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Date: Tue, 18 Dec 2012 15:08:08 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: "Michael S. Tsirkin" <mst@...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
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. It
shouldn't be necessary, because the critical section of
virtscsi_queuecommand_multi will already include the appropriate
compiler barriers, but it is actually clearer this way to me. :)
>> + 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?
>> + 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.
>>
>> /* 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.
Isn't the same thing true for virtio-net mq?
Paolo
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