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Date: Thu, 6 Apr 2017 11:46:35 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>, Mike Galbraith <efault@gmx.de>,
        Ingo Molnar <mingo@elte.hu>, "Rafael J . Wysocki" <rjw@rjwysocki.net>
Subject: Re: [RFC PATCH] kernel: sched: Provide a pointer to the valid CPU
 mask
Message-ID: <20170406094635.iwkaewsbiud35q6b@hirez.programming.kicks-ass.net>
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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? Because
mainline will not get anything other than prempt_disable() for
migrate_disable().

> >         }
> > 
> >         ...
> > 
> >         static inline void migrate_enable(void)
> >         {
> >                 current->migration_disabled--;
> >         }
> > 
> > ... and make sure the scheduler migration code plus the CPU hotplug code considers 
> > the counter.
> 
> So on the sched part I need go through all places where it looks at the
> mask and make sure it ignores the CPU switch decision. This could be
> doable.

Don't sprinkle that around. Use a wrapper like say: tsk_cpus_allowed()
to encapsulate that logic :-)