Re: [PATCH] cpuhp: Expedite synchronize_rcu during SMT switch

[Peter Zijlstra]

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;
 }