Re: [patch 3/21] x86, bts: wait until traced task has beenscheduled out
From: Oleg Nesterov
Date: Tue Mar 31 2009 - 20:22:21 EST
On 03/31, Markus Metzger wrote:
>
> +static void wait_to_unschedule(struct task_struct *task)
> +{
> + unsigned long nvcsw;
> + unsigned long nivcsw;
> +
> + if (!task)
> + return;
> +
> + if (task == current)
> + return;
> +
> + nvcsw = task->nvcsw;
> + nivcsw = task->nivcsw;
> + for (;;) {
> + if (!task_is_running(task))
> + break;
> + /*
> + * The switch count is incremented before the actual
> + * context switch. We thus wait for two switches to be
> + * sure at least one completed.
> + */
> + if ((task->nvcsw - nvcsw) > 1)
> + break;
> + if ((task->nivcsw - nivcsw) > 1)
> + break;
> +
> + schedule();
schedule() is a nop here. We can wait unpredictably long...
Ingo, do have have any ideas to improve this helper?
Not that I really like it, but how about
int force_unschedule(struct task_struct *p)
{
struct rq *rq;
unsigned long flags;
int running;
rq = task_rq_lock(p, &flags);
running = task_running(rq, p);
task_rq_unlock(rq, &flags);
if (running)
wake_up_process(rq->migration_thread);
return running;
}
which should be used instead of task_is_running() ?
We can even do something like
void wait_to_unschedule(struct task_struct *task)
{
struct migration_req req;
rq = task_rq_lock(p, &task);
running = task_running(rq, p);
if (running) {
// make sure __migrate_task() will do nothing
req->dest_cpu = NR_CPUS + 1;
init_completion(&req->done);
list_add(&req->list, &rq->migration_queue);
}
task_rq_unlock(rq, &flags);
if (running) {
wake_up_process(rq->migration_thread);
wait_for_completion(&req.done);
}
}
This way we don't poll, and we need only one helper.
(Can't resist, this patch is not bisect friendly, without the next patches
wait_to_unschedule() is called under write_lock_irq, this is deadlockable).
But anyway, I think we can do this later.
Oleg.
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