Target-devel archive on lore.kernel.org
 help / color / Atom feed
From: Stefan Hajnoczi <stefanha@gmail.com>
To: Mike Christie <michael.christie@oracle.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>, fam <fam@euphon.net>,
	linux-scsi <linux-scsi@vger.kernel.org>,
	Jason Wang <jasowang@redhat.com>,
	qemu-devel <qemu-devel@nongnu.org>,
	Linux Virtualization <virtualization@lists.linux-foundation.org>,
	target-devel <target-devel@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Stefano Garzarella <sgarzare@redhat.com>
Subject: Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq
Date: Fri, 20 Nov 2020 08:45:49 +0000
Message-ID: <CAJSP0QVu4P6c+kdFkhw1S_OEaj7B-eiDqFOVDxWAaSOcsAADrA@mail.gmail.com> (raw)
In-Reply-To: <CAJSP0QUfqd=QNFa-RikH4dVcLmfcP-pYCwznP3W0zobYkM+KDw@mail.gmail.com>

On Thu, Nov 19, 2020 at 5:08 PM Stefan Hajnoczi <stefanha@gmail.com> wrote:
>
> On Thu, Nov 19, 2020 at 4:43 PM Mike Christie
> <michael.christie@oracle.com> wrote:
> >
> > On 11/19/20 10:24 AM, Stefan Hajnoczi wrote:
> > > On Thu, Nov 19, 2020 at 4:13 PM Mike Christie
> > > <michael.christie@oracle.com> wrote:
> > >>
> > >> On 11/19/20 8:46 AM, Michael S. Tsirkin wrote:
> > >>> On Wed, Nov 18, 2020 at 11:31:17AM +0000, Stefan Hajnoczi wrote:
> > > struct vhost_run_worker_info {
> > >      struct timespec *timeout;
> > >      sigset_t *sigmask;
> > >
> > >      /* List of virtqueues to process */
> > >      unsigned nvqs;
> > >      unsigned vqs[];
> > > };
> > >
> > > /* This blocks until the timeout is reached, a signal is received, or
> > > the vhost device is destroyed */
> > > int ret = ioctl(vhost_fd, VHOST_RUN_WORKER, &info);
> > >
> > > As you can see, userspace isn't involved with dealing with the
> > > requests. It just acts as a thread donor to the vhost driver.
> > >
> > > We would want the VHOST_RUN_WORKER calls to be infrequent to avoid the
> > > penalty of switching into the kernel, copying in the arguments, etc.
> >
> > I didn't get this part. Why have the timeout? When the timeout expires,
> > does userspace just call right back down to the kernel or does it do
> > some sort of processing/operation?
> >
> > You could have your worker function run from that ioctl wait for a
> > signal or a wake up call from the vhost_work/poll functions.
>
> An optional timeout argument is common in blocking interfaces like
> poll(2), recvmmsg(2), etc.
>
> Although something can send a signal to the thread instead,
> implementing that in an application is more awkward than passing a
> struct timespec.
>
> Compared to other blocking calls we don't expect
> ioctl(VHOST_RUN_WORKER) to return soon, so maybe the timeout will
> rarely be used and can be dropped from the interface.
>
> BTW the code I posted wasn't a carefully thought out proposal :). The
> details still need to be considered and I'm going to be offline for
> the next week so maybe someone else can think it through in the
> meantime.

One final thought before I'm offline for a week. If
ioctl(VHOST_RUN_WORKER) is specific to a single vhost device instance
then it's hard to support poll-mode (busy waiting) workers because
each device instance consumes a whole CPU. If we stick to an interface
where the kernel manages the worker threads then it's easier to share
workers between devices for polling.

I have CCed Stefano Garzarella, who is looking at similar designs for
vDPA software device implementations.

Stefan

  reply index

Thread overview: 47+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-12 23:18 Mike Christie
2020-11-12 23:19 ` [PATCH 1/1] qemu vhost scsi: add VHOST_SET_VRING_ENABLE support Mike Christie
2020-11-17 11:53   ` Stefan Hajnoczi
2020-12-02  9:59   ` Michael S. Tsirkin
2020-12-02 16:05     ` Michael Christie
2020-11-12 23:19 ` [PATCH 01/10] vhost: remove work arg from vhost_work_flush Mike Christie
2020-11-17 13:04   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 02/10] vhost scsi: remove extra flushes Mike Christie
2020-11-17 13:07   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 03/10] vhost poll: fix coding style Mike Christie
2020-11-17 13:07   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 04/10] vhost: support multiple worker threads Mike Christie
2020-11-12 23:19 ` [PATCH 05/10] vhost: poll support support multiple workers Mike Christie
2020-11-17 15:32   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 06/10] vhost scsi: make SCSI cmd completion per vq Mike Christie
2020-11-17 16:04   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 07/10] vhost, vhost-scsi: flush IO vqs then send TMF rsp Mike Christie
2020-11-17 16:05   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 08/10] vhost: move msg_handler to new ops struct Mike Christie
2020-11-17 16:08   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 09/10] vhost: add VHOST_SET_VRING_ENABLE support Mike Christie
2020-11-17 16:14   ` Stefan Hajnoczi
2020-11-12 23:19 ` [PATCH 10/10] vhost-scsi: create a woker per IO vq Mike Christie
2020-11-17 16:40 ` [PATCH 00/10] vhost/qemu: thread per IO SCSI vq Stefan Hajnoczi
2020-11-17 19:13   ` Mike Christie
2020-11-18  9:54     ` Michael S. Tsirkin
2020-11-19 14:00       ` Stefan Hajnoczi
2020-11-18 11:31     ` Stefan Hajnoczi
2020-11-19 14:46       ` Michael S. Tsirkin
2020-11-19 16:11         ` Mike Christie
2020-11-19 16:24           ` Stefan Hajnoczi
2020-11-19 16:43             ` Mike Christie
2020-11-19 17:08               ` Stefan Hajnoczi
2020-11-20  8:45                 ` Stefan Hajnoczi [this message]
2020-11-20 12:31                   ` Michael S. Tsirkin
2020-12-01 12:59                     ` Stefan Hajnoczi
2020-12-01 13:45                       ` Stefano Garzarella
2020-12-01 17:43                         ` Stefan Hajnoczi
2020-12-02 10:35                           ` Stefano Garzarella
2020-11-23 15:17                   ` Stefano Garzarella
2020-11-18  5:17   ` Jason Wang
2020-11-18  6:57     ` Mike Christie
2020-11-18  7:19       ` Mike Christie
2020-11-18  7:54       ` Jason Wang
2020-11-18 20:06         ` Mike Christie
2020-11-19  4:35           ` Jason Wang
2020-11-19 15:49             ` Mike Christie

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=CAJSP0QVu4P6c+kdFkhw1S_OEaj7B-eiDqFOVDxWAaSOcsAADrA@mail.gmail.com \
    --to=stefanha@gmail.com \
    --cc=fam@euphon.net \
    --cc=jasowang@redhat.com \
    --cc=linux-scsi@vger.kernel.org \
    --cc=michael.christie@oracle.com \
    --cc=mst@redhat.com \
    --cc=pbonzini@redhat.com \
    --cc=qemu-devel@nongnu.org \
    --cc=sgarzare@redhat.com \
    --cc=stefanha@redhat.com \
    --cc=target-devel@vger.kernel.org \
    --cc=virtualization@lists.linux-foundation.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Target-devel archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/target-devel/0 target-devel/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 target-devel target-devel/ https://lore.kernel.org/target-devel \
		target-devel@vger.kernel.org
	public-inbox-index target-devel

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.target-devel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git