Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging normal ones

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jun 30, 2015 at 03:00:15PM -0700, josh@joshtriplett.org wrote:
> On Tue, Jun 30, 2015 at 02:48:05PM -0700, Paul E. McKenney wrote:
> > Hello!
> > 
> > This series contains some highly experimental patches that allow normal
> > grace periods to take advantage of the work done by concurrent expedited
> > grace periods.  This can reduce the overhead incurred by normal grace
> > periods by eliminating the need for force-quiescent-state scans that
> > would otherwise have happened after the expedited grace period completed.
> > It is not clear whether this is a useful tradeoff.  Nevertheless, this
> > series contains the following patches:
> 
> While it makes sense to avoid unnecessarily delaying a normal grace
> period if the expedited machinery has provided the necessary delay, I'm
> also *deeply* concerned that this will create a new class of
> nondeterministic performance issues.  Something that uses RCU may
> perform badly due to grace period latency, but then suddenly start
> performing well because an unrelated task starts hammering expedited
> grace periods.  This seems particularly likely during boot, for
> instance, where RCU grace periods can be a significant component of boot
> time (when you're trying to boot to userspace in small fractions of a
> second).

I will take that as another vote against.  And for a reason that I had
not yet come up with, so good show!  ;-)

							Thanx, Paul