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Date: Tue, 18 Dec 2012 17:03:27 +0200
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
gaowanlong@...fujitsu.com, hutao@...fujitsu.com,
linux-scsi@...r.kernel.org,
virtualization@...ts.linux-foundation.org, rusty@...tcorp.com.au,
asias@...hat.com, stefanha@...hat.com, nab@...ux-iscsi.org
Subject: Re: [PATCH v2 5/5] virtio-scsi: introduce multiqueue support
On Tue, Dec 18, 2012 at 03:08:08PM +0100, Paolo Bonzini wrote:
> Il 18/12/2012 14:57, Michael S. Tsirkin ha scritto:
> >> -static int virtscsi_queuecommand(struct Scsi_Host *sh, struct scsi_cmnd *sc)
> >> +static int virtscsi_queuecommand(struct virtio_scsi *vscsi,
> >> + struct virtio_scsi_target_state *tgt,
> >> + struct scsi_cmnd *sc)
> >> {
> >> - struct virtio_scsi *vscsi = shost_priv(sh);
> >> - struct virtio_scsi_target_state *tgt = &vscsi->tgt[sc->device->id];
> >> struct virtio_scsi_cmd *cmd;
> >> + struct virtio_scsi_vq *req_vq;
> >> int ret;
> >>
> >> struct Scsi_Host *shost = virtio_scsi_host(vscsi->vdev);
> >> @@ -461,7 +533,8 @@ static int virtscsi_queuecommand(struct Scsi_Host *sh, struct scsi_cmnd *sc)
> >> BUG_ON(sc->cmd_len > VIRTIO_SCSI_CDB_SIZE);
> >> memcpy(cmd->req.cmd.cdb, sc->cmnd, sc->cmd_len);
> >>
> >> - if (virtscsi_kick_cmd(tgt, &vscsi->req_vq, cmd,
> >> + req_vq = ACCESS_ONCE(tgt->req_vq);
> >
> > This ACCESS_ONCE without a barrier looks strange to me.
> > Can req_vq change? Needs a comment.
>
> Barriers are needed to order two things. Here I don't have the second thing
> to order against, hence no barrier.
>
> Accessing req_vq lockless is safe, and there's a comment about it, but you
> still want ACCESS_ONCE to ensure the compiler doesn't play tricks.
That's just it.
Why don't you want compiler to play tricks?
ACCESS_ONCE is needed if the value can change
while you access it, this helps ensure
a consistent value is evalutated.
If it can you almost always need a barrier. If it doesn't
you don't need ACCESS_ONCE.
> It
> shouldn't be necessary, because the critical section of
> virtscsi_queuecommand_multi will already include the appropriate
> compiler barriers,
So if there's a barrier then pls add a comment saying where
it is.
> but it is actually clearer this way to me. :)
No barriers are needed I think because
when you queue command req is incremented to req_vq
can not change. But this also means ACCESS_ONCE
is not needed either.
> >> + if (virtscsi_kick_cmd(tgt, req_vq, cmd,
> >> sizeof cmd->req.cmd, sizeof cmd->resp.cmd,
> >> GFP_ATOMIC) == 0)
> >> ret = 0;
> >> @@ -472,6 +545,48 @@ out:
> >> return ret;
> >> }
> >>
> >> +static int virtscsi_queuecommand_single(struct Scsi_Host *sh,
> >> + struct scsi_cmnd *sc)
> >> +{
> >> + struct virtio_scsi *vscsi = shost_priv(sh);
> >> + struct virtio_scsi_target_state *tgt = &vscsi->tgt[sc->device->id];
> >> +
> >> + atomic_inc(&tgt->reqs);
> >
> > And here we don't have barrier after atomic? Why? Needs a comment.
>
> Because we don't write req_vq, so there's no two writes to order. Barrier
> against what?
Between atomic update and command. Once you queue command it
can complete and decrement reqs, if this happens before
increment reqs can become negative even.
> >> + return virtscsi_queuecommand(vscsi, tgt, sc);
> >> +}
> >> +
> >> +static int virtscsi_queuecommand_multi(struct Scsi_Host *sh,
> >> + struct scsi_cmnd *sc)
> >> +{
> >> + struct virtio_scsi *vscsi = shost_priv(sh);
> >> + struct virtio_scsi_target_state *tgt = &vscsi->tgt[sc->device->id];
> >> + unsigned long flags;
> >> + u32 queue_num;
> >> +
> >> + /*
> >> + * Using an atomic_t for tgt->reqs lets the virtqueue handler
> >> + * decrement it without taking the spinlock.
> >> + *
> >> + * We still need a critical section to prevent concurrent submissions
> >> + * from picking two different req_vqs.
> >> + */
> >> + spin_lock_irqsave(&tgt->tgt_lock, flags);
> >> + if (atomic_inc_return(&tgt->reqs) == 1) {
> >> + queue_num = smp_processor_id();
> >> + while (unlikely(queue_num >= vscsi->num_queues))
> >> + queue_num -= vscsi->num_queues;
> >> +
> >> + /*
> >> + * Write reqs before writing req_vq, matching the
> >> + * smp_read_barrier_depends() in virtscsi_req_done.
> >> + */
> >> + smp_wmb();
> >> + tgt->req_vq = &vscsi->req_vqs[queue_num];
> >> + }
> >> + spin_unlock_irqrestore(&tgt->tgt_lock, flags);
> >> + return virtscsi_queuecommand(vscsi, tgt, sc);
> >> +}
> >> +
> >> static int virtscsi_tmf(struct virtio_scsi *vscsi, struct virtio_scsi_cmd *cmd)
> >> {
> >> DECLARE_COMPLETION_ONSTACK(comp);
> >> @@ -541,12 +656,26 @@ static int virtscsi_abort(struct scsi_cmnd *sc)
> >> return virtscsi_tmf(vscsi, cmd);
> >> }
> >>
> >> -static struct scsi_host_template virtscsi_host_template = {
> >> +static struct scsi_host_template virtscsi_host_template_single = {
> >> .module = THIS_MODULE,
> >> .name = "Virtio SCSI HBA",
> >> .proc_name = "virtio_scsi",
> >> - .queuecommand = virtscsi_queuecommand,
> >> .this_id = -1,
> >> + .queuecommand = virtscsi_queuecommand_single,
> >> + .eh_abort_handler = virtscsi_abort,
> >> + .eh_device_reset_handler = virtscsi_device_reset,
> >> +
> >> + .can_queue = 1024,
> >> + .dma_boundary = UINT_MAX,
> >> + .use_clustering = ENABLE_CLUSTERING,
> >> +};
> >> +
> >> +static struct scsi_host_template virtscsi_host_template_multi = {
> >> + .module = THIS_MODULE,
> >> + .name = "Virtio SCSI HBA",
> >> + .proc_name = "virtio_scsi",
> >> + .this_id = -1,
> >> + .queuecommand = virtscsi_queuecommand_multi,
> >> .eh_abort_handler = virtscsi_abort,
> >> .eh_device_reset_handler = virtscsi_device_reset,
> >>
> >> @@ -572,16 +701,27 @@ static struct scsi_host_template virtscsi_host_template = {
> >> &__val, sizeof(__val)); \
> >> })
> >>
> >> +
> >> static void virtscsi_init_vq(struct virtio_scsi_vq *virtscsi_vq,
> >> - struct virtqueue *vq)
> >> + struct virtqueue *vq, bool affinity)
> >> {
> >> spin_lock_init(&virtscsi_vq->vq_lock);
> >> virtscsi_vq->vq = vq;
> >> + if (affinity)
> >> + virtqueue_set_affinity(vq, vq->index - VIRTIO_SCSI_VQ_BASE);
> >
> > I've been thinking about how set_affinity
> > interacts with online/offline CPUs.
> > Any idea?
>
> No, I haven't tried.
We need a TODO, for -net too.
> >>
> >> /* Discover virtqueues and write information to configuration. */
> >> - err = vdev->config->find_vqs(vdev, 3, vqs, callbacks, names);
> >> + err = vdev->config->find_vqs(vdev, num_vqs, vqs, callbacks, names);
> >> if (err)
> >> return err;
> >>
> >> - virtscsi_init_vq(&vscsi->ctrl_vq, vqs[0]);
> >> - virtscsi_init_vq(&vscsi->event_vq, vqs[1]);
> >> - virtscsi_init_vq(&vscsi->req_vq, vqs[2]);
> >> + virtscsi_init_vq(&vscsi->ctrl_vq, vqs[0], false);
> >> + virtscsi_init_vq(&vscsi->event_vq, vqs[1], false);
> >> + for (i = VIRTIO_SCSI_VQ_BASE; i < num_vqs; i++)
> >> + virtscsi_init_vq(&vscsi->req_vqs[i - VIRTIO_SCSI_VQ_BASE],
> >> + vqs[i], vscsi->num_queues > 1);
> >
> > So affinity is true if >1 vq? I am guessing this is not
> > going to do the right thing unless you have at least
> > as many vqs as CPUs.
>
> Yes, and then you're not setting up the thing correctly.
Why not just check instead of doing the wrong thing?
> Isn't the same thing true for virtio-net mq?
>
> Paolo
Last I looked it checked vi->max_queue_pairs == num_online_cpus().
This is even too aggressive I think, max_queue_pairs >=
num_online_cpus() should be enough.
--
MST
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