linux-kernel - Re: [PATCH 0/3] coupled cpuidle state support

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Message-ID: <4EF1AA8A.8060304@linux.intel.com>
Date:	Wed, 21 Dec 2011 10:44:42 +0100
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Colin Cross <ccross@...roid.com>
CC:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-pm@...ts.linux-foundation.org,
	Len Brown <len.brown@...el.com>, Kevin Hilman <khilman@...com>,
	Santosh Shilimkar <santosh.shilimkar@...com>,
	Amit Kucheria <amit.kucheria@...aro.org>,
	Trinabh Gupta <g.trinabh@...il.com>,
	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>,
	linux-omap@...r.kernel.org, linux-tegra@...r.kernel.org
Subject: Re: [PATCH 0/3] coupled cpuidle state support

On 12/21/2011 10:40 AM, Colin Cross wrote:

>> 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 ?
> 
> All the cpus have interrupts off when they increment the counter, so
> they cannot receive an interrupt.  If an interrupt is pending on one
> of those cpus, it will be handled later when WFI aborts due to the
> pending interrupt.

... but this leads to cases where you're aborting before other cpus are
entering..... so your "last guy in" doesn't really work, since while cpu
0 thinks it's the last guy, cpu 1 is already on the way out/out
already...  (heck it might already be going back to sleep if your idle
code can run fast, like in the size of a cache miss)
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