linux-kernel - Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE

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Message-ID: <20110302000703.GK2218@linux.vnet.ibm.com>
Date:	Tue, 1 Mar 2011 16:07:03 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq
 to kthread

On Tue, Mar 01, 2011 at 03:38:11PM +0100, Peter Zijlstra wrote:
> On Mon, 2011-02-28 at 16:13 -0800, Paul E. McKenney wrote:
> 
> > I am OK doing the sched_setscheduler_nocheck() in UP_PREPARE, correct?
> 
> Yeah, it should be perfectly fine to call that.

Cool!

> > Ah, there is the rub -- I am using wait_event(), so I need to wake up the
> > kthread once before anyone uses it (or at least concurrently with anyone
> > using it).  Which I can presumably do from the CPU_STARTING notifier.
> 
> Right, so your kthread is doing:
> 
> static int rcu_cpu_kthread()
> {
>   for (;;) {
>     wait_event_interruptible();
> 
>     /* do stuff */
> 
>   }
>   return 0;
> }
> 
> Which means that all folks wanting to make use of this already need to
> do a wakeup. So I don't see any reason to do that first wakeup from
> CPU_STARTING.

That is good to hear, because doing so seems to result in abject failure.

> wait_event() will only actually wait if the condition is false, in the
> start-up case above it will find the condition true and fall right
> through to do stuff.

So as long as it is OK to call sched_setscheduler_nocheck() before
the kthread is first awakened, we should be OK.

							Thanx, Paul
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