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

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Date:   Sat, 22 Oct 2022 18:34:37 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Alexander Gordeev <agordeev@...ux.ibm.com>
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 Thu, Oct 20, 2022 at 02:47:54PM +0200, Alexander Gordeev wrote:
> On Wed, Oct 19, 2022 at 03:09:15PM -0700, John Stultz wrote:
> 
> Hi John,
> 
> [...]
> 
> > So I'll go ahead and simplify the check to just the LONG_SOFTIRQ_MASK
> > & (active | pending softirqs) check. This should avoid the need to
> > pull the cpu_rq(cpu)->curr value and simplify things.
> 
> 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.

Did I miss something?

thanks,

 - Joel