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

[josh@joshtriplett.org]

On Tue, Jun 30, 2015 at 03:12:24PM -0700, Paul E. McKenney wrote:
> 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!  ;-)

Consider it a fairly weak concern against.  Increasing performance seems
like a good thing in general; I just don't relish the future "feels less
responsive" bug reports that take a long time to track down and turn out
to be "this completely unrelated driver was loaded and started using
expedited grace periods".

Then again, perhaps the more relevant concern would be why drivers use
expedited grace periods in the first place.

- Josh Triplett