linux-kernel - Re: tip: demise of tsk_cpus_allowed() and tsk_nr_cpus

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Date:   Mon, 6 Feb 2017 14:32:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...e.hu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: tip: demise of tsk_cpus_allowed() and tsk_nr_cpus_allowed()

On Mon, Feb 06, 2017 at 01:29:28PM +0100, Ingo Molnar wrote:
> > +/* 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?

cpumasks are a pain, the above avoids allocating more of them.