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

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Message-ID: <20170208114016.GX6500@twins.programming.kicks-ass.net>
Date:   Wed, 8 Feb 2017 12:40:16 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
        Ingo Molnar <mingo@...e.hu>,
        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 Wed, Feb 08, 2017 at 11:20:19AM +0100, Thomas Gleixner wrote:
> On Mon, 6 Feb 2017, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > cpumasks are a pain, the above avoids allocating more of them.
> 
> Indeed.
> 
> > Yeah, so this could then be done by pointerifying ->cpus_allowed - more robust 
> > than the wrappery,
> 
> You mean:
> 
> struct task_struct {
>        cpumask_t	cpus_allowed;
>        cpumask_t	*effective_cpus_allowed;
> };
> 
> and make the scheduler use effective_cpus_allowed instead of cpus_allowed?
> Or what do you have in mind?

That scheme is weird for nr_cpus_allowed. Not to mention that the
pointer to the integer is larger than the integer itself.

I really prefer the current wrappers, they're trivial and consistent
with one another.