Re: [PATCH v3] drivers: hv: vmbus: Use kthread for vmbus interrupts on PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-02-16 17:24:56 [+0100], Jan Kiszka wrote:
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -25,6 +25,7 @@
>  #include <linux/cpu.h>
>  #include <linux/sched/isolation.h>
>  #include <linux/sched/task_stack.h>
> +#include <linux/smpboot.h>
>  
>  #include <linux/delay.h>
>  #include <linux/panic_notifier.h>
> @@ -1350,7 +1351,7 @@ static void vmbus_message_sched(struct hv_per_cpu_context *hv_cpu, void *message
>  	}
>  }
>  
> -void vmbus_isr(void)
> +static void __vmbus_isr(void)
>  {
>  	struct hv_per_cpu_context *hv_cpu
>  		= this_cpu_ptr(hv_context.cpu_context);
> @@ -1363,6 +1364,53 @@ void vmbus_isr(void)
>  
>  	add_interrupt_randomness(vmbus_interrupt);

This is feeding entropy and would like to see interrupt registers. But
since this is invoked from a thread it won't.

>  }
> +
…
> +void vmbus_isr(void)
> +{
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +		vmbus_irqd_wake();
> +	} else {
> +		lockdep_hardirq_threaded();

What clears this? This is wrongly placed. This should go to
sysvec_hyperv_callback() instead with its matching canceling part. The
add_interrupt_randomness() should also be there and not here.
sysvec_hyperv_stimer0() managed to do so.

Different question: What guarantees that there won't be another
interrupt before this one is done? The handshake appears to be
deprecated. The interrupt itself returns ACKing (or not) but the actual
handler is delayed to this thread. Depending on the userland it could
take some time and I don't know how impatient the host is.

> +		__vmbus_isr();
Moving on. This (trying very hard here) even schedules tasklets. Why?
You need to disable BH before doing so. Otherwise it ends in ksoftirqd.
You don't want that.

Couldn't the whole logic be integrated into the IRQ code? Then we could
have mask/ unmask if supported/ provided and threaded interrupts. Then
sysvec_hyperv_reenlightenment() could use a proper threaded interrupt
instead apic_eoi() + schedule_delayed_work(). 

> +	}
> +}
>  EXPORT_SYMBOL_FOR_MODULES(vmbus_isr, "mshv_vtl");
>  
>  static irqreturn_t vmbus_percpu_isr(int irq, void *dev_id)

Sebastian