virtio-blk: dataplane: Don't batch notifications if EVENT_IDX is present

virtio-blk: dataplane: Don't batch notifications if EVENT_IDX is present

Commit 5b2ffbe4d99843fd8305c573a100047a8c962327 ("virtio-blk: dataplane:
notify guest as a batch") deferred guest notification to a BH in order
batch notifications, with purpose of avoiding flooding the guest with
interruptions.

This optimization came with a cost. The average latency perceived in the
guest is increased by a few microseconds, but also when multiple IO
operations finish at the same time, the guest won't be notified until
all completions from each operation has been run. On the contrary,
virtio-scsi issues the notification at the end of each completion.

On the other hand, nowadays we have the EVENT_IDX feature that allows a
better coordination between QEMU and the Guest OS to avoid sending
unnecessary interruptions.

With this change, virtio-blk/dataplane only batches notifications if the
EVENT_IDX feature is not present.

Some numbers obtained with fio (ioengine=sync, iodepth=1, direct=1):
 - Test specs:
   * fio-3.4 (ioengine=sync, iodepth=1, direct=1)
   * qemu master
   * virtio-blk with a dedicated iothread (default poll-max-ns)
   * backend: null_blk nr_devices=1 irqmode=2 completion_nsec=280000
   * 8 vCPUs pinned to isolated physical cores
   * Emulator and iothread also pinned to separate isolated cores
   * variance between runs < 1%

 - Not patched
   * numjobs=1:  lat_avg=327.32  irqs=29998
   * numjobs=4:  lat_avg=337.89  irqs=29073
   * numjobs=8:  lat_avg=342.98  irqs=28643

 - Patched:
   * numjobs=1:  lat_avg=323.92  irqs=30262
   * numjobs=4:  lat_avg=332.65  irqs=29520
   * numjobs=8:  lat_avg=335.54  irqs=29323

Signed-off-by: Sergio Lopez <slp@redhat.com>
Message-id: 20180307114459.26636-1-slp@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>