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    SubjectRe: Pin-pointing the root of unusual application latencies
    [ Adding linux-net to the mix ]

    John Sigler wrote:

    > ( check_dektec_in-1095 |#0): new 271 us user-latency.
    > ( check_dektec_in-1095 |#0): new 275 us user-latency.
    > ( check_dektec_in-1095 |#0): new 290 us user-latency.
    > ( check_dektec_in-1095 |#0): new 297 us user-latency.
    > ( check_dektec_in-1095 |#0): new 345 us user-latency.
    > ( check_dektec_in-1095 |#0): new 358 us user-latency.
    > ( check_dektec_in-1095 |#0): new 384 us user-latency.
    > ( check_dektec_in-1095 |#0): new 392 us user-latency.
    > ( check_dektec_in-1095 |#0): new 395 us user-latency.
    > ( check_dektec_in-1095 |#0): new 396 us user-latency.
    > ( check_dektec_in-1095 |#0): new 1031 us user-latency.
    > ( check_dektec_in-1095 |#0): new 1100 us user-latency.
    > ( check_dektec_in-1095 |#0): new 1105 us user-latency.
    > ( check_dektec_in-1095 |#0): new 1106 us user-latency.
    >
    > Here's the function trace for the 1106-µs latency:
    >
    > http://linux.kernel.free.fr/latency/1106-us-trace.txt
    >
    > These two lines repeat ~2000 times for ~800 µs:
    >
    > softirq--4 0.... 272us : __lock_text_start (rt_run_flush)
    > softirq--4 0.... 272us : rt_spin_unlock (rt_run_flush)
    >
    > With a pair of the following in the middle:
    >
    > softirq--4 0.... 670us : call_rcu (rt_run_flush)
    > softirq--4 0D..1 670us : __rcu_advance_callbacks (call_rcu)

    Could someone explain why the kernel is "spinning" in rt_run_flush.
    http://lxr.linux.no/source/net/ipv4/route.c#L692
    What is the kernel trying to do?

    Is the kernel in the following loop?

    701 for (i = rt_hash_mask; i >= 0; i--) {
    702 spin_lock_bh(rt_hash_lock_addr(i));
    703 rth = rt_hash_table[i].chain;
    704 if (rth)
    705 rt_hash_table[i].chain = NULL;
    706 spin_unlock_bh(rt_hash_lock_addr(i));

    Line 702 would be __lock_text_start (rt_run_flush)
    Line 706 would be rt_spin_unlock
    Is that correct?

    And this would be done 2000 times because rt_hash_mask=2000?

    Regards.
    -
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