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Date: Mon, 9 Nov 2015 14:29:14 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: byungchul.park@lge.com
Cc: mingo@kernel.org, linux-kernel@vger.kernel.org, yuyang.du@intel.com,
        pjt@google.com, efault@gmx.de, tglx@linutronix.de
Subject: Re: [PATCH v4 3/3] sched: optimize migration by forcing rmb() and
 updating to be called once
Message-ID: <20151109132914.GK17308@twins.programming.kicks-ass.net>
References: <1445616981-29904-1-git-send-email-byungchul.park@lge.com>
 <1445616981-29904-4-git-send-email-byungchul.park@lge.com>
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On Sat, Oct 24, 2015 at 01:16:21AM +0900, byungchul.park@lge.com wrote:
> +++ b/kernel/sched/core.c
> @@ -1264,6 +1264,8 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr);
>  
>  void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
>  {
> +	unsigned int prev_cpu = task_cpu(p);
> +
>  #ifdef CONFIG_SCHED_DEBUG
>  	/*
>  	 * We should never call set_task_cpu() on a blocked task,
> @@ -1289,15 +1291,14 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
>  #endif
>  
>  	trace_sched_migrate_task(p, new_cpu);
> +	__set_task_cpu(p, new_cpu);
>  
> -	if (task_cpu(p) != new_cpu) {
> +	if (prev_cpu != new_cpu) {
>  		if (p->sched_class->migrate_task_rq)
> -			p->sched_class->migrate_task_rq(p, new_cpu);
> +			p->sched_class->migrate_task_rq(p, prev_cpu);
>  		p->se.nr_migrations++;
>  		perf_event_task_migrate(p);
>  	}
> -
> -	__set_task_cpu(p, new_cpu);
>  }

I don't think this is safe, see the comment in __set_task_cpu(). We want
that to be last.