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SubjectRe: libata/sata_nv latency on NVIDIA CK804 [was Re: AMD64 X2 lost ticks on PM timer]

* 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.

Ingo
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