Re: [PATCH RT 7/8] sched: migrate_enable: Use select_fallback_rq()

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2019-07-27 00:56:37 [-0500], Scott Wood wrote:
> migrate_enable() currently open-codes a variant of select_fallback_rq().
> However, it does not have the "No more Mr. Nice Guy" fallback and thus
> it will pass an invalid CPU to the migration thread if cpus_mask only
> contains a CPU that is !active.
> 
> Signed-off-by: Scott Wood <swood@redhat.com>
> ---
> This scenario will be more likely after the next patch, since
> the migrate_disable_update check goes away.  However, it could happen
> anyway if cpus_mask was updated to a CPU other than the one we were
> pinned to, and that CPU subsequently became inactive.

I'm unclear about the problem / side effect this has (before and after
the change). It is possible (before and after that change) that a CPU is
selected which is invalid / goes offline after the "preempt_enable()"
statement and before stop_one_cpu() does its job, correct?

> ---
>  kernel/sched/core.c | 25 ++++++++++---------------
>  1 file changed, 10 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index eb27a9bf70d7..3a2d8251a30c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7368,6 +7368,7 @@ void migrate_enable(void)
>  	if (p->migrate_disable_update) {
>  		struct rq *rq;
>  		struct rq_flags rf;
> +		int cpu = task_cpu(p);
>  
>  		rq = task_rq_lock(p, &rf);
>  		update_rq_clock(rq);
> @@ -7377,21 +7378,15 @@ void migrate_enable(void)
>  
>  		p->migrate_disable_update = 0;
>  
> -		WARN_ON(smp_processor_id() != task_cpu(p));
> -		if (!cpumask_test_cpu(task_cpu(p), &p->cpus_mask)) {
> -			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_mask);
> -			arg.task = p;
> -			arg.dest_cpu = dest_cpu;
> +		WARN_ON(smp_processor_id() != cpu);
> +		if (!cpumask_test_cpu(cpu, &p->cpus_mask)) {
> +			struct migration_arg arg = { p };
> +			struct rq_flags rf;
> +
> +			rq = task_rq_lock(p, &rf);
> +			update_rq_clock(rq);
> +			arg.dest_cpu = select_fallback_rq(cpu, p);
> +			task_rq_unlock(rq, p, &rf);
>  
>  			unpin_current_cpu();
>  			preempt_lazy_enable();

Sebastian