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Subject[PATCH 0/2] Limit number of hw queues by nr_cpu_ids for virtio-blk and virtio-scsi
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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@default/

Thank you very much!

Dongli Zhang

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