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

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Date:   Mon, 17 Oct 2022 22:32:16 -0700
From:   John Stultz <jstultz@...gle.com>
To:     Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     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, Oct 17, 2022 at 8:42 PM John Stultz <jstultz@...gle.com> wrote:
>  On Mon, Oct 17, 2022 at 5:40 AM Alexander Gordeev
> <agordeev@...ux.ibm.com> wrote:
> > On Mon, Oct 03, 2022 at 11:20:32PM +0000, John Stultz wrote:
> > > +               preempt_count() & SOFTIRQ_MASK);
> >
> > Could you please clarify this whole check in more detail?
> >
> > What is the point in checking if a remote CPU is handling
> > a "long" softirq while the local one is handling any softirq?
>
> Good question! This looks like an error from my rework of the earlier
> version of the patch.
> In v1 it was:
>    task_thread_info(curr)->preempt_count & SOFTIRQ_MASK));
> And it looks like I erroneously condensed that to preempt_count() &
> SOFTIRQ_MASK  treating curr (from the target cpu rq) as if it were
> current. :P
>
> I'll fix this. Thank you for catching it!

Ah, it's not so trivial to fix as I see part of my motivation for
condensing it was task_thread_info(curr)->preempt_count doesn't build
on x86 (which uses percpu values instead of threadinfo).

So I'll have to think a bit more about how to do this generically, and
if the conditionalization can be otherwise simplified.

Thanks again for pointing the issue out!
-john