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Date: Sun, 19 Jul 2020 00:18:41 -0400
From: Joel Fernandes <joel@...lfernandes.org>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Josh Triplett <josh@...htriplett.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
"Paul E. McKenney" <paulmck@...nel.org>, rcu <rcu@...r.kernel.org>,
Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RFC v2] rcu/segcblist: Add counters to segcblist
datastructure
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@...lfernandes.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@...lfernandes.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@...lfernandes.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
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