linux-kernel - Re: [PATCH] cpuidle: Add "cpuidle.use_deepest" to bypass governor and allow HW to go deep

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Message-ID: <9371b55d-c4eb-c3e0-aadf-f624ec72a83f@tu-dresden.de>
Date:   Thu, 16 Nov 2017 17:11:28 +0100
From:   Thomas Ilsche <thomas.ilsche@...dresden.de>
To:     Len Brown <lenb@...nel.org>
Cc:     dsmythies@...us.net, linux-pm@...r.kernel.org,
        jacob.jun.pan@...ux.intel.com, linux-kernel@...r.kernel.org,
        Len Brown <len.brown@...el.com>
Subject: Re: [PATCH] cpuidle: Add "cpuidle.use_deepest" to bypass governor and
 allow HW to go deep

On 2017-11-09 08:38, Len Brown wrote:
> From: Len Brown <len.brown@...el.com>
> 
> While there are several mechanisms (cmdline, sysfs, PM_QOS) to limit
> cpuidle to shallow idle states, there is no simple mechanism
> to give the hardware permission to enter the deeptest state permitted by PM_QOS.
> 
> Here we create the "cpuidle.use_deepest" modparam to provide this capability.
> 
> "cpuidle.use_deepest=Y" can be set at boot-time, and
> /sys/module/cpuidle/use_deepest can be modified (with Y/N) at run-time.

This is a good option to have and can conveniently prevent idle power consumption
issues. But that wouldn't be a reasonable default, would it?
I still think there is an inherent need for a heuristic and a corresponding
mitigation to avoid staying in a sleep state too long.

Best,
Thomas