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    SubjectRe: libata/sata_nv latency on NVIDIA CK804 [was Re: AMD64 X2 lost ticks on PM timer]
    Ingo Molnar wrote:
    > * Bill Rugolsky Jr. <brugolsky@telemetry-investments.com> wrote:
    >
    >
    >> <...>-2913 0d.h. 8us : raise_softirq_irqoff (blk_complete_request)
    >> <...>-2913 0d.h. 8us : __ata_qc_complete (ata_qc_complete)
    >> <...>-2913 0d.h. 9us : ata_host_intr (nv_interrupt)
    >> <...>-2913 0d.h. 9us!: ata_bmdma_status (ata_host_intr)
    >> <...>-2913 0d.h. 16641us : nv_check_hotplug_ck804 (nv_interrupt)
    >> <...>-2913 0d.h. 16642us : _spin_unlock_irqrestore (nv_interrupt)
    >> <...>-2913 0d.h. 16642us : smp_apic_timer_interrupt (apic_timer_interrupt)
    >> <...>-2913 0d.h. 16642us : exit_idle (smp_apic_timer_interrupt)
    >
    >
    > ouch. The codepath in question (ata_host_intr()) doesnt seem to have any
    > loop that could take 16.6 msecs (!). This very much looks like some
    > hardware-triggered delay - some really screwed up DMA prioritization
    > perhaps, starving the host CPU for 16.6 msecs? But what DMA takes 16.6
    > msecs? That's enough time to transfer dozens of megabytes of data on a
    > midrange system.

    Yeah, I don't see anything offhand either.

    sata_nv's nv_interrupt() should be using spin_lock() rather than
    spin_lock_irqsave(), but I doubt that's a latency cause.

    I would be surprised if the legacy PCI IDE registers, all PIO-based,
    would be implemented via BIOS SMM or some other similarly slow method.
    But its not impossible...

    Jeff



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