linux-kernel - Re: [PATCH] cpuidle: Fix coupled state parameter in cpuidle

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Date:	Tue, 17 May 2016 16:55:35 +0200
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	rjw@...ysocki.net
Cc:	gregory.clement@...e-electrons.com, linux-pm@...r.kernel.org,
	linux@...linux.org.uk, paul.burton@...tec.com,
	stable@...r.kernel.org, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpuidle: Fix coupled state parameter in cpuidle_enter()

On 05/17/2016 04:54 PM, Daniel Lezcano wrote:
> The commit 0b89e9aa2856 rightfully fixed a regression by letting the couple
> idle state framework to handle local interrupt enabling when the cpu is
> exiting an idle state.
>
> The current code check if the idle state is a coupled one and, if so, it will
> let the couple code to enable the interrupt. This way, it can decrement the
> ready-count before handling the interrupts. This mechanism prevents the other
> cpus to wait for a cpu which is handling interrupts.
>
> But the check is done against the state index returned by the back end driver
> 'enter' functions which could be different from the initial index passed as
> parameter to the cpuidle_enter_state() function.
>
>   entered_state = target_state->enter(dev, drv, index);
>
>   [ ... ]
>
>   if (!cpuidle_state_is_coupled(drv, entered_state))
> 	local_irq_enable();
>
>   [ ... ]
>
> If the 'index' is referring to a coupled idle state but the 'entered_state'
> is *not* coupled, then the interrupts are enabled again. All cpus blocked on
> the sync barrier may busy loop longer if the cpu has interrupts to handle
> before decrementing the ready-count. Thus consuming more energy than saving.
>
> Fixes: 0b89e9aa2856 "cpuidle: delay enabling interrupts until all coupled CPUs leave idle"
> Cc: stable@...r.kernel.org
> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
> ---

I don't have a board available with a driver doing couple idle state, so 
I wasn't able to test this patch.

   -- Daniel