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

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Message-ID: <da9d7835-7473-87fc-9766-016a743af5ef@citrix.com>
Date:   Mon, 5 Jun 2017 17:14:34 +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: [Xen-devel] [PATCH] xen-evtchn: Bind dyn evtchn:qemu-dm interrupt
 to next online VCPU

On 05/06/17 16:32, Boris Ostrovsky wrote:
> I believe we do need to take affinity into consideration even if the
> chance that it is non-default is small.

Agreed.

> I am not opposed to having bind_last_selected_cpu percpu, I just wanted
> to understand the reason better. Additional locking would be a downside
> with a global so if you feel that percpu is worth it then I won't object.

If affinity == cpu_online_mask, then percpu will give a better spread. 
atomic set/get can be used, if we want to use a global variable, but I 
think it will be more random than percpu.

>
>> Yes, you are correct. .irq_set_affinity pretty much does the same thing.
>>
>> The code will now looks like this.
>> raw_spin_lock_irqsave(lock, flags);
>> percpu read
>> select_cpu
>> percpu write
>> xen_rebind_evtchn_to_cpu(evtchn, selected_cpu)
>> raw_spin_unlock_irqsave(lock, flags);
> (BTW, I just noticed --- you don't need to initialize desc)

Sorry, I didn't get it. Which desc doesn't need init ?

-Anoob