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

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Message-ID: <20190508094758.GE2606@hirez.programming.kicks-ass.net>
Date:   Wed, 8 May 2019 11:47:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] sched: Provide a pointer to the valid CPU mask

On Tue, Apr 23, 2019 at 04:26:36PM +0200, Sebastian Andrzej Siewior 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.
> 
> I proposed this patch as a series earlier and it was shutdown due to the
> migrate_disable() bits. It has been said that migrate_disable() should
> only be used with RT and thus not introduced without it.
> I hereby propose only the mask CPU-bits.
> 
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

OK I suppose; I still think that accessor was the nicer solution, but
whatever. Queued for after the merge window.