linux-kernel - Re: linux-next-20110923: warning kernel/rcutree.c:1833

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Message-ID: <20111002225019.GA1835@somewhere>
Date:	Mon, 3 Oct 2011 00:50:22 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	linux-kernel@...r.kernel.org, Dipankar Sarma <dipankar@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Lai Jiangshan <laijs@...fujitsu.com>
Subject: Re: linux-next-20110923: warning kernel/rcutree.c:1833

On Fri, Sep 30, 2011 at 12:24:38PM -0700, Paul E. McKenney wrote:
> @@ -328,11 +326,11 @@ static int rcu_implicit_offline_qs(struct rcu_data *rdp)
>  		return 1;
>  	}
>  
> -	/* If preemptible RCU, no point in sending reschedule IPI. */
> -	if (rdp->preemptible)
> -		return 0;
> -
> -	/* The CPU is online, so send it a reschedule IPI. */
> +	/*
> +	 * The CPU is online, so send it a reschedule IPI.  This forces
> +	 * it through the scheduler, and (inefficiently) also handles cases
> +	 * where idle loops fail to inform RCU about the CPU being idle.
> +	 */

If the idle loop forgets to call rcu_idle_enter() before going to
sleep, I don't know if it's a good idea to try to cure that situation
by forcing a quiescent state remotely. It may make the thing worse
because we actually won't notice the lack of call to rcu_idle_enter()
that the rcu stall detector would otherwise report to us.

Also I don't think that works. If the task doesn't have
TIF_RESCHED, it won't go through the scheduler on irq exit.
smp_send_reschedule() doesn't set the flag. And also scheduler_ipi()
returns right away if no wake up is pending.

So, other than resuming the idle loop to sleep again, nothing may happen.

Or am I missing something?

>  	if (rdp->cpu != smp_processor_id())
>  		smp_send_reschedule(rdp->cpu);
>  	else
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