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Date:   Wed, 5 Apr 2017 09:39:43 +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:

> In commit 4b53a3412d66 ("sched/core: Remove the tsk_nr_cpus_allowed()
> wrapper") the tsk_nr_cpus_allowed() wrapper was removed. There was not
> much difference in !RT but in RT we used this to implement
> migrate_disable(). Within a migrate_disable() section the CPU mask is
> restricted to single CPU while the "normal" CPU mask remains untouched.
> 
> As an alternative implementation Ingo suggested to use
> 	struct task_struct {
> 		const cpumask_t		*cpus_ptr;
> 		cpumask_t		cpus_mask;
>         };
> with
> 	t->cpus_allowed_ptr = &t->cpus_allowed;
> 
> In -RT we then can switch the cpus_ptr to
> 	t->cpus_allowed_ptr = &cpumask_of(task_cpu(p));
> 
> in a migration disabled region. The rules are simple:
> - Code that 'uses' ->cpus_allowed would use the pointer.
> - Code that 'modifies' ->cpus_allowed would use the direct mask.
> 
> While converting the existing users I tried to stick with the rules
> above however… well mostly CPUFREQ tries to temporary switch the CPU
> mask to do something on a certain CPU and then switches the mask back it
> its original value. So in theory `cpus_ptr' could or should be used.
> However if this is invoked in a migration disabled region (which is not
> the case because it would require something like preempt_disable() and
> set_cpus_allowed_ptr() might sleep so it can't be) then the "restore"
> part would restore the wrong mask. So it only looks strange and I go for
> the pointer…

So maybe we could add the following facility:

	ptr = sched_migrate_to_cpu_save(cpu);

	...

	sched_migrate_to_cpu_restore(ptr);

... and use it in the cpufreq code. Then -rt could simply define migrate_disable() 
to be:

	ptr = sched_migrate_to_cpu_save(raw_smp_processor_id());

and define migrate_enable() as:

	sched_migrate_to_cpu_restore(ptr);

... or such.

In the cpu == current_cpu case it would be super fast - otherwise it would migrate 
over to the target CPU first. Also note that this facility is strictly a special 
case for single-CPU masks and migrations - i.e. the constant pointer cpumask 
optimization would always apply.

Note that due to the use of the 'ptr' local variable the interface nests 
naturally, so this would be a legitimate use:
	
	ptr = sched_migrate_to_cpu_save(cpu);

	...
		migrate_disable();
		...
		migrate_enable();
	...

	sched_migrate_to_cpu_restore(ptr);

I.e. my proposal would be to essentially upstream the -rt migrate_disable() 
facility in a slightly more generic form that would fit the cpufreq usecase.

I bet a number of the current driver's mucking with cpumask would also fit this 
new API.

Does this make sense?

Thanks,

	Ingo

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