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SubjectRe: route cache DoS testing and softirqs

Andrea Arcangeli writes:

> But I feel like we should change the softirq code so that the irqexit
> runs only the softirq setup by the current (or nested) irq handler. This
> way the timer irq will stop executing the softirqs posted by the network
> stack and it may very well fix the problem completely. This definitely
> will help fairness too.

How would softirq's scheduled be softirq's be run?

For a definitive solution a think Alexey analysis make sense:

"All of them happening outside of ksoftirqd are equally bad, unaccountable,
uncontrollable and will show up in some situation."

FYI. I did some more experiments:

Simple sycall test: user app = ank-time-loop (does gettimeofday in a loop)
=========================================================================
Before
total droppped tsquz throttl bh_enbl ksoftird irqexit other
00000099 00000000 00000000 00000000 0000005d 00000005 000163a2 00000000
00000008 00000000 00000000 00000000 00000009 00000000 000162f6 00000000
ank-time-loop

After 10 sec.
000000c0 00000000 00000000 00000000 0000005d 00000005 0001c3d2 00000000
00000008 00000000 00000000 00000000 00000009 00000000 0001c2f3 00000000
So syscalls "in this context" seems to do_softirq() via irqexit.

Route DoS while BGP tables are loading. ank-time-loop is running.
================================================================
total droppped tsquz throttl bh_enbl ksoftird irqexit other
000000c2 00000000 00000000 00000000 00000042 00000005 0001aef8 00000000
00000008 00000000 00000000 00000000 00000017 00000001 0001adba 00000000
After run:
000b3a10 00000000 00000eda 00000000 000006f6 0000052e 00041c06 00000000
000b0500 00000000 00000ec1 00000000 0000075e 00000197 000419f2 00000000
Most softirq's are run from irqexit but we see local_bh_enable is running
softirq's too. And only a minority of softirq's is running under scheduler
control. As discussed we before with 100Hz timer we would expect lower
rates from irqexit.

Cheers.
--ro


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