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

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Message-ID: <20170406094209.j7zjwxbzgqzh5pt6@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Apr 2017 11:42:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        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 11:35:45AM +0200, Peter Zijlstra wrote:
> On Thu, Apr 06, 2017 at 10:01:39AM +0200, Ingo Molnar wrote:
> > I.e. just have a counter and these two APIs:
> > 
> >         static inline void migrate_disable(void)
> >         {
> >                 current->migration_disabled++;
> >         }
> > 
> >         ...
> > 
> >         static inline void migrate_enable(void)
> >         {
> >                 current->migration_disabled--;
> >         }
> > 
> > ... and make sure the scheduler migration code plus the CPU hotplug code considers 
> > the counter.
> > 
> > Would this work, and would this be the simplest all around solution?
> 
> If you want to add expensive bits to hot paths like wakeups ... :/

Hmm, doesn't that get us back to exactly where we were: ?

tsk_cpus_allowed() 
{
	if (!tsk->migration_disabled)
		return tsk->cpus_allowed;
	else
		return cpumask_of(task_cpu(tsk));
}