linux-kernel - Re: [PATCH v8 02/10] sched: idle: Do not stop the tick upfront in the idle loop

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Date:   Mon, 2 Apr 2018 22:36:39 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Ilsche <thomas.ilsche@...dresden.de>,
        Doug Smythies <dsmythies@...us.net>,
        Rik van Riel <riel@...riel.com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Mike Galbraith <mgalbraith@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Len Brown <len.brown@...el.com>
Subject: Re: [PATCH v8 02/10] sched: idle: Do not stop the tick upfront in
 the idle loop

On Thu, Mar 29, 2018 at 02:01:14PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.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@...nel.org>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

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