RE: [PATCH] Drivers: hv: vmbus: Deferring per cpu tasks

[Michael Kelley]

From: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx> Sent: Wednesday, July 24, 2024 10:26 PM
> 
> Currently on a very large system with 1780 CPUs, hv_acpi_init takes
> around 3 seconds to complete for all the CPUs. This is because of
> sequential synic initialization for each CPU.
> 
> Defer these tasks so that each CPU executes hv_acpi_init in parallel
> to take full advantage of multiple CPUs.
> 
> This solution saves around 2 seconds of boot time on a 1780 CPU system,
> that around 66% improvement in the existing logic.
> 
> Signed-off-by: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx>
> ---
>  drivers/hv/vmbus_drv.c | 33 ++++++++++++++++++++++++++++++---
>  1 file changed, 30 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index c857dc3975be..3395526ad0d0 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -1306,6 +1306,13 @@ static irqreturn_t vmbus_percpu_isr(int irq, void *dev_id)
>  	return IRQ_HANDLED;
>  }
> 
> +static void vmbus_percpu_work(struct work_struct *work)
> +{
> +	unsigned int cpu = smp_processor_id();
> +
> +	hv_synic_init(cpu);
> +}
> +
>  /*
>   * vmbus_bus_init -Main vmbus driver initialization routine.
>   *
> @@ -1316,7 +1323,8 @@ static irqreturn_t vmbus_percpu_isr(int irq, void *dev_id)
>   */
>  static int vmbus_bus_init(void)
>  {
> -	int ret;
> +	int ret, cpu;
> +	struct work_struct __percpu *works;
> 
>  	ret = hv_init();
>  	if (ret != 0) {
> @@ -1355,12 +1363,31 @@ static int vmbus_bus_init(void)
>  	if (ret)
>  		goto err_alloc;
> 
> +	works = alloc_percpu(struct work_struct);
> +	if (!works) {
> +		ret = -ENOMEM;
> +		goto err_alloc;
> +	}
> +
>  	/*
>  	 * Initialize the per-cpu interrupt state and stimer state.
>  	 * Then connect to the host.
>  	 */
> -	ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "hyperv/vmbus:online",
> -				hv_synic_init, hv_synic_cleanup);
> +	cpus_read_lock();
> +	for_each_online_cpu(cpu) {
> +		struct work_struct *work = per_cpu_ptr(works, cpu);
> +
> +		INIT_WORK(work, vmbus_percpu_work);
> +		schedule_work_on(cpu, work);
> +	}
> +
> +	for_each_online_cpu(cpu)
> +		flush_work(per_cpu_ptr(works, cpu));
> +
> +	ret = __cpuhp_setup_state_cpuslocked(CPUHP_AP_ONLINE_DYN, "hyperv/vmbus:online", false,
> +					     hv_synic_init, hv_synic_cleanup, false);

I'd suggest using cpuhp_setup_state_nocalls_cpuslocked().  It appears to be
the interface intended for users outside the cpuhotplug code, whereas
__cpuhp_setup_state_cpuslocked() should be private to the cpuhotplug code.

Michael

> +	cpus_read_unlock();
> +	free_percpu(works);
>  	if (ret < 0)
>  		goto err_alloc;
>  	hyperv_cpuhp_online = ret;
> --
> 2.43.0
>