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

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Message-ID: <20170406115058.pntrn7mgywt7j3va@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Apr 2017 13:50:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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 01:03:16PM +0200, Thomas Gleixner wrote:

> Fair enough, but I prefer to have the ability for a temporary restriction
> of the user space visible cpus allowed mask in the core code, which can be
> used for both migrate_disable() and things like that MIPS FPU stuff rather
> than all those home brewn hackeries which are prone to bitrot, security
> issues and subtle wreckage. cpus_allowed should be solely under sched core
> control and not accessible from anything outside.

So the big difference between what MIPS does and what migrate_disable()
needs here is the amount of storage.

For migrate_disable() we don't need a whole second cpumask bitmap. We
can simply use one of the static cpumask_of() things.

Given that cpumask bitmaps can be quite large, adding another one to
task_struct just for $very_rare_occasion, doesn't seem like a good
thing.