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

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Message-ID: <20200924095138.5318d242@oasis.local.home>
Date:   Thu, 24 Sep 2020 09:51:38 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.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>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
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        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Russell King <linux@...linux.org.uk>,
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        <linux-snps-arc@...ts.infradead.org>,
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        "David S. Miller" <davem@...emloft.net>,
        linux-sparc <sparclinux@...r.kernel.org>
Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of
 kmap_atomic & friends

On Thu, 24 Sep 2020 14:42:41 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> 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.

Yes, understood.

> 
> 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.

Unless of course that running task is in a migrate disable section
itself ;-)

But I guess we will always have that SHC, and there will always be a
scenario that you can't balance properly. But hopefully in practice we
wont see that.

How to handle kmap_local(), will migrate_disable() be used only for
32bit or, for consistency, will it also apply to 64bit?

-- Steve