From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Steven Rostedt <rostedt@goodmis.org>,
"Paul E. McKenney" <paulmck@kernel.org>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
Stefan Metzmacher <metze@samba.org>
Cc: linux-kernel@vger.kernel.org,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: [RFC PATCH v2] Fix: tracepoint: rcu get state and cond sync for static call updates
Date: Tue, 27 Jul 2021 15:13:11 -0400 [thread overview]
Message-ID: <20210727191311.26061-1-mathieu.desnoyers@efficios.com> (raw)
In-Reply-To: <20210727150613.20886-4-mathieu.desnoyers@efficios.com>
State transitions from 1->0->1 and N->2->1 callbacks require RCU
synchronization. Rather than performing the RCU synchronization every
time the state change occurs, which is quite slow when many tracepoints
are registered in batch, instead keep a snapshot of the RCU state on the
most recent transitions which belong to a chain, and conditionally wait
for a grace period on the last transition of the chain if one g.p. has
not elapsed since the last snapshot.
This applies to both RCU and SRCU.
[ Build tested only. ]
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
---
Changes since v1:
- Use tp_rcu_get_state/tp_rcu_cond_sync on 2->1 transition when
tp_funcs[0].data != old[0].data rather than
tracepoint_synchronize_unregister.
---
kernel/tracepoint.c | 81 ++++++++++++++++++++++++++++++++++++++-------
1 file changed, 69 insertions(+), 12 deletions(-)
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index a85e7dc8b490..1140599988a1 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -28,6 +28,44 @@ extern tracepoint_ptr_t __stop___tracepoints_ptrs[];
DEFINE_SRCU(tracepoint_srcu);
EXPORT_SYMBOL_GPL(tracepoint_srcu);
+enum tp_transition_sync {
+ TP_TRANSITION_SYNC_1_0_1,
+ TP_TRANSITION_SYNC_N_2_1,
+
+ _NR_TP_TRANSITION_SYNC,
+};
+
+struct tp_transition_snapshot {
+ unsigned long rcu;
+ unsigned long srcu;
+ bool ongoing;
+};
+
+/* Protected by tracepoints_mutex */
+static struct tp_transition_snapshot tp_transition_snapshot[_NR_TP_TRANSITION_SYNC];
+
+static void tp_rcu_get_state(enum tp_transition_sync sync)
+{
+ struct tp_transition_snapshot *snapshot = &tp_transition_snapshot[sync];
+
+ /* Keep the latest get_state snapshot. */
+ snapshot->rcu = get_state_synchronize_rcu();
+ snapshot->srcu = start_poll_synchronize_srcu(&tracepoint_srcu);
+ snapshot->ongoing = true;
+}
+
+static void tp_rcu_cond_sync(enum tp_transition_sync sync)
+{
+ struct tp_transition_snapshot *snapshot = &tp_transition_snapshot[sync];
+
+ if (!snapshot->ongoing)
+ return;
+ cond_synchronize_rcu(snapshot->rcu);
+ if (!poll_state_synchronize_srcu(&tracepoint_srcu, snapshot->srcu))
+ synchronize_srcu(&tracepoint_srcu);
+ snapshot->ongoing = false;
+}
+
/* Set to 1 to enable tracepoint debug output */
static const int tracepoint_debug;
@@ -311,6 +349,11 @@ static int tracepoint_add_func(struct tracepoint *tp,
*/
switch (nr_func_state(tp_funcs)) {
case TP_FUNC_1: /* 0->1 */
+ /*
+ * Make sure new static func never uses old data after a
+ * 1->0->1 transition sequence.
+ */
+ tp_rcu_cond_sync(TP_TRANSITION_SYNC_1_0_1);
/* Set static call to first function */
tracepoint_update_call(tp, tp_funcs);
/* Both iterator and static call handle NULL tp->funcs */
@@ -326,9 +369,21 @@ static int tracepoint_add_func(struct tracepoint *tp,
* static call update/call.
*/
rcu_assign_pointer(tp->funcs, tp_funcs);
+ /*
+ * Make sure static func never uses incorrect data after a
+ * 1->...->2->1 transition sequence.
+ */
+ if (tp_funcs[0].data != old[0].data)
+ tp_rcu_get_state(TP_TRANSITION_SYNC_N_2_1);
break;
case TP_FUNC_N: /* N->N+1 (N>1) */
rcu_assign_pointer(tp->funcs, tp_funcs);
+ /*
+ * Make sure static func never uses incorrect data after a
+ * N->...->2->1 (N>1) transition sequence.
+ */
+ if (tp_funcs[0].data != old[0].data)
+ tp_rcu_get_state(TP_TRANSITION_SYNC_N_2_1);
break;
default:
WARN_ON_ONCE(1);
@@ -372,29 +427,31 @@ static int tracepoint_remove_func(struct tracepoint *tp,
/* Both iterator and static call handle NULL tp->funcs */
rcu_assign_pointer(tp->funcs, NULL);
/*
- * Make sure new func never uses old data after a 1->0->1
- * transition sequence.
- * Considering that transition 0->1 is the common case
- * and don't have rcu-sync, issue rcu-sync after
- * transition 1->0 to break that sequence by waiting for
- * readers to be quiescent.
+ * Make sure new static func never uses old data after a
+ * 1->0->1 transition sequence.
*/
- tracepoint_synchronize_unregister();
+ tp_rcu_get_state(TP_TRANSITION_SYNC_1_0_1);
break;
case TP_FUNC_1: /* 2->1 */
rcu_assign_pointer(tp->funcs, tp_funcs);
/*
- * On 2->1 transition, RCU sync is needed before setting
- * static call to first callback, because the observer
- * may have loaded any prior tp->funcs after the last one
- * associated with an rcu-sync.
+ * Make sure static func never uses incorrect data after a
+ * N->...->2->1 (N>2) transition sequence.
*/
- tracepoint_synchronize_unregister();
+ if (tp_funcs[0].data != old[0].data)
+ tp_rcu_get_state(TP_TRANSITION_SYNC_N_2_1);
+ tp_rcu_cond_sync(TP_TRANSITION_SYNC_N_2_1);
/* Set static call to first function */
tracepoint_update_call(tp, tp_funcs);
break;
case TP_FUNC_N: /* N->N-1 (N>2) */
rcu_assign_pointer(tp->funcs, tp_funcs);
+ /*
+ * Make sure static func never uses incorrect data after a
+ * N->...->2->1 (N>2) transition sequence.
+ */
+ if (tp_funcs[0].data != old[0].data)
+ tp_rcu_get_state(TP_TRANSITION_SYNC_N_2_1);
break;
default:
WARN_ON_ONCE(1);
--
2.20.1
prev parent reply other threads:[~2021-07-27 19:13 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-27 15:06 [RFC PATCH 0/3] tracepoint static call fixes Mathieu Desnoyers
2021-07-27 15:06 ` [RFC PATCH 1/3] Fix: tracepoint: static call: compare data on transition from 2->1 callees Mathieu Desnoyers
2021-07-27 15:06 ` [RFC PATCH 2/3] Fix: tracepoint: static call function vs data state mismatch Mathieu Desnoyers
2021-07-27 15:06 ` [RFC PATCH 3/3] Fix: tracepoint: rcu get state and cond sync for static call updates Mathieu Desnoyers
2021-07-27 16:18 ` Paul E. McKenney
2021-07-27 19:13 ` Mathieu Desnoyers [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20210727191311.26061-1-mathieu.desnoyers@efficios.com \
--to=mathieu.desnoyers@efficios.com \
--cc=akpm@linux-foundation.org \
--cc=linux-kernel@vger.kernel.org \
--cc=metze@samba.org \
--cc=mingo@redhat.com \
--cc=paulmck@kernel.org \
--cc=peterz@infradead.org \
--cc=rostedt@goodmis.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).