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

[Ingo Molnar]

* Bill Rugolsky Jr. <brugolsky@xxxxxxxxxxxxxxxxxxxxxxxxx> 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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