linux-kernel - Re: [PATCH RT 15/25] sched: migrate_enable: Use select_fallback

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Date:   Mon, 24 Feb 2020 17:05:29 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Tom Zanussi <zanussi@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        John Kacur <jkacur@...hat.com>, Daniel Wagner <wagi@...om.org>
Subject: Re: [PATCH RT 15/25] sched: migrate_enable: Use select_fallback_rq()

On 2020-02-24 09:31:06 [-0600], Tom Zanussi wrote:
> On Mon, 2020-02-24 at 10:43 +0100, Sebastian Andrzej Siewior wrote:
> > On 2020-02-21 15:24:43 [-0600], zanussi@...nel.org wrote:
> > > From: Scott Wood <swood@...hat.com>
> > > 
> > > v4.14.170-rt75-rc1 stable review patch.
> > > If anyone has any objections, please let me know.
> > 
> > This creates bug which is stuffed later via
> > 	sched: migrate_enable: Busy loop until the migration request is
> > completed
> > 
> > So if apply this, please take the bug fix, too. This is Stevens queue
> > for reference:
> > > [PATCH RT 22/30] sched: migrate_enable: Use select_fallback_rq()
> > 
> > ^^ bug introduced
> > 
> 
> Hmm, it seemed from the comment on the 4.19 series that it was '24/32
> sched: migrate_enable: Use stop_one_cpu_nowait()' that required 'sched:
> migrate_enable: Busy loop until the migration request is
> completed' as a bug fix.
> 
>   https://lore.kernel.org/linux-rt-users/20200122083130.kuu3yppckhyjrr4u@linutronix.de/#t
> 
> I didn't take the stop_one_cpu_nowait() one, so didn't take the busy
> loop one either.

Ach, it was the different WARN_ON() then. So this might not introduce
any bug then. *Might*. 
Steven backported the whole pile and you took just this one patch. The
whole set was tested in devel and uncovered a problem which was fixed
later. Taking only a part *may* expose other problems it *may* be fine.

Steven, any opinion on your side?

> Thanks,
> 
> Tom

Sebastian