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

[Ingo Molnar <mingo@elte.hu>]

* Jeff Garzik <jeff@garzik.org> wrote:

> 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.

please see my previous codepath description. No spin_lock() calls in the 
measured codepath. They would show up in the latency trace anyway.

> 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...

yeah. But then again - PIO ops easily take 16 usecs on almost arbitrary 
PC hardware, so an SMM trap might not be all that noticeable - 
especially if this register is considered legacy. (maybe Windows does 
the MMIO variant)

	Ingo
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