Re: [PATCH v2 tip/core/rcu 01/10] rcu: Add call_rcu_tasks()

[Lai Jiangshan <laijs@cn.fujitsu.com>]

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.