Re: [PATCH RFC tip/core/rcu 39/41] rcu: Wait at least a jiffy beforedeclaring a CPU to be offline

From: Paul E. McKenney
Date: Thu Feb 02 2012 - 13:28:04 EST

On Wed, Feb 01, 2012 at 10:12:28PM -0800, Josh Triplett wrote:
> 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?

It really needs to have gone with the addition of of the fqs_ module
parameters to rcutorture.c, but that was some years back. Failing that,
it needs to have gone with the addition of RCU_FAST_NO_HZ, but that
was more than a year ago as well.

But yes, merging with #38 makes sense to me!

The still-ongoing top-to-bottom review of RCU is still yielding results,
and this is one of them. Working through the hotplug code has been
time consuming, buteducational. ;-)

Thanx, Paul

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