linux-kernel - Re: [PATCH RFC tip/core/rcu 39/41] rcu: Wait at least a jiffy before declaring a CPU to be offline

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Date:	Wed, 1 Feb 2012 22:12:28 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 39/41] rcu: Wait at least a jiffy before
 declaring a CPU to be offline

On Wed, Feb 01, 2012 at 11:41:57AM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@...aro.org>
> 
> The force_quiescent_state() function uses a state machine to help
> force grace periods to completion.  One of its responsibilities is to
> detect offline CPUs, and to report quiescent states on their behalf.
> However, the CPU hotplug process is not atomic, in fact, there is
> significant uncertainty as to exactly when a given CPU came online or
> went offline.  For example, once a CPU has marked itself offline and
> executed the CPU_DYING notifiers, it continues executing, entering
> the scheduler and perhaps also the idle loop.
> 
> In the old days, force_quiescent_state() was guaranteed to wait for
> several jiffies before declaring a given CPU offline.  This is no
> longer the case, due to some of the more aggressive rcutorture tests
> and the CONFIG_RCU_FAST_NO_HZ idle-entry code.  Therefore, this commit
> makes force_quiescent_state() explicitly wait for at least a jiffy
> before declaring a CPU to be offline.

This commit seems to implement behavior documented as working in patch
38.  Shouldn't those bits go together?

- Josh Triplett
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