On 08/01/2014 12:09 AM, Paul E. McKenney wrote:
>
>>> + /*
>>> + * There were callbacks, so we need to wait for an
>>> + * RCU-tasks grace period. Start off by scanning
>>> + * the task list for tasks that are not already
>>> + * voluntarily blocked. Mark these tasks and make
>>> + * a list of them in rcu_tasks_holdouts.
>>> + */
>>> + rcu_read_lock();
>>> + for_each_process_thread(g, t) {
>>> + if (t != current && ACCESS_ONCE(t->on_rq) &&
>>> + !is_idle_task(t)) {
>>
>> What happen when the trampoline is on the idle task?
>>
>> I think we need to use schedule_on_each_cpu() to replace one of
>> the synchronize_sched() in this function. (or other stuff which can
>> cause real schedule for *ALL* online CPUs).
>
> Well, that is one of the questions in the 0/10 cover letter. If it turns
> out to be necessary to worry about idle-task trampolines, it should be
> possible to avoid hammering all idle CPUs in the common case. Though maybe
> battery-powered devices won't need RCU-tasks.
>
trampolines on NO_HZ idle CPU can be arbitrary long, (example, SMI happens
inside the trampoline). So only the real schedule on idle CPU is reliable
to me.
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