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SubjectRe: [patch 2/4] nohz: Prevent erroneous tick stop invocations
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();
}



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