Il 05/07/2012 12:29, Jason Wang ha scritto:

Sometimes, virtio device need to configure irq affiniry hint to maximize the
performance. Instead of just exposing the irq of a virtqueue, this patch
introduce an API to set the affinity for a virtqueue.

The api is best-effort, the affinity hint may not be set as expected due to
platform support, irq sharing or irq type. Currently, only pci method were
implemented and we set the affinity according to:

- if device uses INTX, we just ignore the request
- if device has per vq vector, we force the affinity hint
- if the virtqueues share MSI, make the affinity OR over all affinities
 requested

Signed-off-by: Jason Wang <jasowang@redhat.com>

Hmm, I don't see any benefit from this patch, I need to use
irq_set_affinity (which however is not exported) to actually bind IRQs
to CPUs.  Example:

with irq_set_affinity_hint:
 43:   89  107  100   97   PCI-MSI-edge   virtio0-request
 44:  178  195  268  199   PCI-MSI-edge   virtio0-request
 45:   97  100   97  155   PCI-MSI-edge   virtio0-request
 46:  234  261  213  218   PCI-MSI-edge   virtio0-request

with irq_set_affinity:
 43:  721    0    0    1   PCI-MSI-edge   virtio0-request
 44:    0  746    0    1   PCI-MSI-edge   virtio0-request
 45:    0    0  658    0   PCI-MSI-edge   virtio0-request
 46:    0    0    1  547   PCI-MSI-edge   virtio0-request

I gathered these quickly after boot, but real benchmarks show the same
behavior, and performance gets actually worse with virtio-scsi
multiqueue+irq_set_affinity_hint than with irq_set_affinity.

I also tried adding IRQ_NO_BALANCING, but the only effect is that I
cannot set the affinity

The queue steering algorithm I use in virtio-scsi is extremely simple
and based on your tx code.  See how my nice pinning is destroyed:

# taskset -c 0 dd if=/dev/sda bs=1M count=1000 of=/dev/null iflag=direct
# cat /proc/interrupts
 43:  2690 2709 2691 2696   PCI-MSI-edge      virtio0-request
 44:   109  122  199  124   PCI-MSI-edge      virtio0-request
 45:   170  183  170  237   PCI-MSI-edge      virtio0-request
 46:   143  166  125  125   PCI-MSI-edge      virtio0-request

All my requests come from CPU#0 and thus go to the first virtqueue, but
the interrupts are serviced all over the place.

Did you set the affinity manually in your experiments, or perhaps there
is a difference between scsi and networking... (interrupt mitigation?)

Paolo