Re: [patch 2/4] nohz: Prevent erroneous tick stop invocations

[Thomas Gleixner]

On Wed, 27 Dec 2017, Thomas Gleixner wrote:
> On Tue, 26 Dec 2017, Frederic Weisbecker wrote:
> 
> > On Fri, Dec 22, 2017 at 03:51:13PM +0100, Thomas Gleixner wrote:
> > > The conditions in irq_exit() to invoke tick_nohz_irq_exit() are:
> > > 
> > >   if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu))
> > > 
> > > This is too permissive in various aspects:
> > > 
> > >   1) If need_resched() is set, then the tick cannot be stopped whether
> > >      the CPU is idle or in nohz full mode.
> > 
> > That's not exactly true. In nohz full mode the tick is not restarted on the
> > switch from idle to a single task. And if an idle interrupt wakes up a
> > single task and enqueues a timer, we want that timer to be programmed even
> > though we have need_resched().
> 
> Hrmm, so the check for softirq_pending() should be sufficient, right?
> 
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -382,7 +382,8 @@ static inline void tick_irq_exit(void)
>  	int cpu = smp_processor_id();
>  
>  	/* Make sure that timer wheel updates are propagated */
> -	if ((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) {
> +	if (((idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) &&
> +	if ((idle_cpu(cpu) || tick_nohz_full_cpu(cpu)) &&
> +	    !local_softirq_pending()) {
>  		if (!in_interrupt())
>  			tick_nohz_irq_exit();
>  	}

Bah, no. We need to move that into the nohz logic somehow to prevent that
repetitive expiry yesterday reprogramming. Lemme think about it some more.

Thanks,

	tglx