linux-kernel - Re: [RFC PATCH] kernel: sched: Provide a pointer to the valid CPU mask

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Message-ID: <alpine.DEB.2.20.1704061248320.1716@nanos>
Date:   Thu, 6 Apr 2017 12:58:27 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...e.hu>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>
Subject: Re: [RFC PATCH] kernel: sched: Provide a pointer to the valid CPU
 mask

On Thu, 6 Apr 2017, Peter Zijlstra wrote:
> On Thu, Apr 06, 2017 at 11:25:42AM +0200, Sebastian Andrzej Siewior wrote:
> > > I.e. just have a counter and these two APIs:
> > > 
> > >         static inline void migrate_disable(void)
> > >         {
> > >                 current->migration_disabled++;
> > plus
> > 	if (current->migration_disabled == 1)
> > 		get_online_cpus()
> 
> You want that other hotplug fast hack crap you have in rt, no?

As I said in the other mail, we certainly want to make get_online_cpus()
fast independently of RT. That's on our todo list anyway.

> Because mainline will not get anything other than prempt_disable() for
> migrate_disable().

I'm not yet convinced of that. There is enough stuff which pointlessly
disables preemption and works around the restrictions caused by that or
plays games with cpus_allowed just to stay on a particular cpu.

I know you don't like it because it imposes restrictions on schedulability,
but disabling preemption for a long time or playing cpus_allowed games has
the same and worse effects.

Thanks,

	tglx