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Subject: Re: [PATCH RT] tasklet/rt: Prevent tasklets from going into
 infinite spin in RT
From: Mike Galbraith <efault@gmx.de>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>, LKML <linux-kernel@vger.kernel.org>,
        RT <linux-rt-users@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
        "Luis Claudio R. Goncalves" <lclaudio@uudg.org>,
        Clark Williams <williams@redhat.com>
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Date: Wed, 30 Nov 2011 13:45:30 +0100
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On Wed, 2011-11-30 at 11:24 +0100, Thomas Gleixner wrote:
> On Wed, 30 Nov 2011, Mike Galbraith wrote:

> > @@ -486,12 +495,43 @@ extern void softirq_check_pending_idle(v
> >   */
> >  DECLARE_PER_CPU(struct list_head [NR_SOFTIRQS], softirq_work_list);
> >  
> > -DECLARE_PER_CPU(struct task_struct *, ksoftirqd);
> > +struct softirqdata {
> > +	int			mask;
> > +	struct task_struct	*tsk;
> > +};
> > +
> > +DECLARE_PER_CPU(struct softirqdata [NR_SOFTIRQ_THREADS], ksoftirqd);
> > +
> > +static inline bool this_cpu_ksoftirqd(struct task_struct *p)
> > +{
> > +	int i;
> > +
> > +	for (i = 0; i < NR_SOFTIRQ_THREADS; i++) {
> > +		if (p == __get_cpu_var(ksoftirqd)[i].tsk)
> > +			return true;
> 
> You are not serious about that loop, are you ?

Well, it's a short loop, especially when NR_SOFTIRQ_THREADS = 1 (poof).

> > @@ -131,11 +155,18 @@ void softirq_check_pending_idle(void)
> >   */
> >  static void wakeup_softirqd(void)
> >  {
> > -	/* Interrupts are disabled: no need to stop preemption */
> > -	struct task_struct *tsk = __this_cpu_read(ksoftirqd);
> > +	struct task_struct *tsk;
> > +	u32 pending = local_softirq_pending(), mask, i;
> >  
> > -	if (tsk && tsk->state != TASK_RUNNING)
> > -		wake_up_process(tsk);
> > +	/* Interrupts are disabled: no need to stop preemption */
> > +	for (i = 0; pending && i < NR_SOFTIRQ_THREADS; i++) {
> > +		mask = __get_cpu_var(ksoftirqd)[i].mask;
> > +		if (!(pending & mask))
> > +			continue;
> > +		tsk = __get_cpu_var(ksoftirqd)[i].tsk;
> > +		if (tsk && tsk->state != TASK_RUNNING)
> > +			wake_up_process(tsk);
> > +	}
> >  }
> 
> Dammned serious is seems. :)

Deadly serious :)

For the problem at hand (timer wakeups), NR_SOFTIRQ_THREADS could be 2.

> I was looking into that as well, though I did not want to inflict it
> on 3.0 at this point.
> 
> Thanks,
> 
> 	tglx