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

[Jeff Garzik]

Ingo Molnar wrote:
> * 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.

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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