srcu: Parallelize callback handling

Peter Zijlstra proposed using SRCU to reduce mmap_sem contention [1,2],
however, there are workloads that could result in a high volume of
concurrent invocations of call_srcu(), which with current SRCU would
result in excessive lock contention on the srcu_struct structure's
->queue_lock, which protects SRCU's callback lists.  This commit therefore
moves SRCU to per-CPU callback lists, thus greatly reducing contention.

Because a given SRCU instance no longer has a single centralized callback
list, starting grace periods and invoking callbacks are both more complex
than in the single-list Classic SRCU implementation.  Starting grace
periods and handling callbacks are now handled using an srcu_node tree
that is in some ways similar to the rcu_node trees used by RCU-bh,
RCU-preempt, and RCU-sched (for example, the srcu_node tree shape is
controlled by exactly the same Kconfig options and boot parameters that
control the shape of the rcu_node tree).

In addition, the old per-CPU srcu_array structure is now named srcu_data
and contains an rcu_segcblist structure named ->srcu_cblist for its
callbacks (and a spinlock to protect this).  The srcu_struct gets
an srcu_gp_seq that is used to associate callback segments with the
corresponding completion-time grace-period number.  These completion-time
grace-period numbers are propagated up the srcu_node tree so that the
grace-period workqueue handler can determine whether additional grace
periods are needed on the one hand and where to look for callbacks that
are ready to be invoked.

The srcu_barrier() function must now wait on all instances of the per-CPU
->srcu_cblist.  Because each ->srcu_cblist is protected by ->lock,
srcu_barrier() can remotely add the needed callbacks.  In theory,
it could also remotely start grace periods, but in practice doing so
is complex and racy.  And interestingly enough, it is never necessary
for srcu_barrier() to start a grace period because srcu_barrier() only
enqueues a callback when a callback is already present--and it turns out
that a grace period has to have already been started for this pre-existing
callback.  Furthermore, it is only the callback that srcu_barrier()
needs to wait on, not any particular grace period.  Therefore, a new
rcu_segcblist_entrain() function enqueues the srcu_barrier() function's
callback into the same segment occupied by the last pre-existing callback
in the list.  The special case where all the pre-existing callbacks are
on a different list (because they are in the process of being invoked)
is handled by enqueuing srcu_barrier()'s callback into the RCU_DONE_TAIL
segment, relying on the done-callbacks check that takes place after all
callbacks are inovked.

Note that the readers use the same algorithm as before.  Note that there
is a separate srcu_idx that tells the readers what counter to increment.
This unfortunately cannot be combined with srcu_gp_seq because they
need to be incremented at different times.

This commit introduces some ugly #ifdefs in rcutorture.  These will go
away when I feel good enough about Tree SRCU to ditch Classic SRCU.

Some crude performance comparisons, courtesy of a quickly hacked rcuperf
asynchronous-grace-period capability:

			Callback Queuing Overhead
			-------------------------
	# CPUS		Classic SRCU	Tree SRCU
	------          ------------    ---------
	     2              0.349 us     0.342 us
	    16             31.66  us     0.4   us
	    41             ---------     0.417 us

The times are the 90th percentiles, a statistic that was chosen to reject
the overheads of the occasional srcu_barrier() call needed to avoid OOMing
the test machine.  The rcuperf test hangs when running Classic SRCU at 41
CPUs, hence the line of dashes.  Despite the hacks to both the rcuperf code
and that statistics, this is a convincing demonstration of Tree SRCU's
performance and scalability advantages.

[1] https://lwn.net/Articles/309030/
[2] https://patchwork.kernel.org/patch/5108281/

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Fix initialization if synchronize_srcu_expedited() called first. ]

1 files changed, 38 insertions, 4 deletions

diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
index 74b1e7243955..ced8f313fd05 100644
--- a/include/linux/rcu_segcblist.h
+++ b/include/linux/rcu_segcblist.h
@@ -402,6 +402,37 @@ static inline void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
 }
 
 /*
+ * Entrain the specified callback onto the specified rcu_segcblist at
+ * the end of the last non-empty segment.  If the entire rcu_segcblist
+ * is empty, make no change, but return false.
+ *
+ * This is intended for use by rcu_barrier()-like primitives, -not-
+ * for normal grace-period use.  IMPORTANT:  The callback you enqueue
+ * will wait for all prior callbacks, NOT necessarily for a grace
+ * period.  You have been warned.
+ */
+static inline bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,
+					 struct rcu_head *rhp, bool lazy)
+{
+	int i;
+
+	if (rcu_segcblist_n_cbs(rsclp) == 0)
+		return false;
+	WRITE_ONCE(rsclp->len, rsclp->len + 1);
+	if (lazy)
+		rsclp->len_lazy++;
+	smp_mb(); /* Ensure counts are updated before callback is entrained. */
+	rhp->next = NULL;
+	for (i = RCU_NEXT_TAIL; i > RCU_DONE_TAIL; i--)
+		if (rsclp->tails[i] != rsclp->tails[i - 1])
+			break;
+	*rsclp->tails[i] = rhp;
+	for (; i <= RCU_NEXT_TAIL; i++)
+		rsclp->tails[i] = &rhp->next;
+	return true;
+}
+
+/*
  * Extract only the counts from the specified rcu_segcblist structure,
  * and place them in the specified rcu_cblist structure.  This function
  * supports both callback orphaning and invocation, hence the separation
@@ -537,7 +568,8 @@ static inline void rcu_segcblist_advance(struct rcu_segcblist *rsclp,
 	int i, j;
 
 	WARN_ON_ONCE(!rcu_segcblist_is_enabled(rsclp));
-	WARN_ON_ONCE(rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL));
+	if (rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL))
+		return;
 
 	/*
 	 * Find all callbacks whose ->gp_seq numbers indicate that they
@@ -582,8 +614,9 @@ static inline void rcu_segcblist_advance(struct rcu_segcblist *rsclp,
  * them to complete at the end of the earlier grace period.
  *
  * This function operates on an rcu_segcblist structure, and also the
- * grace-period sequence number at which new callbacks would become
- * ready to invoke.
+ * grace-period sequence number seq at which new callbacks would become
+ * ready to invoke.  Returns true if there are callbacks that won't be
+ * ready to invoke until seq, false otherwise.
  */
 static inline bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp,
 					    unsigned long seq)
@@ -591,7 +624,8 @@ static inline bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp,
 	int i;
 
 	WARN_ON_ONCE(!rcu_segcblist_is_enabled(rsclp));
-	WARN_ON_ONCE(rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL));
+	if (rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL))
+		return false;
 
 	/*
 	 * Find the segment preceding the oldest segment of callbacks