linux-kernel - Re: [PATCH] 4.4.86-rt99: fix sync breakage between nr_cpus_allowed and cpus

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Date:   Tue, 21 Nov 2017 09:33:52 -0500
From:   joe.korty@...current-rt.com
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] 4.4.86-rt99: fix sync breakage between nr_cpus_allowed
 and cpus_allowed

On Mon, Nov 20, 2017 at 11:57:51PM -0500, Steven Rostedt wrote:
> On Mon, 20 Nov 2017 23:02:07 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> 
> > Ideally, I would like to stay close to what upstream -rt does. Would
> > you be able to backport the 4.11-rt patch?
> > 
> > I'm currently working on releasing 4.9-rt and 4.4-rt with the latest
> > backports. I could easily add this one too.
> 
> Speaking of which. I just backported this patch to 4.4-rt. Is this what
> you are talking about?

Yes it is.
Thanks for finding that!
Joe

> >From 1dc89be37874bfc7bb4a0ea7c45492d7db39f62b Mon Sep 17 00:00:00 2001
> From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Date: Mon, 19 Jun 2017 09:55:47 +0200
> Subject: [PATCH] sched/migrate disable: handle updated task-mask mg-dis
>  section
> 
> If task's cpumask changes while in the task is in a migrate_disable()
> section then we don't react on it after a migrate_enable(). It matters
> however if current CPU is no longer part of the cpumask. We also miss
> the ->set_cpus_allowed() callback.
> This patch fixes it by setting task->migrate_disable_update once we this
> "delayed" hook.
> This bug was introduced while fixing unrelated issue in
> migrate_disable() in v4.4-rt3 (update_migrate_disable() got removed
> during that).
> 
> Cc: stable-rt@...r.kernel.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
>  include/linux/sched.h |    1 
>  kernel/sched/core.c   |   59 ++++++++++++++++++++++++++++++++++++++++++++------
>  2 files changed, 54 insertions(+), 6 deletions(-)
> 
> Index: stable-rt.git/include/linux/sched.h
> ===================================================================
> --- stable-rt.git.orig/include/linux/sched.h	2017-11-20 23:43:24.214077537 -0500
> +++ stable-rt.git/include/linux/sched.h	2017-11-20 23:43:24.154079278 -0500
> @@ -1438,6 +1438,7 @@ struct task_struct {
>  	unsigned int policy;
>  #ifdef CONFIG_PREEMPT_RT_FULL
>  	int migrate_disable;
> +	int migrate_disable_update;
>  # ifdef CONFIG_SCHED_DEBUG
>  	int migrate_disable_atomic;
>  # endif
> Index: stable-rt.git/kernel/sched/core.c
> ===================================================================
> --- stable-rt.git.orig/kernel/sched/core.c	2017-11-20 23:43:24.214077537 -0500
> +++ stable-rt.git/kernel/sched/core.c	2017-11-20 23:56:05.071687323 -0500
> @@ -1212,18 +1212,14 @@ void set_cpus_allowed_common(struct task
>  	p->nr_cpus_allowed = cpumask_weight(new_mask);
>  }
>  
> -void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
> +static void __do_set_cpus_allowed_tail(struct task_struct *p,
> +				       const struct cpumask *new_mask)
>  {
>  	struct rq *rq = task_rq(p);
>  	bool queued, running;
>  
>  	lockdep_assert_held(&p->pi_lock);
>  
> -	if (__migrate_disabled(p)) {
> -		cpumask_copy(&p->cpus_allowed, new_mask);
> -		return;
> -	}
> -
>  	queued = task_on_rq_queued(p);
>  	running = task_current(rq, p);
>  
> @@ -1246,6 +1242,20 @@ void do_set_cpus_allowed(struct task_str
>  		enqueue_task(rq, p, ENQUEUE_RESTORE);
>  }
>  
> +void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
> +{
> +	if (__migrate_disabled(p)) {
> +		lockdep_assert_held(&p->pi_lock);
> +
> +		cpumask_copy(&p->cpus_allowed, new_mask);
> +#if defined(CONFIG_PREEMPT_RT_FULL) && defined(CONFIG_SMP)
> +		p->migrate_disable_update = 1;
> +#endif
> +		return;
> +	}
> +	__do_set_cpus_allowed_tail(p, new_mask);
> +}
> +
>  static DEFINE_PER_CPU(struct cpumask, sched_cpumasks);
>  static DEFINE_MUTEX(sched_down_mutex);
>  static cpumask_t sched_down_cpumask;
> @@ -3231,6 +3241,43 @@ void migrate_enable(void)
>  	 */
>  	p->migrate_disable = 0;
>  
> +	if (p->migrate_disable_update) {
> +		unsigned long flags;
> +		struct rq *rq;
> +
> +		rq = task_rq_lock(p, &flags);
> +		update_rq_clock(rq);
> +
> +		__do_set_cpus_allowed_tail(p, &p->cpus_allowed);
> +		task_rq_unlock(rq, p, &flags);
> +
> +		p->migrate_disable_update = 0;
> +
> +		WARN_ON(smp_processor_id() != task_cpu(p));
> +		if (!cpumask_test_cpu(task_cpu(p), &p->cpus_allowed)) {
> +			const struct cpumask *cpu_valid_mask = cpu_active_mask;
> +			struct migration_arg arg;
> +			unsigned int dest_cpu;
> +
> +			if (p->flags & PF_KTHREAD) {
> +				/*
> +				 * Kernel threads are allowed on online && !active CPUs
> +				 */
> +				cpu_valid_mask = cpu_online_mask;
> +			}
> +			dest_cpu = cpumask_any_and(cpu_valid_mask, &p->cpus_allowed);
> +			arg.task = p;
> +			arg.dest_cpu = dest_cpu;
> +
> +			unpin_current_cpu();
> +			preempt_lazy_enable();
> +			preempt_enable();
> +			stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg);
> +			tlb_migrate_finish(p->mm);
> +			return;
> +		}
> +	}
> +
>  	unpin_current_cpu();
>  	preempt_enable();
>  	preempt_lazy_enable();