From: Alexander Bluhm <bluhm@openbsd.org>
Subject: btrace netlock
To: tech@openbsd.org
Date: Tue, 24 Jun 2025 20:50:46 +0200

Hi,

I would like to have a trace point in net lock to find the hot spots
that have not been unlocked yet.  Diff below produces output like
this.

@[
nd6_timer+0x130
softclock_thread+0x11b
proc_trampoline+0x10
kernel
]: 475
@[
igmp_slowtimo+0x89
pfslowtimo+0x107
softclock_thread+0x11b
proc_trampoline+0x10
kernel
]: 18
...

What I don't like is adding more includes into systm.h.  If we agree
on the feature, I can try to shuffle the declarations around.

Or should I put it in rw_enter_write() to have generic lock analysis?
We have something similar in refcnt.

bluhm

Index: sys/dev/dt/dt_prov_static.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/dev/dt/dt_prov_static.c,v
diff -u -p -r1.25 dt_prov_static.c
--- sys/dev/dt/dt_prov_static.c	10 Mar 2025 09:28:56 -0000	1.25
+++ sys/dev/dt/dt_prov_static.c	24 Jun 2025 14:10:12 -0000
@@ -94,6 +94,11 @@ DT_STATIC_PROBE0(smr, wakeup);
 DT_STATIC_PROBE2(smr, thread, "uint64_t", "uint64_t");
 
 /*
+ * Network
+ */
+DT_STATIC_PROBE0(net, lock);
+
+/*
  * reference counting, keep in sync with sys/refcnt.h
  */
 DT_STATIC_PROBE0(refcnt, none);
@@ -151,6 +156,8 @@ struct dt_probe *const dtps_static[] = {
 	&_DT_STATIC_P(smr, barrier_exit),
 	&_DT_STATIC_P(smr, wakeup),
 	&_DT_STATIC_P(smr, thread),
+	/* Network */
+	&_DT_STATIC_P(net, lock),
 	/* refcnt */
 	&_DT_STATIC_P(refcnt, none),
 	&_DT_STATIC_P(refcnt, ethmulti),
Index: sys/sys/systm.h
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/sys/systm.h,v
diff -u -p -r1.176 systm.h
--- sys/sys/systm.h	19 Jun 2025 12:01:08 -0000	1.176
+++ sys/sys/systm.h	24 Jun 2025 18:40:22 -0000
@@ -325,6 +325,9 @@ int	uiomove(void *, size_t, struct uio *
 #if defined(_KERNEL)
 
 #include <sys/rwlock.h>
+#include <sys/clockintr.h>
+#include <sys/syslimits.h>
+#include <sys/tracepoint.h>
 
 extern struct rwlock netlock;
 
@@ -333,8 +336,12 @@ extern struct rwlock netlock;
  * by the NET_LOCK().  It's a single non-recursive lock for the whole
  * subsystem.
  */
-#define	NET_LOCK()	do { rw_enter_write(&netlock); } while (0)
-#define	NET_UNLOCK()	do { rw_exit_write(&netlock); } while (0)
+#define NET_LOCK()							\
+do {									\
+	TRACEPOINT(net, lock, NULL);					\
+	rw_enter_write(&netlock);					\
+} while (0)
+#define NET_UNLOCK()	do { rw_exit_write(&netlock); } while (0)
 
 /*
  * Reader version of NET_LOCK().
Index: share/btrace/netlock.bt
===================================================================
RCS file: share/btrace/netlock.bt
diff -N share/btrace/netlock.bt
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ share/btrace/netlock.bt	24 Jun 2025 18:34:38 -0000
@@ -0,0 +1,12 @@
+/*	$OpenBSD$	*/
+
+/*
+ * netlock.bt	Stack sampling of exclusive net lock.
+ *
+ * To produce a FlameGraph process the output with stackcollapse-bpftrace.pl
+ * and flamegraph.pl found in:
+ *	https://github.com/brendangregg/FlameGraph
+ */
+tracepoint:net:lock {
+	@[kstack] = count();
+}