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

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Date:   Wed, 27 Dec 2017 19:22:06 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Ingo Molnar <mingo@...nel.org>, stable@...r.kernel.org
Subject: Re: [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();
 	}