linux-kernel - Re: [RFC PATCH] kernel: sched: Provide a pointer to the valid CPU mask

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Message-ID: <20170406114115.khh3xltd6a7pmls5@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Apr 2017 13:41:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...e.hu>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>
Subject: Re: [RFC PATCH] kernel: sched: Provide a pointer to the valid CPU
 mask

On Thu, Apr 06, 2017 at 12:58:27PM +0200, Thomas Gleixner wrote:
> On Thu, 6 Apr 2017, Peter Zijlstra wrote:

> > Because mainline will not get anything other than prempt_disable() for
> > migrate_disable().
> 
> I'm not yet convinced of that. There is enough stuff which pointlessly
> disables preemption and works around the restrictions caused by that or
> plays games with cpus_allowed just to stay on a particular cpu.
> 
> I know you don't like it because it imposes restrictions on schedulability,
> but disabling preemption for a long time or playing cpus_allowed games has
> the same and worse effects.

There really isn't much code that plays games with cpus_allowed. And
code that disabled preemption for a long time can surely be fixed, we've
done so many a time.

A pure migrate_disable() (without wrapping lock) is a straight up
nightmare, you can end up with a gazillion runnable tasks stuck to one
CPU and all other CPUs with their thumbs up their arses waiting for
work.

The moment you stick migrate_disable() in a lock, you at least get PI to
bail you out of some of the pain (but by no means all of it).

Adding migrate_disable() to the kernel is really just moving pain
around, it doesn't solve anything much.