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Message-ID: <20171226151746.GA6560@lerouge> Date: Tue, 26 Dec 2017 16:17:48 +0100 From: Frederic Weisbecker <frederic@...nel.org> To: Thomas Gleixner <tglx@...utronix.de> 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 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().
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