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

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Message-ID: <Y3OMQ+1D+AHbP72+@li-4a3a4a4c-28e5-11b2-a85c-a8d192c6f089.ibm.com>
Date:   Tue, 15 Nov 2022 13:55:31 +0100
From:   Alexander Gordeev <agordeev@...ux.ibm.com>
To:     John Stultz <jstultz@...gle.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        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 Mon, Nov 14, 2022 at 11:08:36PM -0800, John Stultz wrote:

Hi John,
...
> > Right. So the check to deem a remote CPU unfit would (logically) look like this:
> >
> > (active | pending | ksoftirqd) & LONG_SOFTIRQ_MASK
...
> As run_ksoftirqd() basically looks at the pending set and calls
> __do_softirq() which then moves the bits from the pending mask  to
> active mask while they are being run.
> 
> So (pending|active)&LONG_SOFTIRQ_MASK seems like it should be a
> sufficient check regardless of if the remote cpu is in softirq or
> ksoftirqd, no?

I did not realize run_ksoftirqd()->__do_softirq() covers it.
Sorry for the noise.

Thanks!

> thanks
> -john