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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>
Subject: Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq
Date: Thu, 19 Nov 2020 16:24:49 +0000
Message-ID: <CAJSP0QUvSwX5NCPmfSODV_C+D41E21LZT=oXQ2PLc6baAsGGDQ@mail.gmail.com> (raw)
In-Reply-To: <ceebdc90-3ffc-1563-ff85-12a848bcba18@oracle.com>

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:
> >>> My preference has been:
> >>>
> >>> 1. If we were to ditch cgroups, then add a new interface that would allow
> >>> us to bind threads to a specific CPU, so that it lines up with the guest's
> >>> mq to CPU mapping.
> >>
> >> A 1:1 vCPU/vq->CPU mapping isn't desirable in all cases.
> >>
> >> The CPU affinity is a userspace policy decision. The host kernel should
> >> provide a mechanism but not the policy. That way userspace can decide
> >> which workers are shared by multiple vqs and on which physical CPUs they
> >> should run.
> >
> > So if we let userspace dictate the threading policy then I think binding
> > vqs to userspace threads and running there makes the most sense,
> > no need to create the threads.
> >
>
> Just to make sure I am on the same page, in one of the first postings of
> this set at the bottom of the mail:
>
> https://www.spinics.net/lists/linux-scsi/msg148322.html
>
> I asked about a new interface and had done something more like what
> Stefan posted:
>
>    struct vhost_vq_worker_info {
>        /*
>         * The pid of an existing vhost worker that this vq will be
>         * assigned to. When pid is 0 the virtqueue is assigned to the
>         * default vhost worker. When pid is -1 a new worker thread is
>         * created for this virtqueue. When pid is -2 the virtqueue's
>         * worker thread is unchanged.
>         *
>         * If a vhost worker no longer has any virtqueues assigned to it
>         * then it will terminate.
>         *
>         * The pid of the vhost worker is stored to this field when the
>         * ioctl completes successfully. Use pid -2 to query the current
>         * vhost worker pid.
>         */
>        __kernel_pid_t pid;  /* in/out */
>
>        /* The virtqueue index*/
>        unsigned int vq_idx; /* in */
>    };
>
> This approach is simple and it allowed me to have userspace map queues
> and threads optimally for our setups.
>
> Note: Stefan, in response to your previous comment, I am just using my
> 1:1 mapping as an example and would make it configurable from userspace.
>
> In the email above are you guys suggesting to execute the SCSI/vhost
> requests in userspace? We should not do that because:
>
> 1. It negates part of what makes vhost fast where we do not have to kick
> out to userspace then back to the kernel.
>
> 2. It's not doable or becomes a crazy mess because vhost-scsi is tied to
> the scsi/target layer in the kernel. You can't process the scsi command
> in userspace since the scsi state machine and all its configuration info
> is in the kernel's scsi/target layer.
>
> For example, I was just the maintainer of the target_core_user module
> that hooks into LIO/target on the backend (vhost-scsi hooks in on the
> front end) and passes commands to userspace and there we have a
> semi-shadow state machine. It gets nasty to try and maintain/sync state
> between lio/target core in the kernel and in userspace. We also see the
> perf loss I mentioned in #1.

No, if I understand Michael correctly he has suggested a different approach.

My suggestion was that the kernel continues to manage the worker
threads but an ioctl allows userspace to control the policy.

I think Michael is saying that the kernel shouldn't manage/create
threads. Userspace should create threads and then invoke an ioctl from
those threads.

The ioctl will call into the vhost driver where it will execute
something similar to vhost_worker(). So this ioctl will block while
the kernel is using the thread to process vqs.

What isn't clear to me is how to tell the kernel which vqs are
processed by a thread. We could try to pass that information into the
ioctl. I'm not sure what the cleanest solution is here.

Maybe something like:

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.

Michael: is this the kind of thing you were thinking of?

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 [this message]
2020-11-19 16:43             ` Mike Christie
2020-11-19 17:08               ` Stefan Hajnoczi
2020-11-20  8:45                 ` Stefan Hajnoczi
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

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