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Date:   Sun, 4 Aug 2019 16:48:35 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org,
        dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
        oleg@redhat.com, joel@joelfernandes.org
Subject: Re: [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu_stop()
 enable tick on all online CPUs
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References: <20190802151435.GA1081@linux.ibm.com>
 <20190802151501.13069-14-paulmck@linux.ibm.com>
 <20190804144317.GF2349@hirez.programming.kicks-ass.net>
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On Sun, Aug 04, 2019 at 04:43:17PM +0200, Peter Zijlstra wrote:
> On Fri, Aug 02, 2019 at 08:15:01AM -0700, Paul E. McKenney wrote:
> > The multi_cpu_stop() function relies on the scheduler to gain control from
> > whatever is running on the various online CPUs, including any nohz_full
> > CPUs running long loops in kernel-mode code.  Lack of the scheduler-clock
> > interrupt on such CPUs can delay multi_cpu_stop() for several minutes
> > and can also result in RCU CPU stall warnings.  This commit therefore
> > causes multi_cpu_stop() to enable the scheduler-clock interrupt on all
> > online CPUs.
> 
> This sounds wrong; should we be fixing sched_can_stop_tick() instead to
> return false when the stop task is runnable?

And even without that; I don't understand how we're not instantly
preempted the moment we enqueue the stop task.

Any enqueue, should go through check_preempt_curr() which will be an
instant resched_curr() when we just woke the stop class.