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Date: Mon, 6 Feb 2017 13:29:28 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Mike Galbraith <efault@gmx.de>
Cc: Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: tip: demise of tsk_cpus_allowed() and tsk_nr_cpus_allowed()
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* Mike Galbraith <efault@gmx.de> wrote:

> On Mon, 2017-02-06 at 11:31 +0100, Ingo Molnar wrote:
> > * Mike Galbraith <efault@gmx.de> wrote:
> > 
> > > Hi Ingo,
> > > 
> > > Doing my ~daily tip merge of -rt, I couldn't help noticing $subject, as
> > > they grow more functionality in -rt, which is allegedly slowly but
> > > surely headed toward merge.  I don't suppose they could be left intact?
> > >  I can easily restore them in my local tree, but it seems a bit of a
> > > shame to whack these integration friendly bits.
> > 
> > Oh, I missed that. How is tsk_cpus_allowed() wrapped in -rt right now?
> 
> RT extends them to reflect whether migration is disabled or not.
> 
> +/* Future-safe accessor for struct task_struct's cpus_allowed. */
> +static inline const struct cpumask *tsk_cpus_allowed(struct task_struct *p)
> +{
> +       if (__migrate_disabled(p))
> +               return cpumask_of(task_cpu(p));
> +
> +       return &p->cpus_allowed;
> +}
> +
> +static inline int tsk_nr_cpus_allowed(struct task_struct *p)
> +{
> +       if (__migrate_disabled(p))
> +               return 1;
> +       return p->nr_cpus_allowed;
> +}

So ... I think the cleaner approach in -rt would be to introduce 
->cpus_allowed_saved, and when disabling/enabling migration then saving the 
current mask there and changing ->cpus_allowed - and then restoring it when 
re-enabling migration.

This means ->cpus_allowed could be used by the scheduler directly, no wrappery 
would be required, AFAICS.

( Some extra care would be required in places that change ->cpus_allowed because 
  they'd now have to be aware of ->cpus_allowed_saved. )

Am I missing something?

	Ingo