em interrupt storm

[John Polstra]

On 23-Nov-2005 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:
> 
>    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  >

This is the dreaded interrupt aliasing problem that several of us have
experienced with this chipset.  High-numbered interrupts alias down to
interrupts in the range 16..19 (or maybe 16..23), a multiple of 8 less
than the original interupt.

Nobody knows what causes it, and nobody knows how to fix it.

John