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

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On Tue, Jun 30, 2015 at 05:15:58PM -0700, Paul E. McKenney wrote:
> On Tue, Jun 30, 2015 at 04:46:33PM -0700, josh@joshtriplett.org wrote:
> > 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".
> 
> From what I can see, this one needs a good reason to go in, as opposed
> to a good reason to stay out.
> 
> > Then again, perhaps the more relevant concern would be why drivers use
> > expedited grace periods in the first place.
> 
> Networking uses expedited grace periods when RTNL is held to reduce
> contention on that lock.

Wait, what?  Why is anything using traditional (non-S) RCU while *any*
lock is held?

> Several other places have used it to minimize
> user-visible grace-period slowdown.  But there are probably places that
> would be better served doing something different.  That is after all
> the common case for most synchronization primitives.  ;-)

Sounds likely. :)

- Josh Triplett
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[PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging  normal ones "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-06-30 23:50 +0200
  [PATCH RFC tip/core/rcu 5/5] rcu: Limit expedited helping to every 10 ms or every 4th GP "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-06-30 23:50 +0200
    Re: [PATCH RFC tip/core/rcu 5/5] rcu: Limit expedited helping to  every 10 ms or every 4th GP Eric Dumazet <edumazet@google.com> - 2015-07-01 00:00 +0200
  [PATCH RFC tip/core/rcu 1/5] rcu: Prepare for expedited GP driving normal GP "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-06-30 23:50 +0200
  Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods  encouraging normal ones josh@joshtriplett.org - 2015-07-01 00:10 +0200
    Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging  normal ones Josh Triplett <josh@joshtriplett.org> - 2015-07-01 02:50 +0200
    Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods  encouraging normal ones josh@joshtriplett.org - 2015-07-01 23:30 +0200
      Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging  normal ones "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-07-01 23:50 +0200
    Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods  encouraging normal ones Mike Galbraith <umgwanakikbuti@gmail.com> - 2015-07-02 03:20 +0200
      Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods  encouraging normal ones josh@joshtriplett.org - 2015-07-02 03:40 +0200
        Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods  encouraging normal ones Mike Galbraith <umgwanakikbuti@gmail.com> - 2015-07-02 04:10 +0200

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