linux-kernel - Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu

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Date:	Mon, 4 Aug 2014 04:50:44 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

On Mon, Aug 04, 2014 at 04:18:53PM +0800, Lai Jiangshan wrote:
> On 08/04/2014 03:46 PM, Peter Zijlstra wrote:
> > On Mon, Aug 04, 2014 at 09:28:45AM +0800, Lai Jiangshan wrote:
> >> On 08/01/2014 05:55 AM, Paul E. McKenney wrote:
> >>> +		rcu_read_lock();
> >>> +		for_each_process_thread(g, t) {
> >>> +			if (t != current && ACCESS_ONCE(t->on_rq) &&
> >>> +			    !is_idle_task(t)) {
> >>> +				get_task_struct(t);
> >>> +				t->rcu_tasks_nvcsw = ACCESS_ONCE(t->nvcsw);
> >>> +				ACCESS_ONCE(t->rcu_tasks_holdout) = 1;
> >>> +				list_add(&t->rcu_tasks_holdout_list,
> >>> +					 &rcu_tasks_holdouts);
> >>
> >> This loop will collect all the runnable tasks.  It is too much tasks.
> >> Is it possible to collect only on_cpu tasks or PREEMPT_ACTIVE tasks?
> >> It seems hard to achieve it.
> > 
> > Without taking the rq->lock you cannot do that race-free. And we're not
> > going to be taking rq->lock here.
> 
> It is because we can't fetch task->on_cpu and preempt_count atomically
> so that rq->lock is required.
> 
> 3 bleeding solutions:
> 
> 1) Allocate one bit in preempt_count to stand for not_on_cpu ( = !task->on_cpu)
> 2) allocate one bit in nvcsw to stand for on_scheduled (or not_on_scheduled, see next)
> 3) introduce task->on_scheduled whose semantics is between on_cpu and on_rq,
>    on_scheduled = scheduled on cpu or preempted, (not voluntary scheduled out)
> 
> But the scheduler doesn't need neither of such things.  So these is still no hope.

OK, I will bite...

What kinds of tasks are on a runqueue, but neither ->on_cpu nor
PREEMPT_ACTIVE?

							Thanx, Paul

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