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

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Date:   Thu, 6 Apr 2017 08:16:22 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.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

* Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> On 2017-04-05 09:39:43 [+0200], Ingo Molnar wrote:
> > 
> > So maybe we could add the following facility:
> > 
> > 	ptr = sched_migrate_to_cpu_save(cpu);
> > 
> > 	...
> > 
> > 	sched_migrate_to_cpu_restore(ptr);

BTW., and I'm sure this has come up before, but why doesn't migrate_disable() use 
a simple per task flag that the scheduler migration code takes into account?

It should be functionally equivalent to the current solution, and it appears to 
have a heck of a smaller cross section with the rest of the scheduler.

I.e.:

	static inline void migrate_disable(void)
	{
		current->migration_disabled++;
	}

	...

	static inline void migrate_enable(void)
	{
		current->migration_disabled--;
	}

or so? Then add this flag as a condition to can_migrate_task() et al.

While we generally dislike such flags as they wreck havoc with the scheduler if 
overused, the cpus_allowed based solution has the exact same effect so it's not 
like it's a step backwards - and it should also be much faster and less intrusive.

Am I missing some complication?

Thanks,

	Ingo