c->c_cpu = callout_init_cpu();
}
void
@@ -1379,7 +1401,7 @@
*c = (struct callout ){
.c_lock = lock,
.c_iflags = flags,
- .c_cpu = cc_default_cpu,
+ .c_cpu = callout_init_cpu(),
};
}

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