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Message-ID: <alpine.DEB.2.20.1712271918540.2431@nanos> 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(); }
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