Re: [PATCH RFC v2] rcu/segcblist: Add counters to segcblist datastructure

[Joel Fernandes <joel@joelfernandes.org>]

On Sun, Jul 19, 2020 at 12:06:28AM -0400, Joel Fernandes wrote:
> On Sat, Jul 18, 2020 at 11:55 PM Joel Fernandes (Google)
> <joel@joelfernandes.org> wrote:
> [...]
> >         /* If no callbacks moved, nothing more need be done. */
> > @@ -419,10 +494,9 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq)
> >          * callbacks.  The overall effect is to copy down the later pointers
> >          * into the gap that was created by the now-ready segments.
> >          */
> > -       for (j = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++, j++) {
> > -               if (rsclp->tails[j] == rsclp->tails[RCU_NEXT_TAIL])
> > -                       break;  /* No more callbacks. */
> > +       for (j = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL && j < RCU_NEXT_TAIL; i++, j++) {
> >                 WRITE_ONCE(rsclp->tails[j], rsclp->tails[i]);
> > +               rcu_segcblist_move_seglen(rsclp, i, j);
> >                 rsclp->gp_seq[j] = rsclp->gp_seq[i];
> >         }
> 
> Unfortunately I broke this code, _sigh_.  I need to reinstate the
> if (rsclp->tails[j] == rsclp->tails[RCU_NEXT_TAIL]) , I completely
> misunderstood that.

And hopefully, third time's a charm with various extra newlines removed:

---8<-----------------------

From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Subject: [PATCH v3] rcu/segcblist: Add counters to segcblist datastructure

Add counting of segment lengths of segmented callback list.

This will be useful for a number of things such as knowing how big the
ready-to-execute segment have gotten. The immediate benefit is ability
to trace how the callbacks in the segmented callback list change.

Tested by profusely reading traces when segcblist counts updated.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

v1->v3: minor nits.
---
 include/linux/rcu_segcblist.h |  2 +
 kernel/rcu/rcu_segcblist.c    | 77 +++++++++++++++++++++++++++++++++--
 2 files changed, 76 insertions(+), 3 deletions(-)

diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
index b36afe7b22c9..d462ae5e340a 100644
--- a/include/linux/rcu_segcblist.h
+++ b/include/linux/rcu_segcblist.h
@@ -69,8 +69,10 @@ struct rcu_segcblist {
 	unsigned long gp_seq[RCU_CBLIST_NSEGS];
 #ifdef CONFIG_RCU_NOCB_CPU
 	atomic_long_t len;
+	atomic_long_t seglen[RCU_CBLIST_NSEGS];
 #else
 	long len;
+	long seglen[RCU_CBLIST_NSEGS];
 #endif
 	u8 enabled;
 	u8 offloaded;
diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index 9a0f66133b4b..c5841efcd38e 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -88,6 +88,57 @@ static void rcu_segcblist_set_len(struct rcu_segcblist *rsclp, long v)
 #endif
 }
 
+/* Get the length of a segment of the rcu_segcblist structure. */
+static long rcu_segcblist_get_seglen(struct rcu_segcblist *rsclp, int seg)
+{
+#ifdef CONFIG_RCU_NOCB_CPU
+	return atomic_long_read(&rsclp->seglen[seg]);
+#else
+	return READ_ONCE(rsclp->seglen[seg]);
+#endif
+}
+
+/* Set the length of a segment of the rcu_segcblist structure. */
+static void rcu_segcblist_set_seglen(struct rcu_segcblist *rsclp, int seg, long v)
+{
+#ifdef CONFIG_RCU_NOCB_CPU
+	atomic_long_set(&rsclp->seglen[seg], v);
+#else
+	WRITE_ONCE(rsclp->seglen[seg], v);
+#endif
+}
+
+/* Return number of callbacks in a segment of the segmented callback list. */
+static void rcu_segcblist_add_seglen(struct rcu_segcblist *rsclp, int seg, long v)
+{
+#ifdef CONFIG_RCU_NOCB_CPU
+	smp_mb__before_atomic(); /* Up to the caller! */
+	atomic_long_add(v, &rsclp->seglen[seg]);
+	smp_mb__after_atomic(); /* Up to the caller! */
+#else
+	smp_mb(); /* Up to the caller! */
+	WRITE_ONCE(rsclp->seglen[seg], rsclp->seglen[seg] + v);
+	smp_mb(); /* Up to the caller! */
+#endif
+}
+
+/* Move from's segment length to to's segment. */
+static void rcu_segcblist_move_seglen(struct rcu_segcblist *rsclp, int from, int to)
+{
+	long len = rcu_segcblist_get_seglen(rsclp, from);
+
+	if (!len || from == to)
+		return;
+	rcu_segcblist_add_seglen(rsclp, to, len);
+	rcu_segcblist_set_seglen(rsclp, from, 0);
+}
+
+/* Increment segment's length. */
+static void rcu_segcblist_inc_seglen(struct rcu_segcblist *rsclp, int seg)
+{
+	rcu_segcblist_add_seglen(rsclp, seg, 1);
+}
+
 /*
  * Increase the numeric length of an rcu_segcblist structure by the
  * specified amount, which can be negative.  This can cause the ->len
@@ -149,8 +200,10 @@ void rcu_segcblist_init(struct rcu_segcblist *rsclp)
 	BUILD_BUG_ON(RCU_NEXT_TAIL + 1 != ARRAY_SIZE(rsclp->gp_seq));
 	BUILD_BUG_ON(ARRAY_SIZE(rsclp->tails) != ARRAY_SIZE(rsclp->gp_seq));
 	rsclp->head = NULL;
-	for (i = 0; i < RCU_CBLIST_NSEGS; i++)
+	for (i = 0; i < RCU_CBLIST_NSEGS; i++) {
 		rsclp->tails[i] = &rsclp->head;
+		rcu_segcblist_set_seglen(rsclp, i, 0);
+	}
 	rcu_segcblist_set_len(rsclp, 0);
 	rsclp->enabled = 1;
 }
@@ -245,6 +298,7 @@ void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
 			   struct rcu_head *rhp)
 {
 	rcu_segcblist_inc_len(rsclp);
+	rcu_segcblist_inc_seglen(rsclp, RCU_NEXT_TAIL);
 	smp_mb(); /* Ensure counts are updated before callback is enqueued. */
 	rhp->next = NULL;
 	WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp);
@@ -274,6 +328,7 @@ bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,
 	for (i = RCU_NEXT_TAIL; i > RCU_DONE_TAIL; i--)
 		if (rsclp->tails[i] != rsclp->tails[i - 1])
 			break;
+	rcu_segcblist_inc_seglen(rsclp, i);
 	WRITE_ONCE(*rsclp->tails[i], rhp);
 	for (; i <= RCU_NEXT_TAIL; i++)
 		WRITE_ONCE(rsclp->tails[i], &rhp->next);
@@ -307,6 +362,7 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
 
 	if (!rcu_segcblist_ready_cbs(rsclp))
 		return; /* Nothing to do. */
+	rclp->len = rcu_segcblist_get_seglen(rsclp, RCU_DONE_TAIL);
 	*rclp->tail = rsclp->head;
 	WRITE_ONCE(rsclp->head, *rsclp->tails[RCU_DONE_TAIL]);
 	WRITE_ONCE(*rsclp->tails[RCU_DONE_TAIL], NULL);
@@ -314,6 +370,7 @@ void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
 	for (i = RCU_CBLIST_NSEGS - 1; i >= RCU_DONE_TAIL; i--)
 		if (rsclp->tails[i] == rsclp->tails[RCU_DONE_TAIL])
 			WRITE_ONCE(rsclp->tails[i], &rsclp->head);
+	rcu_segcblist_set_seglen(rsclp, RCU_DONE_TAIL, 0);
 }
 
 /*
@@ -330,11 +387,16 @@ void rcu_segcblist_extract_pend_cbs(struct rcu_segcblist *rsclp,
 
 	if (!rcu_segcblist_pend_cbs(rsclp))
 		return; /* Nothing to do. */
+	rclp->len = rcu_segcblist_get_seglen(rsclp, RCU_WAIT_TAIL) +
+		    rcu_segcblist_get_seglen(rsclp, RCU_NEXT_READY_TAIL) +
+		    rcu_segcblist_get_seglen(rsclp, RCU_NEXT_TAIL);
 	*rclp->tail = *rsclp->tails[RCU_DONE_TAIL];
 	rclp->tail = rsclp->tails[RCU_NEXT_TAIL];
 	WRITE_ONCE(*rsclp->tails[RCU_DONE_TAIL], NULL);
-	for (i = RCU_DONE_TAIL + 1; i < RCU_CBLIST_NSEGS; i++)
+	for (i = RCU_DONE_TAIL + 1; i < RCU_CBLIST_NSEGS; i++) {
 		WRITE_ONCE(rsclp->tails[i], rsclp->tails[RCU_DONE_TAIL]);
+		rcu_segcblist_set_seglen(rsclp, i, 0);
+	}
 }
 
 /*
@@ -359,6 +421,7 @@ void rcu_segcblist_insert_done_cbs(struct rcu_segcblist *rsclp,
 
 	if (!rclp->head)
 		return; /* No callbacks to move. */
+	rcu_segcblist_add_seglen(rsclp, RCU_DONE_TAIL, rclp->len);
 	*rclp->tail = rsclp->head;
 	WRITE_ONCE(rsclp->head, rclp->head);
 	for (i = RCU_DONE_TAIL; i < RCU_CBLIST_NSEGS; i++)
@@ -379,6 +442,8 @@ void rcu_segcblist_insert_pend_cbs(struct rcu_segcblist *rsclp,
 {
 	if (!rclp->head)
 		return; /* Nothing to do. */
+
+	rcu_segcblist_add_seglen(rsclp, RCU_NEXT_TAIL, rclp->len);
 	WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rclp->head);
 	WRITE_ONCE(rsclp->tails[RCU_NEXT_TAIL], rclp->tail);
 }
@@ -403,6 +468,7 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq)
 		if (ULONG_CMP_LT(seq, rsclp->gp_seq[i]))
 			break;
 		WRITE_ONCE(rsclp->tails[RCU_DONE_TAIL], rsclp->tails[i]);
+		rcu_segcblist_move_seglen(rsclp, i, RCU_DONE_TAIL);
 	}
 
 	/* If no callbacks moved, nothing more need be done. */
@@ -423,6 +489,7 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq)
 		if (rsclp->tails[j] == rsclp->tails[RCU_NEXT_TAIL])
 			break;  /* No more callbacks. */
 		WRITE_ONCE(rsclp->tails[j], rsclp->tails[i]);
+		rcu_segcblist_move_seglen(rsclp, i, j);
 		rsclp->gp_seq[j] = rsclp->gp_seq[i];
 	}
 }
@@ -444,7 +511,7 @@ void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq)
  */
 bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq)
 {
-	int i;
+	int i, j;
 
 	WARN_ON_ONCE(!rcu_segcblist_is_enabled(rsclp));
 	if (rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL))
@@ -479,6 +546,10 @@ bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq)
 	if (++i >= RCU_NEXT_TAIL)
 		return false;
 
+	/* Accounting: everything below i is about to get merged into i. */
+	for (j = i + 1; j <= RCU_NEXT_TAIL; j++)
+		rcu_segcblist_move_seglen(rsclp, j, i);
+
 	/*
 	 * Merge all later callbacks, including newly arrived callbacks,
 	 * into the segment located by the for-loop above.  Assign "seq"
-- 
2.28.0.rc0.105.gf9edc3c819-goog