linux-kernel - Re: [PATCH v5] cpuidle: menu: Handle stopped tick more aggressively

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Message-ID: <20180820110254.GX2494@hirez.programming.kicks-ass.net>
Date:   Mon, 20 Aug 2018 13:02:54 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Leo Yan <leo.yan@...aro.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH v5] cpuidle: menu: Handle stopped tick more aggressively

On Tue, Aug 14, 2018 at 12:34:40PM +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> Commit 87c9fe6ee495 (cpuidle: menu: Avoid selecting shallow states
> with stopped tick) missed the case when the target residencies of
> deep idle states of CPUs are above the tick boundary which may cause
> the CPU to get stuck in a shallow idle state for a long time.
> 
> Say there are two CPU idle states available: one shallow, with the
> target residency much below the tick boundary and one deep, with
> the target residency significantly above the tick boundary.  In
> that case, if the tick has been stopped already and the expected
> next timer event is relatively far in the future, the governor will
> assume the idle duration to be equal to TICK_USEC and it will select
> the idle state for the CPU accordingly.  However, that will cause the
> shallow state to be selected even though it would have been more
> energy-efficient to select the deep one.
> 
> To address this issue, modify the governor to always use the time
> till the closest timer event instead of the predicted idle duration
> if the latter is less than the tick period length and the tick has
> been stopped already.  Also make it extend the search for a matching
> idle state if the tick is stopped to avoid settling on a shallow
> state if deep states with target residencies above the tick period
> length are available.
> 
> In addition, make it always indicate that the tick should be stopped
> if it has been stopped already for consistency.
> 
> Fixes: 87c9fe6ee495 (cpuidle: menu: Avoid selecting shallow states with stopped tick)
> Reported-by: Leo Yan <leo.yan@...aro.org>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Seems OK,

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>