linux-kernel - Re: 2.6.23-mm1 breaks C-state support on Intel T7200 x86

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Date:	Thu, 8 Nov 2007 10:07:39 -0800
From:	Mark Gross <mgross@...ux.intel.com>
To:	Valdis.Kletnieks@...edu
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.23-mm1 breaks C-state support on Intel T7200 x86_64

On Thu, Nov 08, 2007 at 12:19:44PM -0500, Valdis.Kletnieks@...edu wrote:
> (Sorry for not reporting this sooner - I haven't been running off battery
> much in the last 3 weeks, so I didn't notice it till now...)
> 
> Dell Latitude D820 laptop, T7200 Core2 Duo CPU, x86_64 kernel.
> 
> As reported by 'powertop' on a basically idle machine:
> 
> 2.6.23-mm1:
> 
> Cn                Avg residency       P-states (frequencies)
> C0 (cpu running)        (100.0%)        2.00 Ghz     0.8%
> C1                0.0ms ( 0.0%)         1.67 Ghz     0.0%
> C2                0.0ms ( 0.0%)         1333 Mhz     0.0%
> C3                0.0ms ( 0.0%)         1000 Mhz    99.2%
> 
> 2.6.23-rc8-mm2:
> 
> Cn                Avg residency       P-states (frequencies)
> C0 (cpu running)        ( 0.3%)         2.00 Ghz     0.0%
> C1                0.0ms ( 0.0%)         1.67 Ghz     0.0%
> C2                0.0ms ( 0.0%)         1333 Mhz     0.0%
> C3               31.5ms (99.7%)         1000 Mhz   100.0%
> 
> In addition, the ACPI power estimate reported about 25 watts for 23-mm1,
> but only 21 watts for -rc8-mm2, a significant regression.

well, thats because you burn less watts if you get into C3.  

> 
> I bisected this down to this set of patches:
> 
> pm-qos-infrastructure-and-interface.patch
> pm-qos-infrastructure-and-interface-fix.patch
> pm-qos-infrastructure-and-interface-vs-git-acpi.patch
> pm-qos-infrastructure-and-interface-vs-git-acpi-2.patch
> latencyc-use-qos-infrastructure.patch

yipes!  I'll look at it right away.  It looks like an integration issue
with CPU-IDLE patches (those control the C-state entry).  I'll get it
fixed up.

> 
> The patch says:
> 
>   To register the default pm_qos target for the specific parameter, the
>   process must open one of /dev/[cpu_dma_latency, network_latency,
>   network_throughput]
> 
>   As long as the device node is held open that process has a registered
>   requirement on the parameter.  The name of the requirement is
>   "process_<PID>" derived from the current->pid from within the open system
>   call.
> 
> I shouldn't have to have a process open a /dev/file, write a number, and then
> stay around forever so the file doesn't close in order to get the same behavior
> I was getting by default before.  What needs to happen to get this to not
> be a behavior regression/change?
>
you won't have such a process (at least I highly doubt you do)  I need
to fix this.  Thanks for taking the time to bisect it and reporting it to me!

--mgross
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