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

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Message-ID: <CAF=yD-K3u0RqGXzyqEEm7hGm0cpKPXxQ_cjnDU33SxOwHiZ1-g@mail.gmail.com>
Date:   Fri, 3 Feb 2017 15:25:50 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Rick Jones <rick.jones2@....com>
Cc:     Benjamin Serebrin <serebrin@...gle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        jasowang@...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 Fri, Feb 3, 2017 at 1:34 PM, Rick Jones <rick.jones2@....com> wrote:
> On 02/03/2017 10:31 AM, Willem de Bruijn wrote:
>>
>> Configuring interrupts and xps from userspace at boot is more robust,
>> as device driver defaults can change. But especially for customers who
>> are unaware of these settings, choosing sane defaults won't hurt.
>
>
> The devil is in finding the sane defaults.

Agreed, of course.

>  For example, the issues we've
> seen with VMs sending traffic getting reordered when the driver took it upon
> itself to enable xps.

That particular reordering issue [1] was with xps on the host, where vm thread
migration causes flows to exit on different tx queues because ooo_okay is
not supported for flows from tap devices.

Within a guest, on the other hand, xps settings should not cause reordering of
packets from the same flow, as ooo_okay is active for tcp flows.

[1] https://patchwork.ozlabs.org/patch/662913/