Re: [PATCH] sched: Fix migration_cpu_stop() return value

Lai Jiangshan
2014-06-04T11:06:21Z

On 06/04/2014 06:41 PM, Peter Zijlstra wrote:
> A while ago I did a similar patch for some debugging, but looking at it
> again today I realized we should probably fix this anyway.
>
> ---
> Subject: sched: Fix migration_cpu_stop() return value
>
> There are a number of migration_cpu_stop() users; and some actually care
> about the success of the migration. So report this.
>
> In particular migrate_task_to() as used from task_numa_migrate()
> actually tests this return value.
>
> Also change set_cpus_allowed_ptr() to propagate this return value, since
> it already returns other errors.
>
> Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Signed-off-by: Peter Zijlstra <peterz@infradead.org>
> ---
> kernel/sched/core.c | 15 +++++++++++----
> 1 file changed, 11 insertions(+), 4 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4633,11 +4633,13 @@ int set_cpus_allowed_ptr(struct task_str
> dest_cpu = cpumask_any_and(cpu_active_mask, new_mask);
> if (p->on_rq) {
> struct migration_arg arg = { p, dest_cpu };
> +
> /* Need help from migration thread: drop lock and wait. */
> task_rq_unlock(rq, p, &flags);
> - stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
> + ret = stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
> tlb_migrate_finish(p->mm);
> - return 0;
> +
> + return ret;
> }
> out:
> task_rq_unlock(rq, p, &flags);
> @@ -4747,19 +4749,24 @@ void sched_setnuma(struct task_struct *p
> * migration_cpu_stop - this will be executed by a highprio stopper thread
> * and performs thread migration by bumping thread off CPU then
> * 'pushing' onto another runqueue.
> + *
> + * Returns 0 on success, -EAGAIN on failure to migrate.
> */
> static int migration_cpu_stop(void *data)
> {
> struct migration_arg *arg = data;
> + int ret = 0;
>
> /*
> * The original target cpu might have gone down and we might
> * be on another cpu but it doesn't matter.
> */
> local_irq_disable();
> - __migrate_task(arg->task, raw_smp_processor_id(), arg->dest_cpu);
> + if (!__migrate_task(arg->task, raw_smp_processor_id(), arg->dest_cpu))
> + ret = -EAGAIN;

Current __migrate_task() returns 0 in these cases:
!cpu_active(dest_cpu):
hotplug subsystem's responsibility to migrate it

!cpumask_test_cpu(dest_cpu, tsk_cpus_allowed(p)):
newer set_cpus_allowed_ptr()'s responsibility

In these cases, current set_cpus_allowed_ptr() doesn't have the responsibility,
I don't think -EAGAIN is proper here since "-EAGAIN" asks the caller to do it again.

> local_irq_enable();
> - return 0;
> +
> + return ret;
> }
>
> #ifdef CONFIG_HOTPLUG_CPU