linux-kernel - Re: [patch V4 4/8] sched: Make migrate

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Message-ID: <20201119091702.59aac061@gandalf.local.home>
Date:   Thu, 19 Nov 2020 09:17:02 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch V4 4/8] sched: Make migrate_disable/enable() independent
 of RT

On Thu, 19 Nov 2020 12:14:13 +0000
Mel Gorman <mgorman@...e.de> wrote:

> * Usage of migrate_disable is heavily discouraged as it is extremely
> * hazardous on PREEMPT_RT kernels and any usage needs to be heavily

I don't believe it's just PREEMPT_RT. It's RT tasks that are concerned,
especially when you are dealing with SCHED_DEADLINE.

PREEMPT_RT just allows better determinism for RT tasks, but the issue with
migrate_disable is not limited to just PREEMPT_RT.

-- Steve


> * justified. Before even thinking about using this, read
> * "Migrate-Disable and why it is undesired" in
> * include/linux/preempt.h and include both a comment and document
> * in the changelog why the use case is an exception.