linux-kernel - Re: tip: demise of tsk_cpus_allowed() and tsk_nr_cpus

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Date:   Thu, 9 Feb 2017 07:45:01 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...e.hu>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: tip: demise of tsk_cpus_allowed() and tsk_nr_cpus_allowed()


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Feb 08, 2017 at 11:20:19AM +0100, Thomas Gleixner wrote:
> > On Mon, 6 Feb 2017, Ingo Molnar wrote:
> > > * Peter Zijlstra <peterz@...radead.org> wrote:
> > > >
> > > > cpumasks are a pain, the above avoids allocating more of them.
> > 
> > Indeed.
> > 
> > > Yeah, so this could then be done by pointerifying ->cpus_allowed - more robust 
> > > than the wrappery,
> > 
> > You mean:
> > 
> > struct task_struct {
> >        cpumask_t	cpus_allowed;
> >        cpumask_t	*effective_cpus_allowed;
> > };

Yeah. I'd name it a bit differently and constify the pointer to get type 
safety and to make sure the mask is never modified through the pointer:

	struct task_struct {
		const cpumask_t		*cpus_ptr;
		cpumask_t		cpus_mask;
	};

( I'd drop the 'allowed' part, it's obvious enough what task->cpus_mask does, 
  right? )

and upstream would essentially just do:

	t->cpus_allowed_ptr = &t->cpus_allowed;

And -rt, when it wants to pin a task, would do:

	t->cpus_allowed_ptr = &cpumask_of(task_cpu(p));

The rules are:

 - Code that 'uses' ->cpus_allowed would use the pointer.

 - Code that 'modifies' ->cpus_allowed would use the direct mask.

The upstream advantages are:

 - The type separation of modifications from usage.

 - Removal of wrappery.

 - Maybe sometime in the future upstream would want to disable migration too ...

In fact -rt gains something too:

 - With this scheme we would AFAICS get slightly more optimal code on -rt.
   (Because there's no __migration_disabled() branching anymore.)

 - Plus all new code is automatically -rt ready - no need to maintain the wrappery 
   space. Much less code path forking.

So as I see it it's win-win for both upstream and for -rt!

> > and make the scheduler use effective_cpus_allowed instead of cpus_allowed? Or 
> > what do you have in mind?
> 
> That scheme is weird for nr_cpus_allowed. Not to mention that the
> pointer to the integer is larger than the integer itself.

So in the new scheme I don't think nr_cpus_allowed would have to be wrapped
at all: whenever the pointer (or mask) is changed in set_cpus_allowed_common() 
nr_cpus_allowed is recalculated as well - like today.

It should be self-maintaining. Am I missing something?

> I really prefer the current wrappers, they're trivial and consistent
> with one another.

I think it's ugly wrappery and we can do better! ;-)

But of course if I cannot suggest a better alternative then it stands.

Thanks,

	Ingo