linux-kernel - Re: [PATCH] cpuidle: Measure idle state durations with monotonic clock

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Message-ID: <50A4C963.9060808@linaro.org>
Date:	Thu, 15 Nov 2012 11:52:19 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Julius Werner <jwerner@...omium.org>
CC:	linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Kevin Hilman <khilman@...com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	linux-acpi@...r.kernel.org, linux-pm@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org,
	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>,
	Trinabh Gupta <g.trinabh@...il.com>,
	Sameer Nanda <snanda@...omium.org>,
	Lists Linaro-dev <linaro-dev@...ts.linaro.org>
Subject: Re: [PATCH] cpuidle: Measure idle state durations with monotonic
 clock

On 11/15/2012 02:56 AM, Julius Werner wrote:
> Many cpuidle drivers measure their time spent in an idle state by
> reading the wallclock time before and after idling and calculating the
> difference. This leads to erroneous results when the wallclock time gets
> updated by another processor in the meantime, adding that clock
> adjustment to the idle state's time counter.
> 
> If the clock adjustment was negative, the result is even worse due to an
> erroneous cast from int to unsigned long long of the last_residency
> variable. The negative 32 bit integer will zero-extend and result in a
> forward time jump of roughly four billion milliseconds or 1.3 hours on
> the idle state residency counter.
> 
> This patch changes all affected cpuidle drivers to either use the
> monotonic clock for their measurements or make use of the generic time
> measurement wrapper in cpuidle.c, which was already working correctly.
> Some superfluous CLIs/STIs in the ACPI code are removed (interrupts
> should always already be disabled before entering the idle function, and
> not get reenabled until the generic wrapper has performed its second
> measurement). It also removes the erroneous cast, making sure that
> negative residency values are applied correctly even though they should
> not appear anymore.
> 
> Signed-off-by: Julius Werner <jwerner@...omium.org>

Tested on a Core 2 Duo (processor_idle driver).

Tested-by: Daniel Lezcano <daniel.lezcano@...aro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@...aro.org>



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