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

Ingo Molnar
2006-03-15T22:01:11Z

* Lee Revell <rlrevell@joe-job.com> wrote:

> On Wed, 2006-03-15 at 16:36 -0500, Bill Rugolsky Jr. wrote:
> > <...>-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)
>
> There's your problem - it looks like ata_bmdma_status() stalled the
> machine for almost 17ms.

i agree. Here's a bit more detailed analysis: the tracer timestamps
function entry points. So what we know is that from the call to
ata_bdma_status(), up to the call to nv_check_hotplug_ck804(), 16.6
msecs passed. The codepath includes:

- the whole of the ata_bdma_status() function

- a small portion of ata_host_intr() [from the point where it returns
from the ops->bdma_status() call up to the return]

- and a small portion of nv_interrupt(), from the ata_host_intr()
return to the ->check_hotplug() call.

in this particular case there's only very simple (and non-IO)
instructions in that codepath (no loops either), except for
ata_bmdma_status() which does IO ops: so i agree with you that the most
likely candidate for the delay is the readb() or the inb() in
ata_bdma_status().

I'm wondering which one of the two. inb()s are known to be horrible on
some systems - but i've never seen them take 16 milliseconds. If it's
the inb(), then that could also involve SMM mode and IO
emulation/bug-workaround BIOS hackery - which could indeed cause such
delays. [but i havent seen such a thing either.]

the other option is that this is a random delay [e.g. DMA starvation]
hitting ata_bmdma_status() only by accident. (That looks a bit unlikely
though, given how related this codepath seems to the whole problem
area.)

Ingo