Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Mike Christie <michael.christie@oracle.com>
Cc: fam@euphon.net, linux-scsi@vger.kernel.org,
	qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org,
	target-devel@vger.kernel.org,
	Stefan Hajnoczi <stefanha@redhat.com>,
	pbonzini@redhat.com
Subject: Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq
Date: Wed, 18 Nov 2020 04:54:07 -0500	[thread overview]
Message-ID: <20201118044620-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <b3343762-bb11-b750-46ec-43b5556f2b8e@oracle.com>

On Tue, Nov 17, 2020 at 01:13:14PM -0600, Mike Christie wrote:
> On 11/17/20 10:40 AM, Stefan Hajnoczi wrote:
> > On Thu, Nov 12, 2020 at 05:18:59PM -0600, Mike Christie wrote:
> >> The following kernel patches were made over Michael's vhost branch:
> >>
> >> https://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git/log/?h=vhost
> >>
> >> and the vhost-scsi bug fix patchset:
> >>
> >> https://lore.kernel.org/linux-scsi/20201112170008.GB1555653@stefanha-x1.localdomain/T/#t
> >>
> >> And the qemu patch was made over the qemu master branch.
> >>
> >> vhost-scsi currently supports multiple queues with the num_queues
> >> setting, but we end up with a setup where the guest's scsi/block
> >> layer can do a queue per vCPU and the layers below vhost can do
> >> a queue per CPU. vhost-scsi will then do a num_queue virtqueues,
> >> but all IO gets set on and completed on a single vhost-scsi thread.
> >> After 2 - 4 vqs this becomes a bottleneck.
> >>
> >> This patchset allows us to create a worker thread per IO vq, so we
> >> can better utilize multiple CPUs with the multiple queues. It
> >> implments Jason's suggestion to create the initial worker like
> >> normal, then create the extra workers for IO vqs with the
> >> VHOST_SET_VRING_ENABLE ioctl command added in this patchset.
> > 
> > How does userspace find out the tids and set their CPU affinity?
> > 
> 
> When we create the worker thread we add it to the device owner's cgroup,
> so we end up inheriting those settings like affinity.
> 
> However, are you more asking about finer control like if the guest is
> doing mq, and the mq hw queue is bound to cpu0, it would perform
> better if we could bind vhost vq's worker thread to cpu0? I think the
> problem might is if you are in the cgroup then we can't set a specific
> threads CPU affinity to just one specific CPU. So you can either do
> cgroups or not.

Something we wanted to try for a while is to allow userspace
to create threads for us, then specify which vqs it processes.

That would address this set of concerns ...

-- 
MST

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