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

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Message-ID: <5a5d9355-34fc-57aa-825c-81123f6bb74e@citrix.com>
Date:   Mon, 5 Jun 2017 11:14:42 +0100
From:   Anoob Soman <anoob.soman@...rix.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.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

On 02/06/17 17:24, Boris Ostrovsky wrote:
>>   
>>   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.

Sure, I will do it.

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

The reason for percpu instead of global, was to avoid locking. We can 
have a global variable (last_cpu) without locking, but value of last_cpu 
wont be consistent, without locks. Moreover, since irq_affinity is also 
used in the calculation of cpu to bind, having a percpu or global 
wouldn't really matter, as the result (selected_cpu) is more likely to 
be random (because different irqs can have different affinity). What do 
you guys suggest.

>
>> +
>>   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
>

I think we would still require spin_lock(). spin_lock is for irq_desc.

>> +	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();
>> +}
>> +
>>