RE: em interrupt storm

From: Scott Long
> >>On Tue, 2005-11-22 at 22:03 -0500, Kris Kennaway wrote:
> >>
> >>>I am seeing the em driver undergoing an interrupt storm whenever the
> >>>amr driver receives interrupts.  In this case I was running newfs on
> >>>the amr array and em0 was not in use:
<...>
> >>>This is on both 6.0-RELEASE and 6.0-STABLE.
> 
> This is apparently a side effect of how we process interupts, which is
> different from Windows and Linux.  Since we mask the interrupt in the
> APIC while the ithread runs, the Intel hardware tries to outsmart us
> and continue delivering the interrupt via irq16.  There are been
> rumors on ways to turn off this 'feature', but none of them seem to
> work.  Since ithreads are integral to SMPng, and masking the APIC pins
> in integral to making ithreads work, the solution will probably be to
> be more aggressive in adopting MSI, and in doing filtered interrupt 
> handlers that don't require the APIC to be masked.  Note that Solaris
> and Darwin would likely exhibit the same problem since they handle
> interrupts similar to us.

Scott, from your message the problem is going to on any E7520 chipset,
regardless of the NICs, disk controllers, etc. used in the system.  Does
this problem also exist for the E7525 chipset?  I know the two are almost
identical, but I figured it would be good to make sure.

I ask because I have a machine with a Supermicro X6DAL-G (which uses the
E7525) on which I got messages about interrupt storms during builds.  This
was back before if_em was fixed, so they were shelved along with use of the
em interface until if_em was fixed.  My apologies for not having the exact
messages.  I don't having access to the logs.

Until this gets "fixed" in FreeBSD, what should those of us who are
effectively stuck with this hardware do to avoid the problem?  Does the
problem exist in RELENG_4?