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

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Message-ID: <000901d3654a$76411b80$62c35280$@net>
Date:   Fri, 24 Nov 2017 09:34:18 -0800
From:   "Doug Smythies" <dsmythies@...us.net>
To:     "'Thomas Ilsche'" <thomas.ilsche@...dresden.de>,
        "'Len Brown'" <lenb@...nel.org>
Cc:     <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 2017.11.16 08:11 Thomas Ilsche wrote:
> 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.

I think so, yes. The potential energy savings, depending on work flow, are significant.
For the idle state 0 case, I have some supporting data that I will send on
the other e-mail thread.

... Doug