linux-kernel - Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap

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Message-ID: <20200924124241.GK2628@hirez.programming.kicks-ass.net>
Date:   Thu, 24 Sep 2020 14:42:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Paul McKenney <paulmck@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
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        Vincent Guittot <vincent.guittot@...aro.org>,
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        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
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        Russell King <linux@...linux.org.uk>,
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        linux-sparc <sparclinux@...r.kernel.org>
Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of
 kmap_atomic & friends

On Thu, Sep 24, 2020 at 08:32:41AM -0400, Steven Rostedt wrote:
> Anyway, instead of blocking. What about having a counter of number of
> migrate disabled tasks per cpu, and when taking a migrate_disable(), and there's
> already another task with migrate_disabled() set, and the current task has
> an affinity greater than 1, it tries to migrate to another CPU?

That doesn't solve the problem. On wakeup we should already prefer an
idle CPU over one running a (RT) task, but you can always wake more
tasks than there's CPUs around and you'll _have_ to stack at some point.

The trick is how to unstack them correctly. We need to detect when a
migrate_disable() task _should_ start running again, and migrate away
whoever is in the way at that point.

It turns out, that getting selected for pull-balance is exactly that
condition, and clearly a migrate_disable() task cannot be pulled, but we
can use that signal to try and pull away the running task that's in the
way.