Re: AIC7xxx panic

[Gérard Roudier]

On Sun, 7 Oct 2001, David M. Grimes wrote:

> On Sun, Oct 07, 2001 at 07:28:57AM -0400, Jim Crilly wrote:
> > Both disks on the controller are Seagate Cheetahs, the one being worked
> > during the panic is a ST39204LW, the other disk is a ST318451LW.
>
> I've seen this on a 2-disk system (both Seagate ST150176LW) on a
> VA-Systems onboad AIC 7xxx.  I enabled TCQ, and noticed the default
> depth increased sometime around 2.4.10, not exactly sure when (it used
> to be 8, now much higher).  I've seen it on both disks.
>
> In drivers/scsi/aic7xxx/aic7xxx_osm.h is the #define for NSEG, and I
> changed it from 128 to 512, and it stopped the problem.  Question is,
> why was the TCQ depth increased, and should NSEG have been upped with
> it?

The default TCQ depth was 8 in Doug Ledford's aic7xxx driver but was 253
in Justin Gibbs' aic7xxx driver. As seen from driver developpers the TCQ
depth haven't been changed. :-)

The max number of DMA segments and TCQ depths are totally unrelated items.
Your guessed work-around may just indicate that their interaction may
trigger some software bug. Using larger TCQ depths make more pressure on
memory and disk IOs, leading to more memory being locked for IO pending
and memory segmentation being more likely.

> > I did have TCQ enabled and I left it at the default of 255, I'll try a
> > lower value tomorrow, since it's so late.
>
> This also fixed my problem, I left NSEG at 128 and lowered the TCQ depth
> back to 8.  This worked fine as well.
>
> I'll be intereted to see what the eventual outcome of this is, so I can
> apply the "right" fix!

The right fix might well not apply to the driver code. Btw, I donnot plan
to look into the problem, as Justin may just be studying it, in my
guessing.  I just wanted to suggest to also look into upper layers and not
to only focus on the low-level driver.

  Gérard.

> Anyhow, thought you might want another datapoint.
>
>   Thanks,
>
>   Dave
>
> >
> > Jim
> >
> > On Sun, 2001-10-07 at 06:48, Rob Turk wrote:
> > > "Jim Crilly" <[e-mail]> wrote in message
> > > news:cistron.1002451051.3718.20.camel@warblade...
> > > > I got a reproducible panic while running dbench simulating 25+ clients,
> > > > the new aic7xxx driver panics with "Too few segs for dma mapping.
> > > > "Increase AHC_NSEG". The partition in question is FAT32 and on a
> > > > different disk than /, I'm not using HIGHMEM. I am using XFS and the
> > > > preempt patches, but I don't think they're related to the panic.
> > > >
> > > > The odd thing, is if I run dbench in the same manner on my / partition,
> > > > which is on a different disk on the same controller, it goes fine. It
> > > > seems, to my untrained eye anyway, to be a bad interaction between the
> > > > vfat driver and the aic7xxx driver.
> > > >
> > > > I'm using the old aic7xxx driver right now and it's fine, has anyone
> > > > else seen anything like this?
> > > >
> > > > Jim
> > >
> > > Since this seems to fail on just one disk, it might have to do with one of the
> > > disk characteristics, like command queue depth. Did you enable Tagged Command
> > > Queueing, and if so, can you try playing around with the maximum depth?
> > >
> > > Rob
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