From: Yuri Benditovich <yuri.benditovich@daynix.com>
To: Akihiko Odaki <akihiko.odaki@daynix.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
Jason Wang <jasowang@redhat.com>,
qemu-devel@nongnu.org, Andrew Melnychenko <andrew@daynix.com>
Subject: Re: [PATCH v6 11/21] virtio-net: Return an error when vhost cannot enable RSS
Date: Mon, 13 Nov 2023 13:44:10 +0200 [thread overview]
Message-ID: <CAOEp5Oee2qinrZJgMMxUQt6zmPVFPCnThfqnLFSWqsSyAoHpjQ@mail.gmail.com> (raw)
In-Reply-To: <8439be4e-a739-4cbd-a569-89b6c7f68ab9@daynix.com>
[-- Attachment #1: Type: text/plain, Size: 4127 bytes --]
On Sat, Nov 11, 2023 at 5:28 PM Akihiko Odaki <akihiko.odaki@daynix.com>
wrote:
> On 2023/11/03 22:14, Yuri Benditovich wrote:
> >
> >
> > On Fri, Nov 3, 2023 at 11:55 AM Akihiko Odaki <akihiko.odaki@daynix.com
> > <mailto:akihiko.odaki@daynix.com>> wrote:
> >
> > On 2023/11/03 18:35, Yuri Benditovich wrote:
> > >
> > >
> > > On Thu, Nov 2, 2023 at 4:56 PM Akihiko Odaki
> > <akihiko.odaki@daynix.com <mailto:akihiko.odaki@daynix.com>
> > > <mailto:akihiko.odaki@daynix.com
> > <mailto:akihiko.odaki@daynix.com>>> wrote:
> > >
> > > On 2023/11/02 19:20, Yuri Benditovich wrote:
> > > >
> > > >
> > > > On Thu, Nov 2, 2023 at 11:33 AM Michael S. Tsirkin
> > > <mst@redhat.com <mailto:mst@redhat.com>
> > <mailto:mst@redhat.com <mailto:mst@redhat.com>>
> > > > <mailto:mst@redhat.com <mailto:mst@redhat.com>
> > <mailto:mst@redhat.com <mailto:mst@redhat.com>>>> wrote:
> > > >
> > > > On Thu, Nov 02, 2023 at 11:09:27AM +0200, Yuri
> > Benditovich wrote:
> > > > > Probably we mix two different patches in this
> > discussion.
> > > > > Focusing on the patch in the e-mail header:
> > > > >
> > > > > IMO it is not acceptable to fail QEMU run for one
> > feature
> > > that we
> > > > can't make
> > > > > active when we silently drop all other features in
> > such a
> > > case.
> > > >
> > > > If the feature is off by default then it seems more
> > reasonable
> > > > and silent masking can be seen as a bug.
> > > > Most virtio features are on by default this is why it's
> > > > reasonable to mask them.
> > > >
> > > >
> > > > If we are talking about RSS: setting it initially off is
> the
> > > development
> > > > time decision.
> > > > When it will be completely stable there is no reason to
> > keep it
> > > off by
> > > > default, so this is more a question of time and of a
> > readiness of
> > > libvirt.
> > >
> > > It is not ok to make "on" the default; that will enable RSS
> > even when
> > > eBPF steering support is not present and can result in
> > performance
> > > degradation.
> > >
> > >
> > > Exactly as it is today - with vhost=on the host does not suggest
> RSS
> > > without eBPF.
> > > I do not understand what you call "performance degradation", can
> you
> > > describe the scenario?
> >
> > I was not clear, but I was talking about the case of vhost=off or
> peers
> > other than tap (e.g., user). rss=on employs in-qemu RSS, which incurs
> > overheads for such configurations.
> >
> >
> > So, vhost=off OR peers other than tap:
> >
> > In the case of peers other than tap (IMO) we're not talking about
> > performance at all.
> > Backends like "user" (without vnet_hdr) do not support _many_
> > performance-oriented features.
> > If RSS is somehow "supported" for such backends this is rather a
> > misunderstanding (IMO again).
>
> We do not need to ensure good performance when RSS is enabled by the
> guest for backends without eBPF steering program as you say. In-QEMU RSS
> is only useful for testing and not meant to improve the performance.
>
> However, if you set rss=on, QEMU will advertise the availability of RSS
> feature. The guest will have no mean to know if it's implemented in a
> way not performance-wise so it may decide to use the feature to improve
> the performance, which can result in performance degradation. Therefore,
> it's better not to set rss=on for such backends.
>
I still do not understand what is the scenario where you see or suspect the
mentioned "performance degradation".
We can discuss whether such a problem exists as soon as you explain it.
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next prev parent reply other threads:[~2023-11-13 11:44 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-30 5:12 [PATCH v6 00/21] virtio-net RSS/hash report fixes and improvements Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 01/21] tap: Remove tap_probe_vnet_hdr_len() Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 02/21] tap: Remove qemu_using_vnet_hdr() Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 03/21] net: Move virtio-net header length assertion Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 04/21] net: Remove receive_raw() Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 05/21] tap: Remove tap_receive() Akihiko Odaki
2023-10-30 18:52 ` Zhang, Chen
2023-10-31 4:57 ` Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 06/21] net: Remove flag propagation Akihiko Odaki
2023-11-10 7:35 ` Pavel Dovgalyuk
2023-11-11 14:27 ` Akihiko Odaki
2023-11-13 8:14 ` Pavel Dovgalyuk
2023-10-30 5:12 ` [PATCH v6 07/21] tap: Shrink zeroed virtio-net header Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 08/21] virtio-net: Copy header only when necessary Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 09/21] virtio-net: Disable RSS on reset Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 10/21] virtio-net: Unify the logic to update NIC state for RSS Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 11/21] virtio-net: Return an error when vhost cannot enable RSS Akihiko Odaki
2023-10-30 12:14 ` Yuri Benditovich
2023-10-30 12:21 ` Akihiko Odaki
2023-10-30 12:51 ` Yuri Benditovich
2023-10-30 13:14 ` Akihiko Odaki
2023-11-01 4:19 ` Jason Wang
2023-11-01 4:50 ` Akihiko Odaki
2023-11-01 6:38 ` Michael S. Tsirkin
2023-11-01 8:35 ` Akihiko Odaki
2023-11-01 9:09 ` Michael S. Tsirkin
2023-11-01 9:15 ` Akihiko Odaki
2023-11-02 9:09 ` Yuri Benditovich
2023-11-02 9:33 ` Michael S. Tsirkin
2023-11-02 10:20 ` Yuri Benditovich
2023-11-02 11:26 ` Michael S. Tsirkin
2023-11-02 12:00 ` Yuri Benditovich
2023-11-02 13:18 ` Michael S. Tsirkin
2023-11-02 14:56 ` Akihiko Odaki
2023-11-03 9:35 ` Yuri Benditovich
2023-11-03 9:55 ` Akihiko Odaki
2023-11-03 13:14 ` Yuri Benditovich
2023-11-11 15:28 ` Akihiko Odaki
2023-11-13 11:44 ` Yuri Benditovich [this message]
2023-11-13 12:44 ` Akihiko Odaki
2023-11-13 17:26 ` Yuri Benditovich
2023-11-14 7:03 ` Akihiko Odaki
2023-11-14 22:09 ` Yuri Benditovich
2023-11-15 6:09 ` Akihiko Odaki
2023-11-16 5:14 ` Jason Wang
2023-10-30 5:12 ` [PATCH v6 12/21] virtio-net: Enable software RSS Akihiko Odaki
2023-10-30 12:42 ` Yuri Benditovich
2023-10-30 5:12 ` [PATCH v6 13/21] virtio-net: Always set populate_hash Akihiko Odaki
2023-10-30 19:02 ` Zhang, Chen
2023-10-31 4:47 ` Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 14/21] virtio-net: Do not write hashes to peer buffer Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 15/21] virtio-net: Do not clear VIRTIO_NET_F_HASH_REPORT Akihiko Odaki
2023-11-03 13:26 ` Yuri Benditovich
2023-10-30 5:12 ` [PATCH v6 16/21] ebpf: Fix RSS error handling Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 17/21] ebpf: Use standard section name Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 18/21] ebpf: Simplify error handling Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 19/21] ebpf: Return 0 when configuration fails Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 20/21] ebpf: Refactor tun_rss_steering_prog() Akihiko Odaki
2023-10-30 5:12 ` [PATCH v6 21/21] ebpf: Add a separate target for skeleton Akihiko Odaki
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