From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752023AbdBIIxH (ORCPT <rfc822;w@1wt.eu>);
        Thu, 9 Feb 2017 03:53:07 -0500
Received: from merlin.infradead.org ([205.233.59.134]:57078 "EHLO
        merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751950AbdBIIxG (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 9 Feb 2017 03:53:06 -0500
Date: Thu, 9 Feb 2017 09:51:34 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>, Mike Galbraith <efault@gmx.de>,
        Ingo Molnar <mingo@elte.hu>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: tip: demise of tsk_cpus_allowed() and tsk_nr_cpus_allowed()
Message-ID: <20170209085134.GY6500@twins.programming.kicks-ass.net>
References: <1486355037.10462.17.camel@gmx.de>
 <20170206103156.GA18908@gmail.com>
 <1486383511.10462.43.camel@gmx.de>
 <20170206122928.GB9404@gmail.com>
 <20170206133242.GK6515@twins.programming.kicks-ass.net>
 <20170206222313.GA6061@gmail.com>
 <alpine.DEB.2.20.1702081107450.3536@nanos>
 <20170208114016.GX6500@twins.programming.kicks-ass.net>
 <20170209064501.GA27072@gmail.com>
 <20170209065727.GA6902@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170209065727.GA6902@gmail.com>
User-Agent: Mutt/1.5.23.1 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, Feb 09, 2017 at 07:57:27AM +0100, Ingo Molnar wrote:
> And -rt would do something like this in migration_disable()/enable():
>  
> 	t->cpus_ptr = &cpumask_of(task_cpu(p));
> 	t->nr_cpus = 1;
> 
> 	...
> 
> 	t->cpus_ptr = &t->cpus_mask;
> 	t->nr_cpus = cpumask_weight(t->cpus_mask);
> 
> In addition to that we could cache the weight of the cpumask as an additional 
> optimization:
> 
> 	t->cpus_ptr = &t->cpus_mask;
> 	t->nr_cpus = t->cpus_mask_weight;
> 
> It all looks like a pretty natural construct to me. The migration_disabled() flag 
> spreads almost a hundred branches all across the scheduler.

Could work I suppose. But please then implement this instead of ripping
out the current thing, because taking out the accessors leaves RT in a
bind without recourse.