linux-kernel - RE: [PATCH] cpuidle: psd: add power sleep demotion prevention for fast I/O devices

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Message-ID: <SJ2PR11MB7670B2AEFF7C0DABE6856F258DA62@SJ2PR11MB7670.namprd11.prod.outlook.com>
Date: Wed, 26 Mar 2025 15:04:18 +0000
From: "King, Colin" <colin.king@...el.com>
To: Bart Van Assche <bvanassche@....org>, Christian Loehle
	<christian.loehle@....com>, Jens Axboe <axboe@...nel.dk>, "Rafael J. Wysocki"
	<rafael@...nel.org>, Daniel Lezcano <daniel.lezcano@...aro.org>,
	"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] cpuidle: psd: add power sleep demotion prevention for
 fast I/O devices

Hi,

> -----Original Message-----
> From: Bart Van Assche <bvanassche@....org>
> Sent: 23 March 2025 12:36
> To: King, Colin <colin.king@...el.com>; Christian Loehle
> <christian.loehle@....com>; Jens Axboe <axboe@...nel.dk>; Rafael J.
> Wysocki <rafael@...nel.org>; Daniel Lezcano <daniel.lezcano@...aro.org>;
> linux-block@...r.kernel.org; linux-pm@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Subject: Re: [PATCH] cpuidle: psd: add power sleep demotion prevention for
> fast I/O devices
> 
> On 3/17/25 3:03 AM, King, Colin wrote:
> > This code is optional, one can enable it or disable it via the config
> > option. Also, even when it is built-in one can disable it by writing 0 to the
> sysfs file
> >    /sys/devices/system/cpu/cpuidle/psd_cpu_lat_timeout_ms
> 
> I'm not sure we need even more configuration knobs in sysfs. 

It's useful for enabling / disabling the functionality, as well as some form of tuning for slower I/O devices, so I think it is justifiable.

> How are users
> expected to find this configuration option? How should they decide whether
> to enable or to disable it?

I can send a V2 with some documentation if that's required.

> 
> Please take a look at this proposal and let me know whether this would solve
> the issue that you are looking into: "[LSF/MM/BPF Topic] Energy- Efficient I/O"
> (https://lore.kernel.org/linux-block/ad1018b6-7c0b-4d70-
> b845-c869287d3cf3@....org/). The only disadvantage of this approach
> compared to the cpuidle patch is that it requires RPM (runtime power
> management) to be enabled. Maybe I should look into modifying the
> approach such that it does not rely on RPM.

I've had a look, the scope of my patch is a bit wider.  If my patch gets accepted I'm 
going to also look at putting the psd call into other devices (such as network devices) to 
also stop deep states while these devices are busy.  Since the code is very lightweight I
was hoping this was going to be relatively easy and simple to use in various devices in the future.

Colin

> 
> Thanks,
> 
> Bart.