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SubjectRe: Linux interrupt latency
Mark Spencer wrote:
>
> Can anyone suggest what might be causing the problem on non-Intel
> chipsets, particularly what event might be occuring once per second and
> disabling interrupts for a couple of hundred microseconds? Thanks!

I have a gizmo which will find this for you.

http://www.uow.edu.au/~andrewm/linux/#timepegs

- Grab the 2.4.1-pre10 patch and tpt.
- Apply patch. Under `Kernel hacking', enable timepegs
and `Interrupt latency'. Make sure you enable IO-APIC
on Uniprocessor. Rebuild kernel. Reboot.
- Run `tpt' to zero all the counters.
- Use the system for a few minutes in a normal manner
- Run `tpt -s | sort -nr +5'

You'll get something like this:

do_IRQ.in:0 -> softirq.c:71 6059 7.14 18.56 8.45 51237.71
do_IRQ.in:0 -> do_IRQ.out:0 255 11.60 16.69 13.47 3437.13
irq.c:476 -> irq.c:481 1 14.31 14.31 14.31 14.31
exit.c:384 -> exit.c:418 4 5.94 10.53 8.40 33.63
ll_rw_blk.c:759 -> ll_rw_blk.c:856 3709 .64 8.96 1.01 3754.22
3c59x.c:1835 -> 3c59x.c:1855 81405 2.53 8.63 2.92 238321.05
ide.c:513 -> ide.c:522 3709 .52 8.10 1.42 5300.37
irq.c:523 -> irq.c:542 1 7.14 7.14 7.14 7.14
signal.c:528 -> signal.c:546 13 1.95 6.10 4.22 55.00
page_alloc.c:181 -> page_alloc.c:198 4407 .41 5.86 .75 3355.55
sched.c:541 -> sched.c:596 8238 .35 5.22 1.22 10055.84
skbuff.c:121 -> skbuff.c:123 206065 .32 5.03 .36 74765.68
signal.c:602 -> signal.c:604 17 1.28 5.03 2.91 49.63
dev.c:1127 -> dev.c:1139 43268 .34 4.86 .41 17836.39
sched.c:713 -> sched.c:748 5497 .31 4.78 1.10 6055.92
ll_rw_blk.c:377 -> ide.c:1357 78 .93 4.63 3.36 262.83
timer.c:205 -> timer.c:209 43465 .33 4.61 .43 19044.11
slab.c:1298 -> slab.c:1322 129153 .32 4.59 .39 51402.35

So the worst interrupt latency path on this machine was 18 usecs,
from the entry into do_IRQ to the enabling of interrupts in do_softirq.
Traversed 6059 times, min,max,avg=7,18,8 usecs. Aggregate irq
blockage 51 msecs.
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