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

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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().