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

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Date:   Thu, 6 Apr 2017 11:46:35 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        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, 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 :-)