[PATCH 3/8][RFC] ftrace: Optimize the function tracer list loop

Steven Rostedt
2012-11-02T22:46:46Z

[-- Attachment #1: 0003-ftrace-Optimize-the-function-tracer-list-loop.patch --]
[-- Type: text/plain, Size: 4886 bytes --]

From: Steven Rostedt <srostedt@redhat.com>

There is lots of places that perform:

op = rcu_dereference_raw(ftrace_control_list);
while (op != &ftrace_list_end) {

Add a helper macro to do this, and also optimize for a single
entity. That is, gcc will optimize a loop for either no iterations
or more than one iteration. But usually only a single callback
is registered to the function tracer, thus the optimized case
should be a single pass. to do this we now do:

op = rcu_dereference_raw(list);
do {
[...]
} while (likely(op = rcu_dereference_raw((op)->next)) &&
unlikely((op) != &ftrace_list_end));

An op is always registered (ftrace_list_end when no callbacks is
registered), thus when a single callback is registered, the link
list looks like:

top => callback => ftrace_list_end => NULL.

The likely(op = op->next) still must be performed due to the race
of removing the callback, where the first op assignment could
equal ftrace_list_end. In that case, the op->next would be NULL.
But this is unlikely (only happens in a race condition when
removing the callback).

But it is very likely that the next op would be ftrace_list_end,
unless more than one callback has been registered. This tells
gcc what the most common case is and makes the fast path with
the least amount of branches.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
kernel/trace/ftrace.c | 48 ++++++++++++++++++++++++++----------------------
1 file changed, 26 insertions(+), 22 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 67e886e..cd6cd3c 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -111,6 +111,26 @@ static void ftrace_ops_no_ops(unsigned long ip, unsigned long parent_ip);
#define ftrace_ops_list_func ((ftrace_func_t)ftrace_ops_no_ops)
#endif

+/*
+ * Traverse the ftrace_global_list, invoking all entries. The reason that we
+ * can use rcu_dereference_raw() is that elements removed from this list
+ * are simply leaked, so there is no need to interact with a grace-period
+ * mechanism. The rcu_dereference_raw() calls are needed to handle
+ * concurrent insertions into the ftrace_global_list.
+ *
+ * Silly Alpha and silly pointer-speculation compiler optimizations!
+ */
+#define do_for_each_ftrace_op(op, list) \
+ op = rcu_dereference_raw(list); \
+ do
+
+/*
+ * Optimized for just a single item in the list (as that is the normal case).
+ */
+#define while_for_each_ftrace_op(op) \
+ while (likely(op = rcu_dereference_raw((op)->next)) && \
+ unlikely((op) != &ftrace_list_end))
+
/**
* ftrace_nr_registered_ops - return number of ops registered
*
@@ -132,15 +152,6 @@ int ftrace_nr_registered_ops(void)
return cnt;
}

-/*
- * Traverse the ftrace_global_list, invoking all entries. The reason that we
- * can use rcu_dereference_raw() is that elements removed from this list
- * are simply leaked, so there is no need to interact with a grace-period
- * mechanism. The rcu_dereference_raw() calls are needed to handle
- * concurrent insertions into the ftrace_global_list.
- *
- * Silly Alpha and silly pointer-speculation compiler optimizations!
- */
static void
ftrace_global_list_func(unsigned long ip, unsigned long parent_ip,
struct ftrace_ops *op, struct pt_regs *regs)
@@ -149,11 +160,9 @@ ftrace_global_list_func(unsigned long ip, unsigned long parent_ip,
return;

trace_recursion_set(TRACE_GLOBAL_BIT);
- op = rcu_dereference_raw(ftrace_global_list); /*see above*/
- while (op != &ftrace_list_end) {
+ do_for_each_ftrace_op(op, ftrace_global_list) {
op->func(ip, parent_ip, op, regs);
- op = rcu_dereference_raw(op->next); /*see above*/
- };
+ } while_for_each_ftrace_op(op);
trace_recursion_clear(TRACE_GLOBAL_BIT);
}

@@ -4104,14 +4113,11 @@ ftrace_ops_control_func(unsigned long ip, unsigned long parent_ip,
*/
preempt_disable_notrace();
trace_recursion_set(TRACE_CONTROL_BIT);
- op = rcu_dereference_raw(ftrace_control_list);
- while (op != &ftrace_list_end) {
+ do_for_each_ftrace_op(op, ftrace_control_list) {
if (!ftrace_function_local_disabled(op) &&
ftrace_ops_test(op, ip))
op->func(ip, parent_ip, op, regs);
-
- op = rcu_dereference_raw(op->next);
- };
+ } while_for_each_ftrace_op(op);
trace_recursion_clear(TRACE_CONTROL_BIT);
preempt_enable_notrace();
}
@@ -4139,12 +4145,10 @@ __ftrace_ops_list_func(unsigned long ip, unsigned long parent_ip,
* they must be freed after a synchronize_sched().
*/
preempt_disable_notrace();
- op = rcu_dereference_raw(ftrace_ops_list);
- while (op != &ftrace_list_end) {
+ do_for_each_ftrace_op(op, ftrace_ops_list) {
if (ftrace_ops_test(op, ip))
op->func(ip, parent_ip, op, regs);
- op = rcu_dereference_raw(op->next);
- };
+ } while_for_each_ftrace_op(op);
preempt_enable_notrace();
trace_recursion_clear(TRACE_INTERNAL_BIT);
}
--
1.7.10.4