linux-kernel - Re: Prevent RT-throttling for idle-injection threads

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Message-ID: <20230908095527.GA2475625@hu-atulpant-hyd.qualcomm.com>
Date:   Fri, 8 Sep 2023 15:25:27 +0530
From:   Atul Kumar Pant <quic_atulpant@...cinc.com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     <rafael@...nel.org>, <daniel.lezcano@...aro.org>,
        <mingo@...hat.com>, <peterz@...radead.org>,
        <juri.lelli@...hat.com>, <vincent.guittot@...aro.org>,
        <dietmar.eggemann@....com>, <bsegall@...gle.com>,
        <mgorman@...e.de>, <bristot@...hat.com>, <vschneid@...hat.com>,
        <linux-pm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <quic_ashayj@...cinc.com>, <quic_rgottimu@...cinc.com>,
        <quic_shashim@...cinc.com>
Subject: Re: Prevent RT-throttling for idle-injection threads

On Fri, Aug 11, 2023 at 11:17:19AM -0400, Steven Rostedt wrote:
> On Tue,  8 Aug 2023 16:55:23 +0530
> Atul Pant <quic_atulpant@...cinc.com> wrote:
> 
> > Hi all,
> > We are trying to implement a solution for thermal mitigation by using
> > idle injection on CPUs. However we face some limitations with the current
> > idle-inject framework. As per our need, we want to start injecting idle
> > cycles on a CPU for indefinite time (until the temperature/power of the
> > CPU falls below a threshold). This will help to keep the hot CPUs in the
> > sleep state until we see improvement in temperature/power. If we set the
> > idle duration to a large value or have an idle-injection ratio of 100%,
> > then the idle-inject RT thread suffers from RT throttling. This results
> > in the CPU exiting from the sleep state and consuming some power.
> > 
> > The above situation can be avoided, if we can prevent RT throttling on
> > the injected CPU. With the currently available sysctl parameters,
> > sched_rt_runtime_us and sched_rt_period_us, we can prevent RT throttling
> > by either setting sched_rt_runtime_us equal to sched_rt_period_us or,
> > setting sched_rt_runtime_us to -1. Since these parameters are system
> > wide, so it will affect the RT tasks on non idle-injected CPUs as well.
> > To overcome this, will it be feasible to have these two parameters on a
> > per CPU basis? This will allow to selectively disable RT throttling on
> > idle-injected CPUs.
> 
> I wonder if the deadline scheduler that Daniel is working on would help in this case?

		Are you referring to this thread regarding SCHED_DEADLINE server?
		https://lore.kernel.org/all/cover.1686239016.git.bristot@kernel.org/T/#u

> 
> -- Steve
>