Linux Virtualization - Mar 2019 - [PATCH 2/2] scsi: virtio_scsi: limit number of hw queues by nr_cpu_ids

When tag_set->nr_maps is 1, the block layer limits the number of hw queues
by nr_cpu_ids. No matter how many hw queues are use by
virtio-blk/virtio-scsi, as they both have (tag_set->nr_maps == 1), they
can use at most nr_cpu_ids hw queues.

In addition, specifically for pci scenario, when the 'num-queues'
specified
by qemu is more than maxcpus, virtio-blk/virtio-scsi would not be able to
allocate more than maxcpus vectors in order to have a vector for each
queue. As a result, they fall back into MSI-X with one vector for config
and one shared for queues.

Considering above reasons, this patch set limits the number of hw queues
used by nr_cpu_ids for both virtio-blk and virtio-scsi.

-------------------------------------------------------------

Here is test result of virtio-scsi:

qemu cmdline:

-smp 2,maxcpus=4, \
-device virtio-scsi-pci,id=scsi0,num_queues=8, \
-device scsi-hd,drive=drive0,bus=scsi0.0,channel=0,scsi-id=0,lun=0, \
-drive file=test.img,if=none,id=drive0

Although maxcpus=4 and num_queues=8, 4 queues are used while 2 interrupts
are allocated.

# cat /proc/interrupts
... ...
 24:          0          0   PCI-MSI 65536-edge      virtio0-config
 25:          0        369   PCI-MSI 65537-edge      virtio0-virtqueues
... ...

# /sys/block/sda/mq/
0  1  2  3   ------> 4 queues although qemu sets num_queues=8


With the patch set, there is per-queue interrupt.

# cat /proc/interrupts
 24:          0          0   PCI-MSI 65536-edge      virtio0-config
 25:          0          0   PCI-MSI 65537-edge      virtio0-control
 26:          0          0   PCI-MSI 65538-edge      virtio0-event
 27:        296          0   PCI-MSI 65539-edge      virtio0-request
 28:          0        139   PCI-MSI 65540-edge      virtio0-request
 29:          0          0   PCI-MSI 65541-edge      virtio0-request
 30:          0          0   PCI-MSI 65542-edge      virtio0-request

# ls /sys/block/sda/mq
0  1  2  3

-------------------------------------------------------------

Here is test result of virtio-blk:

qemu cmdline:

-smp 2,maxcpus=4,
-device virtio-blk-pci,drive=drive-virtio-disk0,id=virtio-disk0,num-queues=8
-drive test.img,format=raw,if=none,id=drive-virtio-disk0

Although maxcpus=4 and num-queues=8, 4 queues are used while 2 interrupts
are allocated.

# cat /proc/interrupts
... ...
 24:          0          0   PCI-MSI 65536-edge      virtio0-config
 25:          0         65   PCI-MSI 65537-edge      virtio0-virtqueues
... ...

# ls /sys/block/vda/mq
0  1  2  3    -------> 4 queues although qemu sets num_queues=8


With the patch set, there is per-queue interrupt.

# cat /proc/interrupts
 24:          0          0   PCI-MSI 65536-edge      virtio0-config
 25:         64          0   PCI-MSI 65537-edge      virtio0-req.0
 26:          0      10290   PCI-MSI 65538-edge      virtio0-req.1
 27:          0          0   PCI-MSI 65539-edge      virtio0-req.2
 28:          0          0   PCI-MSI 65540-edge      virtio0-req.3

# ls /sys/block/vda/mq/
0  1  2  3


Reference: https://lore.kernel.org/lkml/e4afe4c5-0262-4500-aeec-60f30734b4fc at
default/

Thank you very much!

Dongli Zhang

When tag_set->nr_maps is 1, the block layer limits the number of hw queues
by nr_cpu_ids. No matter how many hw queues are used by virtio-blk, as it
has (tag_set->nr_maps == 1), it can use at most nr_cpu_ids hw queues.

In addition, specifically for pci scenario, when the 'num-queues'
specified
by qemu is more than maxcpus, virtio-blk would not be able to allocate more
than maxcpus vectors in order to have a vector for each queue. As a result,
it falls back into MSI-X with one vector for config and one shared for
queues.

Considering above reasons, this patch limits the number of hw queues used
by virtio-blk by nr_cpu_ids.

Signed-off-by: Dongli Zhang <dongli.zhang at oracle.com>
---
 drivers/block/virtio_blk.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 4bc083b..b83cb45 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
 	if (err)
 		num_vqs = 1;
 
+	num_vqs = min_t(unsigned int, nr_cpu_ids, num_vqs);
+
 	vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
 	if (!vblk->vqs)
 		return -ENOMEM;
-- 
2.7.4

When tag_set->nr_maps is 1, the block layer limits the number of hw queues
by nr_cpu_ids. No matter how many hw queues are used by virtio-scsi, as it
has (tag_set->nr_maps == 1), it can use at most nr_cpu_ids hw queues.

In addition, specifically for pci scenario, when the 'num_queues'
specified
by qemu is more than maxcpus, virtio-scsi would not be able to allocate
more than maxcpus vectors in order to have a vector for each queue. As a
result, it falls back into MSI-X with one vector for config and one shared
for queues.

Considering above reasons, this patch limits the number of hw queues used
by virtio-scsi by nr_cpu_ids.

Signed-off-by: Dongli Zhang <dongli.zhang at oracle.com>
---
 drivers/scsi/virtio_scsi.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
index 8af0177..9c4a3e1 100644
--- a/drivers/scsi/virtio_scsi.c
+++ b/drivers/scsi/virtio_scsi.c
@@ -793,6 +793,7 @@ static int virtscsi_probe(struct virtio_device *vdev)
 
 	/* We need to know how many queues before we allocate. */
 	num_queues = virtscsi_config_get(vdev, num_queues) ? : 1;
+	num_queues = min_t(unsigned int, nr_cpu_ids, num_queues);
 
 	num_targets = virtscsi_config_get(vdev, max_target) + 1;
 
-- 
2.7.4

ping?

Thank you very much!

Dongli Zhang

On 03/27/2019 06:36 PM, Dongli Zhang wrote:
> When tag_set->nr_maps is 1, the block layer limits the number of hw
queues
> by nr_cpu_ids. No matter how many hw queues are use by
> virtio-blk/virtio-scsi, as they both have (tag_set->nr_maps == 1), they
> can use at most nr_cpu_ids hw queues.
> 
> In addition, specifically for pci scenario, when the 'num-queues'
specified
> by qemu is more than maxcpus, virtio-blk/virtio-scsi would not be able to
> allocate more than maxcpus vectors in order to have a vector for each
> queue. As a result, they fall back into MSI-X with one vector for config
> and one shared for queues.
> 
> Considering above reasons, this patch set limits the number of hw queues
> used by nr_cpu_ids for both virtio-blk and virtio-scsi.
> 
> -------------------------------------------------------------
> 
> Here is test result of virtio-scsi:
> 
> qemu cmdline:
> 
> -smp 2,maxcpus=4, \
> -device virtio-scsi-pci,id=scsi0,num_queues=8, \
> -device scsi-hd,drive=drive0,bus=scsi0.0,channel=0,scsi-id=0,lun=0, \
> -drive file=test.img,if=none,id=drive0
> 
> Although maxcpus=4 and num_queues=8, 4 queues are used while 2 interrupts
> are allocated.
> 
> # cat /proc/interrupts
> ... ...
>  24:          0          0   PCI-MSI 65536-edge      virtio0-config
>  25:          0        369   PCI-MSI 65537-edge      virtio0-virtqueues
> ... ...
> 
> # /sys/block/sda/mq/
> 0  1  2  3   ------> 4 queues although qemu sets num_queues=8
> 
> 
> With the patch set, there is per-queue interrupt.
> 
> # cat /proc/interrupts
>  24:          0          0   PCI-MSI 65536-edge      virtio0-config
>  25:          0          0   PCI-MSI 65537-edge      virtio0-control
>  26:          0          0   PCI-MSI 65538-edge      virtio0-event
>  27:        296          0   PCI-MSI 65539-edge      virtio0-request
>  28:          0        139   PCI-MSI 65540-edge      virtio0-request
>  29:          0          0   PCI-MSI 65541-edge      virtio0-request
>  30:          0          0   PCI-MSI 65542-edge      virtio0-request
> 
> # ls /sys/block/sda/mq
> 0  1  2  3
> 
> -------------------------------------------------------------
> 
> Here is test result of virtio-blk:
> 
> qemu cmdline:
> 
> -smp 2,maxcpus=4,
> -device
virtio-blk-pci,drive=drive-virtio-disk0,id=virtio-disk0,num-queues=8
> -drive test.img,format=raw,if=none,id=drive-virtio-disk0
> 
> Although maxcpus=4 and num-queues=8, 4 queues are used while 2 interrupts
> are allocated.
> 
> # cat /proc/interrupts
> ... ...
>  24:          0          0   PCI-MSI 65536-edge      virtio0-config
>  25:          0         65   PCI-MSI 65537-edge      virtio0-virtqueues
> ... ...
> 
> # ls /sys/block/vda/mq
> 0  1  2  3    -------> 4 queues although qemu sets num_queues=8
> 
> 
> With the patch set, there is per-queue interrupt.
> 
> # cat /proc/interrupts
>  24:          0          0   PCI-MSI 65536-edge      virtio0-config
>  25:         64          0   PCI-MSI 65537-edge      virtio0-req.0
>  26:          0      10290   PCI-MSI 65538-edge      virtio0-req.1
>  27:          0          0   PCI-MSI 65539-edge      virtio0-req.2
>  28:          0          0   PCI-MSI 65540-edge      virtio0-req.3
> 
> # ls /sys/block/vda/mq/
> 0  1  2  3
> 
> 
> Reference:
https://lore.kernel.org/lkml/e4afe4c5-0262-4500-aeec-60f30734b4fc at default/
> 
> Thank you very much!
> 
> Dongli Zhang
>

On Wed, Mar 27, 2019 at 06:36:34PM +0800, Dongli Zhang
wrote:
> When tag_set->nr_maps is 1, the block layer limits the number of hw
queues
> by nr_cpu_ids. No matter how many hw queues are used by virtio-blk, as it
> has (tag_set->nr_maps == 1), it can use at most nr_cpu_ids hw queues.
> 
> In addition, specifically for pci scenario, when the 'num-queues'
specified
> by qemu is more than maxcpus, virtio-blk would not be able to allocate more
> than maxcpus vectors in order to have a vector for each queue. As a result,
> it falls back into MSI-X with one vector for config and one shared for
> queues.
> 
> Considering above reasons, this patch limits the number of hw queues used
> by virtio-blk by nr_cpu_ids.
> 
> Signed-off-by: Dongli Zhang <dongli.zhang at oracle.com>
> ---
>  drivers/block/virtio_blk.c | 2 ++
>  1 file changed, 2 insertions(+)
Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
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On Wed, Mar 27, 2019 at 06:36:35PM +0800, Dongli Zhang
wrote:
> When tag_set->nr_maps is 1, the block layer limits the number of hw
queues
> by nr_cpu_ids. No matter how many hw queues are used by virtio-scsi, as it
> has (tag_set->nr_maps == 1), it can use at most nr_cpu_ids hw queues.
> 
> In addition, specifically for pci scenario, when the 'num_queues'
specified
> by qemu is more than maxcpus, virtio-scsi would not be able to allocate
> more than maxcpus vectors in order to have a vector for each queue. As a
> result, it falls back into MSI-X with one vector for config and one shared
> for queues.
> 
> Considering above reasons, this patch limits the number of hw queues used
> by virtio-scsi by nr_cpu_ids.
> 
> Signed-off-by: Dongli Zhang <dongli.zhang at oracle.com>
> ---
>  drivers/scsi/virtio_scsi.c | 1 +
>  1 file changed, 1 insertion(+)
Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
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On 3/27/19 4:36 AM, Dongli Zhang wrote:
> When tag_set->nr_maps is 1, the block layer limits the number of hw
queues
> by nr_cpu_ids. No matter how many hw queues are use by
> virtio-blk/virtio-scsi, as they both have (tag_set->nr_maps == 1), they
> can use at most nr_cpu_ids hw queues.
> 
> In addition, specifically for pci scenario, when the 'num-queues'
specified
> by qemu is more than maxcpus, virtio-blk/virtio-scsi would not be able to
> allocate more than maxcpus vectors in order to have a vector for each
> queue. As a result, they fall back into MSI-X with one vector for config
> and one shared for queues.
> 
> Considering above reasons, this patch set limits the number of hw queues
> used by nr_cpu_ids for both virtio-blk and virtio-scsi.
I picked both up for 5.1.

-- 
Jens Axboe