linux-kernel - Re: [PATCH v5] nohz: set isolcpus when nohz

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Date:	Thu, 9 Apr 2015 19:12:51 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] nohz: set isolcpus when nohz_full is set

On Thu, Apr 09, 2015 at 12:59:39PM -0400, Chris Metcalf wrote:
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 6d77432e14ff..18a961b9beba 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -323,6 +323,7 @@ struct task_struct;
>  extern int lockdep_tasklist_lock_is_held(void);
>  #endif /* #ifdef CONFIG_PROVE_RCU */
>  
> +extern void sched_isolated_map_add(const struct cpumask *);
>  extern void sched_init(void);
>  extern void sched_init_smp(void);
>  extern asmlinkage void schedule_tail(struct task_struct *prev);
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f0f831e8a345..b055c5e0e65c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5824,6 +5824,11 @@ static int __init isolated_cpu_setup(char *str)
>  
>  __setup("isolcpus=", isolated_cpu_setup);
>  
> +void sched_isolated_map_add(const struct cpumask *cpumask)
> +{
> +	cpumask_or(cpu_isolated_map, cpu_isolated_map, cpumask);
> +}
> +
>  struct s_data {
>  	struct sched_domain ** __percpu sd;
>  	struct root_domain	*rd;
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index a4c4edac4528..b0092d02ca3f 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -385,6 +385,9 @@ void __init tick_nohz_init(void)
>  	for_each_cpu(cpu, tick_nohz_full_mask)
>  		context_tracking_cpu_set(cpu);
>  
> +	/* It's not meaningful to be nohz without disabling the scheduler. */
> +	sched_isolated_map_add(tick_nohz_full_mask);
> +
>  	cpu_notifier(tick_nohz_cpu_down_callback, 0);
>  	pr_info("NO_HZ: Full dynticks CPUs: %*pbl.\n",
>  		cpumask_pr_args(tick_nohz_full_mask));

Right, this could work. Although I would suggest adding a comment
somewhere that we should be careful with init order. I checked, this
appears to be ordered right, but...
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