jump label: Introduce static_branch() interface

Introduce:

static __always_inline bool static_branch(struct jump_label_key *key);

instead of the old JUMP_LABEL(key, label) macro.

In this way, jump labels become really easy to use:

Define:

        struct jump_label_key jump_key;

Can be used as:

        if (static_branch(&jump_key))
                do unlikely code

enable/disale via:

        jump_label_inc(&jump_key);
        jump_label_dec(&jump_key);

that's it!

For the jump labels disabled case, the static_branch() becomes an
atomic_read(), and jump_label_inc()/dec() are simply atomic_inc(),
atomic_dec() operations. We show testing results for this change below.

Thanks to H. Peter Anvin for suggesting the 'static_branch()' construct.

Since we now require a 'struct jump_label_key *key', we can store a pointer into
the jump table addresses. In this way, we can enable/disable jump labels, in
basically constant time. This change allows us to completely remove the previous
hashtable scheme. Thanks to Peter Zijlstra for this re-write.

Testing:

I ran a series of 'tbench 20' runs 5 times (with reboots) for 3
configurations, where tracepoints were disabled.

jump label configured in
avg: 815.6

jump label *not* configured in (using atomic reads)
avg: 800.1

jump label *not* configured in (regular reads)
avg: 803.4

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20110316212947.GA8792@redhat.com>
Signed-off-by: Jason Baron <jbaron@redhat.com>
Suggested-by: H. Peter Anvin <hpa@linux.intel.com>
Tested-by: David Daney <ddaney@caviumnetworks.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 10 insertions, 12 deletions

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 97c84a58efb8..d530a4460a0b 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -29,7 +29,7 @@ struct tracepoint_func {
 
 struct tracepoint {
         const char *name;               /* Tracepoint name */
-        int state;                      /* State. */
+        struct jump_label_key key;
         void (*regfunc)(void);
         void (*unregfunc)(void);
         struct tracepoint_func __rcu *funcs;
@@ -146,9 +146,7 @@ void tracepoint_update_probe_range(struct tracepoint * const *begin,
         extern struct tracepoint __tracepoint_##name;                   \
         static inline void trace_##name(proto)                          \
         {                                                               \
-                JUMP_LABEL(&__tracepoint_##name.state, do_trace);       \
-                return;                                                 \
-do_trace:                                                               \
+                if (static_branch(&__tracepoint_##name.key))            \
                         __DO_TRACE(&__tracepoint_##name,                \
                                 TP_PROTO(data_proto),                   \
                                 TP_ARGS(data_args),                     \
@@ -176,14 +174,14 @@ do_trace:								\
  * structures, so we create an array of pointers that will be used for iteration
  * on the tracepoints.
  */
-#define DEFINE_TRACE_FN(name, reg, unreg)                               \
-        static const char __tpstrtab_##name[]                           \
-        __attribute__((section("__tracepoints_strings"))) = #name;      \
-        struct tracepoint __tracepoint_##name                           \
-        __attribute__((section("__tracepoints"))) =                     \
-                { __tpstrtab_##name, 0, reg, unreg, NULL };             \
-        static struct tracepoint * const __tracepoint_ptr_##name __used \
-        __attribute__((section("__tracepoints_ptrs"))) =                \
+#define DEFINE_TRACE_FN(name, reg, unreg)                                \
+        static const char __tpstrtab_##name[]                            \
+        __attribute__((section("__tracepoints_strings"))) = #name;       \
+        struct tracepoint __tracepoint_##name                            \
+        __attribute__((section("__tracepoints"))) =                      \
+                { __tpstrtab_##name, JUMP_LABEL_INIT, reg, unreg, NULL };\
+        static struct tracepoint * const __tracepoint_ptr_##name __used  \
+        __attribute__((section("__tracepoints_ptrs"))) =                 \
                 &__tracepoint_##name;
 
 #define DEFINE_TRACE(name)                                              \