Hi Peter.
On 1/19/26 5:13 PM, Peter Zijlstra wrote:
On Mon, Jan 19, 2026 at 04:17:40PM +0530, Vishal Chourasia wrote:
Expedite synchronize_rcu() during the cpuhp_smt_[enable|disable] path to
accelerate the operation.
Bulk CPU hotplug operations—such as switching SMT modes across all
cores—require hotplugging multiple CPUs in rapid succession. On large
systems, this process takes significant time, increasing as the number
of CPUs to hotplug during SMT switch grows, leading to substantial
delays on high-core-count machines. Analysis [1] reveals that the
majority of this time is spent waiting for synchronize_rcu().
You seem to have left out all the useful bits from your changelog again
:/
Anyway, ISTR Joel posted a patch hoisting a lock; it was a icky, but not
something we can't live with either.
Also, memory got jogged and I think something like the below will remove
2/3 of your rcu woes as well.
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 8df2d773fe3b..1365c19444b2 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2669,6 +2669,7 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
int cpu, ret = 0;
cpu_maps_update_begin();
+ rcu_sync_enter(&cpu_hotplug_lock.rss);
for_each_online_cpu(cpu) {
if (topology_is_primary_thread(cpu))
continue;
@@ -2698,6 +2699,7 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
}
if (!ret)
cpu_smt_control = ctrlval;
+ rcu_sync_exit(&cpu_hotplug_lock.rss);
cpu_maps_update_done();
return ret;
}
@@ -2715,6 +2717,7 @@ int cpuhp_smt_enable(void)
int cpu, ret = 0;
cpu_maps_update_begin();
+ rcu_sync_enter(&cpu_hotplug_lock.rss);
cpu_smt_control = CPU_SMT_ENABLED;
for_each_present_cpu(cpu) {
/* Skip online CPUs and CPUs on offline nodes */
@@ -2728,6 +2731,7 @@ int cpuhp_smt_enable(void)
/* See comment in cpuhp_smt_disable() */
cpuhp_online_cpu_device(cpu);
}
+ rcu_sync_exit(&cpu_hotplug_lock.rss);
cpu_maps_update_done();
return ret;
}
Currently, cpuhp_smt_[enable/disable] calls _cpu_up/_cpu_down
which does the same in cpus_write_lock/unlock. though it is per
cpu enable/disable one after another.
How hoisting this up will help?
