linux-kernel - Re: [PATCH] xen-evtchn: Bind dyn evtchn:qemu-dm interrupt to next online VCPU

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Date:   Fri, 2 Jun 2017 12:24:40 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Anoob Soman <anoob.soman@...rix.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     jgross@...e.com
Subject: Re: [PATCH] xen-evtchn: Bind dyn evtchn:qemu-dm interrupt to next
 online VCPU


>  
>  static int set_affinity_irq(struct irq_data *data, const struct cpumask *dest,
>  			    bool force)
> diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
> index 10f1ef5..1192f24 100644
> --- a/drivers/xen/evtchn.c
> +++ b/drivers/xen/evtchn.c
> @@ -58,6 +58,8 @@
>  #include <xen/xen-ops.h>
>  #include <asm/xen/hypervisor.h>
>  
> +static DEFINE_PER_CPU(int, bind_last_selected_cpu);

This should be moved into evtchn_bind_interdom_next_vcpu() since that's
the only place referencing it.

Why is it a percpu variable BTW? Wouldn't making it global result in
better interrupt distribution?

> +
>  struct per_user_data {
>  	struct mutex bind_mutex; /* serialize bind/unbind operations */
>  	struct rb_root evtchns;
> @@ -421,6 +423,36 @@ static void evtchn_unbind_from_user(struct per_user_data *u,
>  	del_evtchn(u, evtchn);
>  }
>  
> +static void evtchn_bind_interdom_next_vcpu(int evtchn)
> +{
> +	unsigned int selected_cpu, irq;
> +	struct irq_desc *desc = NULL;
> +	unsigned long flags;
> +
> +	irq = irq_from_evtchn(evtchn);
> +	desc = irq_to_desc(irq);
> +
> +	if (!desc)
> +		return;
> +
> +	raw_spin_lock_irqsave(&desc->lock, flags);
> +	selected_cpu = this_cpu_read(bind_last_selected_cpu);
> +	selected_cpu = cpumask_next_and(selected_cpu,
> +			desc->irq_common_data.affinity, cpu_online_mask);
> +
> +	if (unlikely(selected_cpu >= nr_cpu_ids))
> +		selected_cpu = cpumask_first_and(desc->irq_common_data.affinity,
> +				cpu_online_mask);
> +
> +	raw_spin_unlock_irqrestore(&desc->lock, flags);

I think if you follow Juergen's suggestion of wrapping everything into
irq_enable/disable you can drop the lock altogether (assuming you keep
bind_last_selected_cpu percpu).

-boris


> +	this_cpu_write(bind_last_selected_cpu, selected_cpu);
> +
> +	local_irq_disable();
> +	/* unmask expects irqs to be disabled */
> +	xen_rebind_evtchn_to_cpu(evtchn, selected_cpu);
> +	local_irq_enable();
> +}
> +
>