em interrupt storm

[Kris Kennaway]

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:

   28 root        1 -68 -187     0K     8K CPU1   1   0:32 53.98% irq16: em0
   36 root        1 -64 -183     0K     8K RUN    1   0:37 27.75% irq24: amr0

# vmstat -i
interrupt                          total       rate
irq1: atkbd0                           2          0
irq4: sio0                           199          1
irq6: fdc0                            32          0
irq13: npx0                            1          0
irq14: ata0                           47          0
irq15: ata1                          931          5
irq16: em0                       6321801      37187
irq24: amr0                        28023        164
cpu0: timer                       337533       1985
cpu1: timer                       337285       1984
Total                            7025854      41328

When newfs finished (i.e. amr was idle), em0 stopped storming.

MPTable: <INTEL    SE7520BD22  >
Timecounter "i8254" frequency 1193182 Hz quality 0
CPU: Intel(R) Xeon(TM) CPU 2.80GHz (2793.02-MHz 686-class CPU)
  Origin = "GenuineIntel"  Id = 0xf34  Stepping = 4
  Features=0xbfebfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,DTS,ACPI,MMX,FXSR,SSE,S
SE2,SS,HTT,TM,PBE>
  Features2=0x441d<SSE3,RSVD2,MON,DS_CPL,CNTX-ID,<b14>>
  AMD Features=0x20000000<LM>
  Hyperthreading: 2 logical CPUs
real memory  = 3757965312 (3583 MB)
avail memory = 3682967552 (3512 MB)
FreeBSD/SMP: Multiprocessor System Detected: 2 CPUs
 cpu0 (BSP): APIC ID:  0
 cpu1 (AP): APIC ID:  6
ioapic0: Assuming intbase of 0
ioapic1: Assuming intbase of 24
ioapic2: Assuming intbase of 48
ioapic0 <Version 2.0> irqs 0-23 on motherboard
ioapic1 <Version 2.0> irqs 24-47 on motherboard
ioapic2 <Version 2.0> irqs 48-71 on motherboard
[...]
pcib0: <MPTable Host-PCI bridge> pcibus 0 on motherboard
pci0: <PCI bus> on pcib0
pci0: <unknown> at device 0.1 (no driver attached)
pci0: <base peripheral> at device 1.0 (no driver attached)
pcib1: <PCI-PCI bridge> irq 16 at device 2.0 on pci0
pci1: <PCI bus> on pcib1
pcib2: <PCI-PCI bridge> at device 0.0 on pci1
pci2: <PCI bus> on pcib2
pcib3: <MPTable PCI-PCI bridge> at device 3.0 on pci2
pci3: <PCI bus> on pcib3
amr0: <LSILogic MegaRAID 1.53> mem 0xfc3f0000-0xfc3fffff,0xfc780000-0xfc7fffff irq 24 at device 0.0 on pci3
amr0: delete logical drives supported by controller
amr0: <LSILogic MegaRAID SCSI 320-2X> Firmware 413G, BIOS H414, 128MB RAM
pci1: <base peripheral, interrupt controller> at device 0.1 (no driver attached)
pcib4: <PCI-PCI bridge> at device 0.2 on pci1
pci4: <PCI bus> on pcib4
pci1: <base peripheral, interrupt controller> at device 0.3 (no driver attached)
pcib5: <PCI-PCI bridge> irq 16 at device 4.0 on pci0
pci5: <PCI bus> on pcib5
pcib6: <PCI-PCI bridge> irq 16 at device 5.0 on pci0
pci6: <PCI bus> on pcib6
pci6: <network, ethernet> at device 0.0 (no driver attached)
pcib7: <PCI-PCI bridge> irq 16 at device 6.0 on pci0
pci7: <PCI bus> on pcib7
pcib8: <MPTable PCI-PCI bridge> at device 30.0 on pci0
pci8: <PCI bus> on pcib8
em0: <Intel(R) PRO/1000 Network Connection, Version - 2.1.7> port 0xec00-0xec3f mem 0xfeaa0000-0xfeabffff irq 16 at device 4.0 on pci8
em0: Ethernet address: 00:02:b3:e8:fc:8a

This is on both 6.0-RELEASE and 6.0-STABLE.

Kris
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