linux-kernel - Re: [PATCH RFC v4 2/3] sched: Avoid placing RT threads on cores handling long softirqs

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Message-ID: <Y1TxCTq5gxh4fIEd@li-4a3a4a4c-28e5-11b2-a85c-a8d192c6f089.ibm.com>
Date:   Sun, 23 Oct 2022 09:45:13 +0200
From:   Alexander Gordeev <agordeev@...ux.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     John Stultz <jstultz@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Connor O'Brien" <connoro@...gle.com>,
        John Dias <joaodias@...gle.com>, Rick Yiu <rickyiu@...gle.com>,
        John Kacur <jkacur@...hat.com>,
        Qais Yousef <qais.yousef@....com>,
        Chris Redpath <chris.redpath@....com>,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>, kernel-team@...roid.com,
        "J . Avila" <elavila@...gle.com>
Subject: Re: [PATCH RFC v4 2/3] sched: Avoid placing RT threads on cores
 handling long softirqs

On Sat, Oct 22, 2022 at 06:34:37PM +0000, Joel Fernandes wrote:
> > In my reading of your approach if you find a way to additionally
> > indicate long softirqs being handled by the remote ksoftirqd, it
> > would cover all obvious/not-corner cases.
> 
> How will that help? The long softirq executing inside ksoftirqd will disable
> preemption and prevent any RT task from executing.

Right. So the check to deem a remote CPU unfit would (logically) look like this:

(active | pending | ksoftirqd) & LONG_SOFTIRQ_MASK

> Did I miss something?

Or me :)

> thanks,
> 
>  - Joel
>