netdev - Re: [PATCH v2 net-next] virtio: Fix affinity for #VCPUs != #queue pairs

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Message-ID: <CAN+hb0X4UWmDvA0=5gpC6+Qt7vDgz0u5B79CM4KqQ-yuJmAO7Q@mail.gmail.com>
Date:   Wed, 15 Feb 2017 13:38:48 -0800
From:   Benjamin Serebrin <serebrin@...gle.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Network Development <netdev@...r.kernel.org>,
        Jason Wang <jasowang@...hat.com>,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>,
        Venkatesh Srinivas <venkateshs@...gle.com>,
        "Jon Olson (Google Drive)" <jonolson@...gle.com>,
        Rick Jones <rick.jones2@....com>,
        James Mattson <jmattson@...gle.com>,
        linux-s390 <linux-s390@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v2 net-next] virtio: Fix affinity for #VCPUs != #queue pairs

On Wed, Feb 15, 2017 at 11:17 AM, Michael S. Tsirkin <mst@...hat.com> wrote:

> Right. But userspace knows it's random at least. If kernel supplies
> affinity e.g. the way your patch does, userspace ATM accepts this as a
> gospel.

The existing code supplies the same affinity gospels in the #vcpu ==
#queue case today.
And the patch (unless it has a bug in it) should not affect the #vcpu
== #queue case's
behavior.  I don't quite understand what property we'd be changing
with the patch.

Here's the same dump of smp_affinity_list, on a 16 VCPU machine with
unmodified kernel:

0
0
1
1
2
2
[..]
15
15

And xps_cpus
00000001
00000002
[...]
00008000

This patch causes #vcpu != #queue case to follow the same pattern.


Thanks again!
Ben