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

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Message-ID: <20140804074620.GH9918@twins.programming.kicks-ass.net>
Date:	Mon, 4 Aug 2014 09:46:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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 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.

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