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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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