linux-kernel - Re: [PATCH v2 5/5] virtio-scsi: introduce multiqueue support

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Date:	Tue, 18 Dec 2012 16:51:28 +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 16:03, Michael S. Tsirkin ha scritto:
> 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?

Because I want the lockless access to occur exactly when I write it.
Otherwise I have one more thing to think about, i.e. what a crazy
compiler writer could do with my code.  And having been on the other
side of the trench, compiler writers can have *really* crazy ideas.

Anyhow, I'll reorganize the code to move the ACCESS_ONCE closer to the
write and make it clearer.

>>>> +	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.

This is not a problem.  Please read Documentation/memory-barrier.txt:

   The following also do _not_ imply memory barriers, and so may
   require explicit memory barriers under some circumstances
   (smp_mb__before_atomic_dec() for instance):

        atomic_add();
        atomic_sub();
        atomic_inc();
        atomic_dec();

   If they're used for statistics generation, then they probably don't
   need memory barriers, unless there's a coupling between statistical
   data.

This is the single-queue case, so it falls under this case.

>>>>  	/* 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?

The right thing could be to set the affinity with a stride, e.g. CPUs
0-4 for virtqueue 0 and so on until CPUs 3-7 for virtqueue 3.

Paolo

>> 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.
> 

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