probable vm exit latency issue?

[Joe Buehler <aspam@cox.net>]

I am debugging large latency issues in a realtime KVM instance, trying to make an existing VM (debugged on another type of box) run on an HP proliant now.

cyclictest came up with the following snippet:

kworker/-71      0.....11 13483223us : ata_sff_exec_command <-ata_bmdma_setup
kworker/-71      0.....11 13483228us : ata_sff_sync <-ata_sff_exec_command
kworker/-71      0.....11 13483231us : delay_tsc <-__const_udelay
kworker/-71      0.....11 13483231us*: ata_bmdma_start <-ata_bmdma_qc_issue
kworker/-71      0d....11 13496181us : smp_apic_timer_interrupt <-apic_timer_interrupt
kworker/-71      0d....11 13496181us : irq_enter <-smp_apic_timer_interrupt
kworker/-71      0d....11 13496182us : rcu_irq_enter <-irq_enter

I don't have a lot of experience with this; is this a DMA start for disk I/O, presumably causing an exit from the VM?  For almost 13 milliseconds????

I haven't been able to find much obvious about eliminating this -- any suggestions?  I set caching to writeback without effect.

    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='writeback'/>
      <source file='/boot.qcow2'/>
      <backingStore type='file' index='1'>
        <format type='raw'/>
        <source file='/root.qcow2'/>
        <backingStore/>
      </backingStore>
      <target dev='hdb' bus='ide'/>
      <alias name='ide0-0-1'/>
      <address type='drive' controller='0' bus='0' target='0' unit='1'/>
    </disk>

Joe Buehler