linux-kernel - Re: [Patch] Idle balancer: cache align nohz structure to improve idle load balancing scalability

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Message-ID: <20111102135414.GC6820@linux.vnet.ibm.com>
Date:	Wed, 2 Nov 2011 19:24:14 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	Venki Pallipadi <venki@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Patch] Idle balancer: cache align nohz structure to improve
 idle load balancing scalability

* Suresh Siddha <suresh.b.siddha@...el.com> [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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