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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

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?