linux-kernel - Re: [nohz] 2a16fc93d2c: kernel lockup on idle injection

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Date:	Tue, 16 Dec 2014 23:54:51 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	"Pan, Jacob jun" <jacob.jun.pan@...el.com>,
	LKML <linux-kernel@...r.kernel.org>, LKP <lkp@...org>
Subject: Re: [nohz] 2a16fc93d2c: kernel lockup on idle injection

On Tue, 16 Dec 2014, Peter Zijlstra wrote:
> On Tue, Dec 16, 2014 at 10:21:27PM +0100, Thomas Gleixner wrote:
> > /* rq->lock is held for evaluating rq->nr_running */
> > static void sched_ttwu_remote_nohz(struct rq *rq)
> > {
> > 	if (nohz_full_disabled())
> > 		return;
> > 
> > 	if (rq->nr_running != 2)
> > 	   	return;
> > 	/*
> > 	 * Force smp_send_reschedule(). irq_exit() on the
> > 	 * remote cpu will handle the rest.
> > 	 */
> 
> smp_send_reschedule() is magic and does not guarantee irq_{enter,exit}()
> being called, although we could audit and fix that.

I know. I did not want to do all the work for the nohz
folks. Otherwise I would have simply sent a patch. :)
 
> > 	if (!task_uses_posix_timers(task))
> > 		clear_bit(NOHZ_POSIXTIMER_NEEDS_TICK,
> > 			  this_cpu_ptr(nohz_full_must_tick));
> > 	else
> > 		set_bit(NOHZ_POSIXTIMER_NEEDS_TICK,
> > 			this_cpu_ptr(nohz_full_must_tick));
> > 
> 
> /me hands you a few spare {} :-)

Without the proper instruction manual they are pretty useless.
 
> Arguably test state before doing a possibly pointless update?
> 
> > 	local_irq_disable();
> > 	tick_full_nohz_update_state();
> > 	local_irq_enable();
> > }
> 
> But yes, that should work just fine..

So I'm not the only one who thinks that this needs a proper
reimplementation :)

Thanks,

	tglx

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