linux-kernel - Re: [PATCH v2 tip/core/rcu 01/10] rcu: Add call_rcu

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Message-ID: <20140731162030.GY19379@twins.programming.kicks-ass.net>
Date:	Thu, 31 Jul 2014 18:20:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.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 v2 tip/core/rcu 01/10] rcu: Add call_rcu_tasks()

On Thu, Jul 31, 2014 at 09:09:24AM -0700, Paul E. McKenney wrote:
> 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.

So do we really need this to be a full blown RCU implementation? Could
we not live with something like a task_barrier() function that
guarantees the same as this rcu_task_synchronize() or somesuch?

So for rostedt's case he could patch out all trampolines, call
task_barrier() and then free them.

We can make task_barrier() force each cpu to resched once -- which, if
we do that after the task's have all scheduled once, could be the empty
set.

It also pushes the cost of this machinery into the caller of
task_barrier() and not in some random kthread (yet another one).

Now, if only task_work would work for kernel threads, then we could do
something far nicer...

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