linux-kernel - Re: [PATCH 1/3] idle: add support for tasks that inject idle

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Message-ID: <20161114150156.GQ3157@twins.programming.kicks-ass.net>
Date:   Mon, 14 Nov 2016 16:01:56 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Zhang Rui <rui.zhang@...el.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        "Chen, Yu C" <yu.c.chen@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [PATCH 1/3] idle: add support for tasks that inject idle

On Mon, Nov 14, 2016 at 03:57:21PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 09, 2016 at 11:05:10AM -0800, Jacob Pan wrote:
> >  static inline bool is_idle_task(const struct task_struct *p)
> >  {
> > -	return p->pid == 0;
> > +	return !!(p->flags & PF_IDLE);
> >  }
> 
> > +void play_idle()
> > +{
> > +	/*
> > +	 * Only FIFO tasks can disable the tick since they don't need the forced
> > +	 * preemption.
> > +	 */
> > +	WARN_ON_ONCE(current->policy != SCHED_FIFO);
> > +	WARN_ON_ONCE(current->nr_cpus_allowed != 1);
> > +	WARN_ON_ONCE(!(current->flags & PF_KTHREAD));
> > +	WARN_ON_ONCE(!(current->flags & PF_NO_SETAFFINITY));
> > +	rcu_sleep_check();
> > +
> > +	preempt_disable();
> > +	current->flags |= PF_IDLE;
> > +	do_idle();
> > +	current->flags &= ~PF_IDLE;
> 
> Whoops, that will clear PF_IDLE from the idle thread. Maybe do something
> like:
> 
> 	unsigned int idle_flag = (~current->flags & PF_IDLE)
> 
> 	current->flags |= PF_IDLE;
> 
> 	current->flags ^= idle_flag;
> 
> > +
> > +	preempt_fold_need_resched();
> > +	preempt_enable();
> > +}
> > +EXPORT_SYMBOL_GPL(play_idle);

n/m I cannot read, the actual idle loop below uses do_idle(), not
play_idle.

> > @@ -299,5 +313,7 @@ void cpu_startup_entry(enum cpuhp_state state)
> >  #endif
> >  	arch_cpu_idle_prepare();
> >  	cpuhp_online_idle(state);
> > -	cpu_idle_loop();
> > +	while (1)
> > +		do_idle();
> > +
> >  }