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

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Message-ID: <20230811111719.17f9965a@gandalf.local.home>
Date:   Fri, 11 Aug 2023 11:17:19 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Atul Pant <quic_atulpant@...cinc.com>
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 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?

-- Steve