netdev - Re: [PATCH net-next] virtio: Fix affinity for >32 VCPUs

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Date:   Mon, 6 Feb 2017 15:45:38 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Benjamin Serebrin <serebrin@...gle.com>
Cc:     netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>,
        Venkatesh Srinivas <venkateshs@...gle.com>,
        James Mattson <jmattson@...gle.com>
Subject: Re: [PATCH net-next] virtio: Fix affinity for >32 VCPUs



On 2017年02月06日 15:28, Benjamin Serebrin wrote:
> On Sun, Feb 5, 2017 at 11:24 PM, Jason Wang<jasowang@...hat.com>  wrote:
>> On 2017年02月03日 14:19, Ben Serebrin wrote:
>>> From: Benjamin Serebrin<serebrin@...gle.com>
>>>
>>> If the number of virtio queue pairs is not equal to the
>>> number of VCPUs, the virtio guest driver doesn't assign
>>> any CPU affinity for the queue interrupts or the xps
>>> aggregation interrupt.
>> So this in fact is not a affinity fixing for #cpus > 32 but adding  affinity
>> for #cpus != #queue pairs.
> Fair enough.  I'll adjust the title line in the subsequent version.
>
>
>>> Google Compute Engine currently provides 1 queue pair for
>>> every VCPU, but limits that at a maximum of 32 queue pairs.
>>>
>>> This code assigns interrupt affinity even when there are more than
>>> 32 VCPUs.
>>>
>>> Tested:
>>>
>>> (on a 64-VCPU VM with debian 8, jessie-backports 4.9.2)
>>>
>>> Without the fix we see all queues affinitized to all CPUs:
>> [...]
>>
>>>    +     /* If there are more cpus than queues, then assign the queues'
>>> +        * interrupts to the first cpus until we run out.
>>> +        */
>>>          i = 0;
>>>          for_each_online_cpu(cpu) {
>>> +               if (i == vi->max_queue_pairs)
>>> +                       break;
>>>                  virtqueue_set_affinity(vi->rq[i].vq, cpu);
>>>                  virtqueue_set_affinity(vi->sq[i].vq, cpu);
>>> -               netif_set_xps_queue(vi->dev, cpumask_of(cpu), i);
>>>                  i++;
>>>          }
>>>    +     /* Stripe the XPS affinities across the online CPUs.
>>> +        * Hyperthread pairs are typically assigned such that Linux's
>>> +        * CPU X and X + (numcpus / 2) are hyperthread twins, so we cause
>>> +        * hyperthread twins to share TX queues, in the case where there
>>> are
>>> +        * more cpus than queues.
>> Since we use combined queue pairs, why not use the same policy for RX?
> XPS is for transmit only.
>
>

Yes, but I mean, e.g consider you let hyperthread twins to share TX 
queues (XPS), why not share TX and RX queue interrupts (affinity)?

Thanks