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From: Christian Borntraeger <borntraeger@de.ibm.com>
To: Suresh Siddha <suresh.b.siddha@intel.com>
Subject: Re: [PATCH v2] NOHZ: fix nohz on cpu unplug
Date: Tue, 3 Feb 2009 09:48:13 +0100
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Cc: Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
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       Heiko Carstens <heiko.carstens@de.ibm.com>,
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Am Tuesday 03 February 2009 00:57:03 schrieb Suresh Siddha:
> Does this patch fix the issue? Thanks.

I fixed up a compile error. (See below). After that, the patch does indeed fix 
the problem.

Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>


> diff --git a/kernel/sched.c b/kernel/sched.c
> index 96439a4..8a0419b 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -4026,19 +4026,24 @@ int select_nohz_load_balancer(int stop_tick)
>  	int cpu = smp_processor_id();
>
>  	if (stop_tick) {
> -		cpumask_set_cpu(cpu, nohz.cpu_mask);
>  		cpu_rq(cpu)->in_nohz_recently = 1;
>
> -		/*
> -		 * If we are going offline and still the leader, give up!
> -		 */
> -		if (!cpu_active(cpu) &&
> -		    atomic_read(&nohz.load_balancer) == cpu) {
> +		if (!cpu_active(cpu) {

That does not compile...I used the following:
		if (!cpu_active(cpu)) {

> +			if (atomic_read(&nohz.load_balancer) != cpu)
> +				return 0;
> +
> +			/*
> +			 * If we are going offline and still the leader,
> +			 * give up!
> +			 */
>  			if (atomic_cmpxchg(&nohz.load_balancer, cpu, -1) != cpu)
>  				BUG();
> +
>  			return 0;
>  		}
>
> +		cpumask_set_cpu(cpu, nohz.cpu_mask);
> +
>  		/* time for ilb owner also to sleep */
>  		if (cpumask_weight(nohz.cpu_mask) == num_online_cpus()) {
>  			if (atomic_read(&nohz.load_balancer) == cpu)