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

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Date:	Tue, 29 Jul 2014 10:12:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, 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 RFC tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

On Mon, Jul 28, 2014 at 03:56:12PM -0700, Paul E. McKenney wrote:
> +		/*
> +		 * Each pass through the following loop scans the list
> +		 * of holdout tasks, removing any that are no longer
> +		 * holdouts.  When the list is empty, we are done.
> +		 */
> +		while (!list_empty(&rcu_tasks_holdouts)) {
> +			schedule_timeout_interruptible(HZ / 10);
> +			flush_signals(current);
> +			rcu_read_lock();
> +			list_for_each_entry_rcu(t, &rcu_tasks_holdouts,
> +						rcu_tasks_holdout_list) {
> +				if (smp_load_acquire(&t->rcu_tasks_holdout))
> +					continue;
> +				list_del_init(&t->rcu_tasks_holdout_list);
> +				/* @@@ need to check for usermode on CPU. */
> +			}
> +			rcu_read_unlock();
> +		}

That's a potential CPU runtime sink.. imagine having to scan 100k tasks
10 times a second. Polling O(nr_tasks) is not good.
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