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Date: Thu, 6 Apr 2017 12:57:36 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        linux-kernel@vger.kernel.org, Mike Galbraith <efault@gmx.de>,
        Ingo Molnar <mingo@elte.hu>, "Rafael J . Wysocki" <rjw@rjwysocki.net>
Subject: Re: [RFC PATCH] kernel: sched: Provide a pointer to the valid CPU
 mask
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On Thu, Apr 06, 2017 at 12:47:21PM +0200, Thomas Gleixner wrote:
> On Thu, 6 Apr 2017, Peter Zijlstra wrote:

> > IIRC MIPS has a case where only 1 in N cores has an FPU. And once a task
> > uses FPU, it gets affined to the core that has one or something like
> > that.
> >
> > Of course, nothing then stops someone else breaking that affinity. But I
> > suspect it will simply fault on the next FPU instruction and 'reset' the
> > mask or something. I've no clue and no real desire to know.
> 
> It does nasty games with it's own storage of p->thread.user_cpus_allowed
> and a fully seperate implementation of sys_sched_set|getaffinity.
> 
> Plus a magic trap handler which forces the thread to a CPU with FPU when
> the user_cpus_allowed mask intersects with the cpus_with_fpu_mask...
> 
> Magic crap, which could all be replaced by a simple function in the
> scheduler which allows to push a task to a FPU CPU and then disable
> migration.

If its even halfway coherent, I'd much rather let it stay where it it.

I really want to limit migrate_disable() to PREEMPT_RT=y where its used
to preserve spinlock semantics and not allow random other
migrate_disable() usage in the kernel.

Also note, that per the above, it can actually migrate to any core that
has an FPU on, so its not a good match for migrate_disable() in any
case.