On Thu, Jun 18, 2020 at 04:09:34PM -0700, Paul E. McKenney wrote: > On Thu, Jun 18, 2020 at 03:11:19PM -0700, Paul E. McKenney wrote: > > On Thu, Jun 18, 2020 at 04:29:49PM -0400, Joel Fernandes (Google) wrote: > > > > First, this looks like a very nice optimization, thank you! Thanks! > > > Cc: urezki@xxxxxxxxx > > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > > As discussed over IRC, I updated the patch as shown below. Does that > work for you? Yes, that works for me. Thanks! - Joel > Thanx, Paul > > ------------------------------------------------------------------------ > > commit ec037e1f438074eb16fd68a63d699fc419c9ba0c > Author: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > Date: Thu Jun 18 16:29:49 2020 -0400 > > rcu/segcblist: Prevent useless GP start if no CBs to accelerate > > The rcu_segcblist_accelerate() function returns true iff it is necessary > to request another grace period. A tracing session showed that this > function unnecessarily requests grace periods. > > For exmaple, consider the following sequence of events: > 1. Callbacks are queued only on the NEXT segment of CPU A's callback list. > 2. CPU A runs RCU_SOFTIRQ, accelerating these callbacks from NEXT to WAIT. > 3. Thus rcu_segcblist_accelerate() returns true, requesting grace period N. > 4. RCU's grace-period kthread wakes up on CPU B and starts grace period N. > 4. CPU A notices the new grace period and invokes RCU_SOFTIRQ. > 5. CPU A's RCU_SOFTIRQ again invokes rcu_segcblist_accelerate(), but > there are no new callbacks. However, rcu_segcblist_accelerate() > nevertheless (uselessly) requests a new grace period N+1. > > This extra grace period results in additional lock contention and also > additional wakeups, all for no good reason. > > This commit therefore adds a check to rcu_segcblist_accelerate() that > prevents the return of true when there are no new callbacks. > > This change reduces the number of grace periods (GPs) and wakeups in each > of eleven five-second rcutorture runs as follows: > > +----+-------------------+-------------------+ > | # | Number of GPs | Number of Wakeups | > +====+=========+=========+=========+=========+ > | 1 | With | Without | With | Without | > +----+---------+---------+---------+---------+ > | 2 | 75 | 89 | 113 | 119 | > +----+---------+---------+---------+---------+ > | 3 | 62 | 91 | 105 | 123 | > +----+---------+---------+---------+---------+ > | 4 | 60 | 79 | 98 | 110 | > +----+---------+---------+---------+---------+ > | 5 | 63 | 79 | 99 | 112 | > +----+---------+---------+---------+---------+ > | 6 | 57 | 89 | 96 | 123 | > +----+---------+---------+---------+---------+ > | 7 | 64 | 85 | 97 | 118 | > +----+---------+---------+---------+---------+ > | 8 | 58 | 83 | 98 | 113 | > +----+---------+---------+---------+---------+ > | 9 | 57 | 77 | 89 | 104 | > +----+---------+---------+---------+---------+ > | 10 | 66 | 82 | 98 | 119 | > +----+---------+---------+---------+---------+ > | 11 | 52 | 82 | 83 | 117 | > +----+---------+---------+---------+---------+ > > The reduction in the number of wakeups ranges from 5% to 40%. > > Cc: urezki@xxxxxxxxx > [ paulmck: Rework commit log and comment. ] > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c > index 9a0f661..2d2a6b6b9 100644 > --- a/kernel/rcu/rcu_segcblist.c > +++ b/kernel/rcu/rcu_segcblist.c > @@ -475,8 +475,16 @@ bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq) > * Also advance to the oldest segment of callbacks whose > * ->gp_seq[] completion is at or after that passed in via "seq", > * skipping any empty segments. > + * > + * Note that segment "i" (and any lower-numbered segments > + * containing older callbacks) will be unaffected, and their > + * grace-period numbers remain unchanged. For example, if i == > + * WAIT_TAIL, then neither WAIT_TAIL nor DONE_TAIL will be touched. > + * Instead, the CBs in NEXT_TAIL will be merged with those in > + * NEXT_READY_TAIL and the grace-period number of NEXT_READY_TAIL > + * would be updated. NEXT_TAIL would then be empty. > */ > - if (++i >= RCU_NEXT_TAIL) > + if (rcu_segcblist_restempty(rsclp, i) || ++i >= RCU_NEXT_TAIL) > return false; > > /*
