linux-kernel - Re: Increased memory usage with scsi-mq

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Date:   Thu, 10 Aug 2017 15:16:34 +0100
From:   "Richard W.M. Jones" <rjones@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, linux-scsi@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Martin K. Petersen" <martin.petersen@...cle.com>
Subject: Re: Increased memory usage with scsi-mq

On Thu, Aug 10, 2017 at 02:53:58PM +0200, Paolo Bonzini wrote:
> can_queue and cmd_per_lun are different.  can_queue should be set to the
> value of vq->vring.num where vq is the command virtqueue (the first one
> is okay if there's >1).
> 
> If you want to change it, you'll have to do so in QEMU.

Here are a couple more patches I came up with, the first to Linux,
the second to qemu.

There are a few problems ...

(1) Setting virtqueue_size to < 128 causes a BUG_ON failure in
virtio_ring.c:virtqueue_add at:

    BUG_ON(total_sg > vq->vring.num);

I initially thought that I should also set cmd_per_lun to the same
value, but that doesn't help.  Is there some relationship between
virtqueue_size and other settings?

(2) Can/should the ctrl, event and cmd queue sizes be set to the same
values?  Or should ctrl & event be left at 128?

(3) It seems like it might be a problem that virtqueue_size is not
passed through the virtio_scsi_conf struct (which is ABI between the
kernel and qemu AFAICT and so not easily modifiable).  However I think
this is not a problem because virtqueue size is stored in the
Virtqueue Descriptor table, which is how the kernel gets the value.
Is that right?

Rich.


--- kernel patch ---

diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index 9be211d68b15..d6b4ff634c0d 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -972,6 +972,8 @@ static int virtscsi_probe(struct virtio_device *vdev)
 	if (err)
 		goto virtscsi_init_failed;
 
+	shost->can_queue = virtqueue_get_vring_size(vscsi->req_vqs[0].vq);
+
 	cmd_per_lun = virtscsi_config_get(vdev, cmd_per_lun) ?: 1;
 	shost->cmd_per_lun = min_t(u32, cmd_per_lun, shost->can_queue);
 	shost->max_sectors = virtscsi_config_get(vdev, max_sectors) ?: 0xFFFF;


--- qemu patch ---

diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index eb639442d1..aadd99aad1 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -867,10 +867,10 @@ void virtio_scsi_common_realize(DeviceState *dev,
     s->sense_size = VIRTIO_SCSI_SENSE_DEFAULT_SIZE;
     s->cdb_size = VIRTIO_SCSI_CDB_DEFAULT_SIZE;
 
-    s->ctrl_vq = virtio_add_queue(vdev, VIRTIO_SCSI_VQ_SIZE, ctrl);
-    s->event_vq = virtio_add_queue(vdev, VIRTIO_SCSI_VQ_SIZE, evt);
+    s->ctrl_vq = virtio_add_queue(vdev, s->conf.virtqueue_size, ctrl);
+    s->event_vq = virtio_add_queue(vdev, s->conf.virtqueue_size, evt);
     for (i = 0; i < s->conf.num_queues; i++) {
-        s->cmd_vqs[i] = virtio_add_queue(vdev, VIRTIO_SCSI_VQ_SIZE, cmd);
+        s->cmd_vqs[i] = virtio_add_queue(vdev, s->conf.virtqueue_size, cmd);
     }
 }
 
@@ -917,6 +917,7 @@ static void virtio_scsi_device_unrealize(DeviceState *dev, Error **errp)
 
 static Property virtio_scsi_properties[] = {
     DEFINE_PROP_UINT32("num_queues", VirtIOSCSI, parent_obj.conf.num_queues, 1),
+    DEFINE_PROP_UINT32("virtqueue_size", VirtIOSCSI, parent_obj.conf.virtqueue_size, 128),
     DEFINE_PROP_UINT32("max_sectors", VirtIOSCSI, parent_obj.conf.max_sectors,
                                                   0xFFFF),
     DEFINE_PROP_UINT32("cmd_per_lun", VirtIOSCSI, parent_obj.conf.cmd_per_lun,
diff --git a/include/hw/virtio/virtio-scsi.h b/include/hw/virtio/virtio-scsi.h
index de6ae5a9f6..e30a92d3e7 100644
--- a/include/hw/virtio/virtio-scsi.h
+++ b/include/hw/virtio/virtio-scsi.h
@@ -48,6 +48,7 @@ typedef struct virtio_scsi_config VirtIOSCSIConfig;
 
 struct VirtIOSCSIConf {
     uint32_t num_queues;
+    uint32_t virtqueue_size;
     uint32_t max_sectors;
     uint32_t cmd_per_lun;
 #ifdef CONFIG_VHOST_SCSI


-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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