From: Arjan van de Ven <arjan@linux.intel.com>
To: Colin Cross <ccross@android.com>
Cc: linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
linux-pm@lists.linux-foundation.org,
Len Brown <len.brown@intel.com>, Kevin Hilman <khilman@ti.com>,
Santosh Shilimkar <santosh.shilimkar@ti.com>,
Amit Kucheria <amit.kucheria@linaro.org>,
Trinabh Gupta <g.trinabh@gmail.com>,
Deepthi Dharwar <deepthi@linux.vnet.ibm.com>,
linux-omap@vger.kernel.org, linux-tegra@vger.kernel.org
Subject: Re: [PATCH 0/3] coupled cpuidle state support
Date: Wed, 21 Dec 2011 10:02:44 +0100 [thread overview]
Message-ID: <4EF1A0B4.5080307@linux.intel.com> (raw)
In-Reply-To: <1324426147-16735-1-git-send-email-ccross@android.com>
On 12/21/2011 1:09 AM, Colin Cross wrote:
> On some ARM SMP SoCs (OMAP4460, Tegra 2, and probably more), the
> cpus cannot be independently powered down, either due to
> sequencing restrictions (on Tegra 2, cpu 0 must be the last to
> power down), or due to HW bugs (on OMAP4460, a cpu powering up
> will corrupt the gic state unless the other cpu runs a work
> around). Each cpu has a power state that it can enter without
> coordinating with the other cpu (usually Wait For Interrupt, or
> WFI), and one or more "coupled" power states that affect blocks
> shared between the cpus (L2 cache, interrupt controller, and
> sometimes the whole SoC). Entering a coupled power state must
> be tightly controlled on both cpus.
>
> The easiest solution to implementing coupled cpu power states is
> to hotplug all but one cpu whenever possible, usually using a
> cpufreq governor that looks at cpu load to determine when to
> enable the secondary cpus. This causes problems, as hotplug is an
> expensive operation, so the number of hotplug transitions must be
> minimized, leading to very slow response to loads, often on the
> order of seconds.
>
> This patch series implements an alternative solution, where each
> cpu will wait in the WFI state until all cpus are ready to enter
> a coupled state, at which point the coupled state function will
> be called on all cpus at approximately the same time.
>
> Once all cpus are ready to enter idle, they are woken by an smp
> cross call. At this point, there is a chance that one of the
> cpus will find work to do, and choose not to enter suspend. A
> final pass is needed to guarantee that all cpus will call the
> power state enter function at the same time. During this pass,
> each cpu will increment the ready counter, and continue once the
> ready counter matches the number of online coupled cpus. If any
> cpu exits idle, the other cpus will decrement their counter and
> retry.
this smells fundamentally racey to me; you can get an interrupt one
cycle after you think you're done, but before the last guy enters WFI...
how do you solve that issue ?
next prev parent reply other threads:[~2011-12-21 9:02 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-21 0:09 [PATCH 0/3] coupled cpuidle state support Colin Cross
2011-12-21 0:09 ` [PATCH 1/3] cpuidle: refactor out cpuidle_enter_state Colin Cross
2012-01-04 14:08 ` [linux-pm] " Jean Pihet
2011-12-21 0:09 ` [PATCH 2/3] cpuidle: fix error handling in __cpuidle_register_device Colin Cross
2011-12-21 0:09 ` [PATCH 3/3] cpuidle: add support for states that affect multiple cpus Colin Cross
2011-12-21 9:02 ` Arjan van de Ven [this message]
2011-12-21 9:40 ` [PATCH 0/3] coupled cpuidle state support Colin Cross
2011-12-21 9:44 ` Arjan van de Ven
2011-12-21 9:55 ` Colin Cross
2011-12-21 12:12 ` Arjan van de Ven
2011-12-21 19:05 ` Colin Cross
2011-12-21 19:36 ` Arjan van de Ven
2011-12-21 19:42 ` [linux-pm] " Colin Cross
2011-12-22 8:35 ` Shilimkar, Santosh
2011-12-22 8:53 ` Arjan van de Ven
2011-12-22 9:30 ` Shilimkar, Santosh
2011-12-22 21:20 ` Colin Cross
2012-03-14 0:39 ` Colin Cross
2012-01-04 0:41 ` Kevin Hilman
2012-01-04 17:27 ` Shilimkar, Santosh
2012-01-20 8:46 ` Daniel Lezcano
2012-01-20 20:40 ` Colin Cross
2012-01-25 14:04 ` Daniel Lezcano
2012-01-31 14:13 ` Daniel Lezcano
2012-01-27 8:54 ` [linux-pm] " Vincent Guittot
2012-01-27 17:32 ` Colin Cross
2012-02-01 12:13 ` Vincent Guittot
2012-02-01 14:59 ` Lorenzo Pieralisi
2012-02-01 17:30 ` Colin Cross
2012-02-01 18:07 ` Lorenzo Pieralisi
2012-02-03 1:19 ` Colin Cross
[not found] ` <8762e8kqi6.fsf@ti.com>
2012-03-14 0:28 ` Colin Cross
2012-03-14 0:47 ` Colin Cross
2012-03-14 14:23 ` [linux-pm] " Kevin Hilman
2012-03-14 2:04 ` Arjan van de Ven
2012-03-14 2:21 ` Colin Cross
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