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    SubjectRe: [tip:sched/core] sched: don't rebalance if attached on NULL domain

    On Thu, 5 Mar 2009, Frederic Weisbecker wrote:

    > sched: don't rebalance if attached on NULL domain
    > Impact: fix function graph trace hang / drop pointless softirq on UP
    > While debugging a function graph trace hang on an old PII, I saw
    > that it consumed most of its time on the timer interrupt. And
    > the domain rebalancing softirq was the most concerned.
    > The timer interrupt calls trigger_load_balance() which will
    > decide if it is worth to schedule a rebalancing softirq.
    > In case of builtin UP kernel, no problem arises because there is
    > no domain question.
    > In case of builtin SMP kernel running on an SMP box, still no
    > problem, the softirq will be raised each time we reach the
    > next_balance time.
    > In case of builtin SMP kernel running on a UP box (most distros
    > provide default SMP kernels, whatever the box you have), then
    > the CPU is attached to the NULL sched domain. So a kind of
    > unexpected behaviour happen:
    > trigger_load_balance() -> raises the rebalancing softirq later
    > on softirq: run_rebalance_domains() -> rebalance_domains() where
    > the for_each_domain(cpu, sd) is not taken because of the NULL
    > domain we are attached at. Which means rq->next_balance is never
    > updated. So on the next timer tick, we will enter
    > trigger_load_balance() which will always reschedule() the
    > rebalacing softirq:
    > if (time_after_eq(jiffies, rq->next_balance))
    > raise_softirq(SCHED_SOFTIRQ);
    > So for each tick, we process this pointless softirq.

    Nice catch!

    -- Steve

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