Re: [PATCH 1/7] rcu/segcblist: Prevent useless GP start if no CBs to accelerate

["Paul E. McKenney" <paulmck@kernel.org>]

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!
> 
> > rcu_segcblist_accelerate() returns true if a GP is to be
> > started/requested and false if not. During tracing, I found that it is
> > asking that GPs be requested
> 
> s/GPs/unnecessary GPs/?  Plus "." at end of the sentence.
> 
> > The exact flow seems to be something like:
> > 1. Callbacks are queued on CPU A - into the NEXT list.
> > 2. softirq runs on CPU A, accelerate all CBs from NEXT->WAIT and request a GP X.
> > 3. GP thread wakes up on another CPU, starts the GP X and requests QS from CPU A.
> > 4. CPU A's softirq runs again presumably because of #3.
> 
> Yes, that is one reason RCU softirq might run again.
> 
> > 5. CPU A's softirq now does acceleration again, this time no CBs are
> >    accelerated since last attempt, but it still requests GP X+1 which
> >    could be useless.
> 
> I can't think of a case where this request helps.  How about: "but
> it still unnecessarily requests GP X+1"?
> 
> > The fix is, prevent the useless GP start if we detect no CBs are there
> > to accelerate.
> > 
> > With this, we have the following improvement in short runs of
> > rcutorture (5 seconds each):
> > +----+-------------------+-------------------+
> > | #  | 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 |
> > +----+---------+---------+---------+---------+
> 
> So the reductions in wakeups ranges from 5% to 40%, with almost a 20%
> overall reduction in wakeups across all the runs.  That should be of
> some use to someone.  ;-)
> 
> I do run rcutorture quite a bit, but is there a more real-world
> benchmark that could be tried?
> 
> > Cc: urezki@gmail.com
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

As discussed over IRC, I updated the patch as shown below.  Does that
work for you?

							Thanx, Paul

------------------------------------------------------------------------

commit ec037e1f438074eb16fd68a63d699fc419c9ba0c
Author: Joel Fernandes (Google) <joel@joelfernandes.org>
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@gmail.com
    [ paulmck: Rework commit log and comment. ]
    Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

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;
 
 	/*