On 07/11, Mathieu Desnoyers wrote:
>
> This patch adds the ability to protect critical sections from migration to
> another CPU without disabling preemption.
>
> This will be useful to minimize the amount of preemption disabling for the -rt
> patch. It will help leveraging improvements brought by the local_t types in
> asm/local.h (see Documentation/local_ops.txt). Note that the updates done to
> variables protected by migrate_disable must be either atomic or protected from
> concurrent updates done by other threads.
>
> Typical use:
>
> migrate_disable();
> local_inc(&__get_cpu_var(&my_local_t_var));
> migrate_enable();
>
> Which will increment the variable atomically wrt the local CPU.
Well, I am not a maintainer, but I personally think this patch is too complex.
Mathieu, please use "diff -p", it is very difficult to read it. I am not sure
I understand this patch correctly, I don't have a -git tree.
> static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
> {
> @@ -4829,13 +4888,19 @@
>
> double_rq_lock(rq_src, rq_dest);
> /* Already moved. */
> - if (task_cpu(p) != src_cpu)
> + if (task_cpu(p) != src_cpu) {
> + ret = 1;
> goto out;
> + }
This is a strange change. Why we return success when migration failed ?
OK, I guess this is a special hack for the modified migration_thread()...
> /* Affinity changed (again). */
> if (!cpu_isset(dest_cpu, p->cpus_allowed))
> goto out;
>
> on_rq = p->se.on_rq;
> +#ifdef CONFIG_PREEMPT
> + if (!on_rq && task_thread_info(p)->migrate_count)
> + goto out;
> +#endif
This means that move_task_off_dead_cpu() will spin until the task will be scheduled
on the dead CPU. Given that we hold tasklist_lock and irqs are disabled, this may
never happen.
(This patch adds a lot of #ifdef's, I think it could be simplified if you add
get_migrate_count() which is defined as 0 when !CONFIG_PREEMPT).
> @@ -4891,10 +4957,22 @@
> list_del_init(head->next);
>
> spin_unlock(&rq->lock);
> - __migrate_task(req->task, cpu, req->dest_cpu);
> + migrated = __migrate_task(req->task, cpu, req->dest_cpu);
> local_irq_enable();
> -
> - complete(&req->done);
> + if (!migrated) {
> + /*
> + * If the process has not been migrated, let it run
> + * until it reaches a migration_check() so it can
> + * wake us up.
> + */
> + spin_lock_irq(&rq->lock);
> + head = &rq->migration_queue;
> + list_add(&req->list, head);
> + set_tsk_thread_flag(req->task, TIF_NEED_MIGRATE);
> + spin_unlock_irq(&rq->lock);
> + wake_up_process(req->task);
> + } else
> + complete(&req->done);
I guess this is migration_thread(). The wake_up_process(req->task) looks strange,
why? It can't help if the task waits for the event/mutex.
And this is racy. What if check_migrate() is in progress, and the task has already
checked TIF_NEED_MIGRATE?
Hm. We re-add "req" to rq->migration_queue. This means that migration_thread() will
do a busy-wait loop. Not good and deadlockable, migration/X is SCHED_FIFO.
And what if __migrate_task() failed because ->cpus_allowed was changed in between?
Oleg.
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