Re: [Patch] Idle balancer: cache align nohz structure to improveidle load balancing scalability

[Srivatsa Vaddagiri]

* Suresh Siddha <suresh.b.siddha@xxxxxxxxx> [2011-11-01 16:52:38]:

> +	/*
> +	 * We were recently in tickless idle mode. We will do the delayed
> +	 * update of busy mode now (first busy tick after returning from idle).
> +	 */
> +	if (unlikely(rq->tick_stopped)) {
> +		cpumask_clear_cpu(cpu, nohz.idle_cpus_mask);
> +
> +		if (cpumask_bits(nohz.idle_cpus_mask)[BIT_WORD(cpu)] == 0 &&
> +		    cpumask_empty(nohz.idle_cpus_mask))
> +			clear_bit(NOHZ_NEED_BALANCING, &nohz.bits);

Can't this clear_bit race with set_bit() in select_nohz_load_balancer()?

   CPU0				CPU1

cpumask_clear_cpu()
if ( ...)
				cpumask_set_cpu();
				set_bit();

	clear_bit();

?

- vatsa

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