Date: Tue, 29 Jul 2014 10:12:11 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com,
        oleg@redhat.com, bobby.prani@gmail.com
Subject: Re: [PATCH RFC tip/core/rcu 1/9] rcu: Add call_rcu_tasks()
Message-ID: <20140729081211.GP20603@laptop.programming.kicks-ass.net>
References: <20140728225556.GA19493@linux.vnet.ibm.com>
 <1406588180-21933-1-git-send-email-paulmck@linux.vnet.ibm.com>
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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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