linux-kernel - Re: [PATCH v2 tip/core/rcu 01/22] smpboot: Add common code for notification from dying CPU

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Message-ID: <20150317165621.GF24151@twins.programming.kicks-ass.net>
Date:	Tue, 17 Mar 2015 17:56:21 +0100
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,
	linux-api@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v2 tip/core/rcu 01/22] smpboot: Add common code for
 notification from dying CPU

On Tue, Mar 17, 2015 at 03:08:46PM +0100, Peter Zijlstra wrote:
> On Tue, Mar 17, 2015 at 04:36:48AM -0700, Paul E. McKenney wrote:
> > On Tue, Mar 17, 2015 at 09:18:07AM +0100, Peter Zijlstra wrote:
> > > On Mon, Mar 16, 2015 at 11:37:45AM -0700, Paul E. McKenney wrote:
> > > > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > > > 
> > > > RCU ignores offlined CPUs, so they cannot safely run RCU read-side code.
> > > > (They -can- use SRCU, but not RCU.)  This means that any use of RCU
> > > > during or after the call to arch_cpu_idle_dead().  Unfortunately,
> > > > commit 2ed53c0d6cc99 added a complete() call, which will contain RCU
> > > > read-side critical sections if there is a task waiting to be awakened.
> > > 
> > > Got a little more detail there?
> > 
> > Quite possibly.  But exactly what sort of detail are you looking for?
> 
> What exact RCU usage you ran into that was problematic. It seems to
> imply that calling complete() -- from a dead cpu -- which ends up in
> try_to_wake_up() was the problem?

Hmm, I'm thinking its select_task_rq_*(). And yes, 'fixing' this in the
wake-up path will penalize everybody for the benefit of the very rare
case someone is doing a hotplug.

So yeah, maybe this is the best solution.. Ulgy though :/
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