Re: [PATCH RFC tip/core/rcu 39/41] rcu: Wait at least a jiffy beforedeclaring a CPU to be offline
From: Josh Triplett
Date: Thu Feb 02 2012 - 01:12:45 EST
On Wed, Feb 01, 2012 at 11:41:57AM -0800, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@xxxxxxxxxx>
> 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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