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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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