Extremely high-load -- due to pcm's IRQ-handling

[Mikhail Teterin]

kmail prompted me for password, which I supplied. Then, all of a sudden,
the system slowed to a crawl -- even the mouse curse was barely moving.

It took many minutes to login from the outside and some more to start top.

Below you can see kmail in kserel keeping the processor(s) EXTREMELY busy.
Note also, the "load" figures.

According to `systat -vm', exactly 50% of the time is spent in "user", and the
other 50% -- either "sys" or in "irq" -- all of them in the pcm driver.

[...]
 1.6%Sys  43.7%Intr 49.1%User  0.0%Nice  5.7%Idl  1903516 inact       12: psm
|    |    |    |    |    |    |    |    |    |     122920 cache       15: ata
=++++++++++++++++++++++>>>>>>>>>>>>>>>>>>>>>>>>     15688 free        16: ahc
                                                          daefr 86002 17: pcm
[...]

The computer has two Opterons (single chip 275 with two cores) and is running
a recent 6.1-stable (amd64).

This happens about once a week. Sometimes a reboot is required, sometimes
killing the stuck process and kldunload-ing the snd_ich is sufficient...

Kmail is not using the audio in any way and the irq-17 is not shared with any
other device (dmesg attached). When snd_ich is loaded, it says, as expected:

pcm0: <AMD-8111> port 0xc800-0xc8ff,0xcc00-0xcc3f irq 17 at device 7.5 on pci0
pcm0: <Avance Logic ALC655 AC97 Codec>

What's going on? Thanks!

	-mi

last pid:  5150;  load averages: 49.23, 52.22, 45.23   up 13+03:13:30  16:01:30
158 processes: 2 running, 154 sleeping, 1 stopped, 1 zombie
CPU states: 50.0% user,  0.0% nice,  1.7% system, 22.6% interrupt, 25.8% idle
Mem: 1487M Active, 1859M Inact, 303M Wired, 120M Cache, 214M Buf, 16M Free
Swap: 4465M Total, 52K Used, 4465M Free

  PID USERNAME  THR PRI NICE   SIZE    RES STATE  C   TIME   WCPU COMMAND
 5893 mi          3  20    0   298M   177M kserel 1 323:18 7634.18% kmail
90397 root        1   5    0  7400K  1768K ttyin  1  97:01  1.12% systat
80809 i           1 120    0   238M   224M select 0 160:18  0.68% firefox-bin
 2265 root        1  96    0   261M   144M select 0  49.0H  0.00% Xorg
 2267 root        1  96    0   184M 76428K select 0 149:40  0.00% Xorg
18203 mi          4  20    0   179M 67696K kserel 0 135:45  0.00% kopete
 2336 i           1  96    0   134M 32132K select 0  71:43  0.00% kdeinit
87056 i           1  96    0   118M 64392K select 0  52:09  0.00% acroread
[...]