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Date: Mon, 2 Apr 2018 22:36:39 +0200
From: Frederic Weisbecker <frederic@kernel.org>
To: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Linux PM <linux-pm@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Thomas Ilsche <thomas.ilsche@tu-dresden.de>,
        Doug Smythies <dsmythies@telus.net>, Rik van Riel <riel@surriel.com>,
        Aubrey Li <aubrey.li@linux.intel.com>,
        Mike Galbraith <mgalbraith@suse.de>,
        LKML <linux-kernel@vger.kernel.org>, Len Brown <len.brown@intel.com>
Subject: Re: [PATCH v8 02/10] sched: idle: Do not stop the tick upfront in
 the idle loop
Message-ID: <20180402203638.GA19895@lerouge>
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On Thu, Mar 29, 2018 at 02:01:14PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> Push the decision whether or not to stop the tick somewhat deeper
> into the idle loop.
> 
> Stopping the tick upfront leads to unpleasant outcomes in case the
> idle governor doesn't agree with the nohz code on the duration of the
> upcoming idle period.  Specifically, if the tick has been stopped and
> the idle governor predicts short idle, the situation is bad regardless
> of whether or not the prediction is accurate.  If it is accurate, the
> tick has been stopped unnecessarily which means excessive overhead.
> If it is not accurate, the CPU is likely to spend too much time in
> the (shallow, because short idle has been predicted) idle state
> selected by the governor [1].
> 
> As the first step towards addressing this problem, change the code
> to make the tick stopping decision inside of the loop in do_idle().
> In particular, do not stop the tick in the cpu_idle_poll() code path.
> Also don't do that in tick_nohz_irq_exit() which doesn't really have
> enough information on whether or not to stop the tick.
> 
> Link: https://marc.info/?l=linux-pm&m=150116085925208&w=2 # [1]
> Link: https://tu-dresden.de/zih/forschung/ressourcen/dateien/projekte/haec/powernightmares.pdf
> Suggested-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>