Re: [PATCH v3 1/2] cpuhp: Optimize SMT switch operation by batching lock acquisition

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, Feb 18 2026 at 14:09, Vishal Chourasia wrote:
> From: Joel Fernandes <joelagnelf@nvidia.com>
>
> Bulk CPU hotplug operations, such as an SMT switch operation, requires
> hotplugging multiple CPUs. The current implementation takes
> cpus_write_lock() for each individual CPU, causing multiple slow grace
> period requests.
>
> Introduce cpu_up_locked() and cpu_down_locked() that assume the caller
> already holds cpus_write_lock(). The cpuhp_smt_enable() and
> cpuhp_smt_disable() functions are updated to hold the lock once around
> the entire loop, rather than for each individual CPU.
>
> Link: https://lore.kernel.org/all/20260113090153.GS830755@noisy.programming.kicks-ass.net/
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Vishal Chourasia <vishalc@linux.ibm.com>

You dropped Joel's Signed-off-by ....

> -/* Requires cpu_add_remove_lock to be held */
> -static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
> +/* Requires cpu_add_remove_lock and cpus_write_lock to be held */
> +static int __ref cpu_down_locked(unsigned int cpu, int tasks_frozen,
>  			   enum cpuhp_state target)

No line break required. You have 100 chars. If you still need one:

  https://www.kernel.org/doc/html/latest/process/maintainer-tip.html

>  	 */
>  	if (cpumask_any_and(cpu_online_mask,
>  			    housekeeping_cpumask(HK_TYPE_DOMAIN)) >= nr_cpu_ids) {
> -		ret = -EBUSY;
> -		goto out;
> +		return -EBUSY;
>  	}

Please remove the brackets. They are not longer required. All over the place.

> +static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
> +			   enum cpuhp_state target)
> +{
> +
> +	int ret;
> +	cpus_write_lock();

Coding style...

> +	ret = cpu_down_locked(cpu, tasks_frozen, target);
>  	cpus_write_unlock();
>  	arch_smt_update();
>  	return ret;
> @@ -2659,6 +2674,16 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
>  	int cpu, ret = 0;
>  
>  	cpu_maps_update_begin();
> +	if (cpu_hotplug_offline_disabled) {
> +		ret = -EOPNOTSUPP;
> +		goto out;
> +	}
> +	if (cpu_hotplug_disabled) {
> +		ret = -EBUSY;
> +		goto out;
> +	}
> +	/* Hold cpus_write_lock() for entire batch operation. */
> +	cpus_write_lock();

 .... for the entire ...

And please visiually separate things. Newlines exist for a reason.

Thanks,

        tglx