[PATCH tip/perf/core 3/3] tracing/perf: Avoid perf_trace_buf_*() in perf_trace_##call() whenpossible

From: Steven Rostedt
Date: Wed Aug 14 2013 - 22:03:56 EST


From: Oleg Nesterov <oleg@xxxxxxxxxx>

perf_trace_buf_prepare() + perf_trace_buf_submit(task => NULL)
make no sense if hlist_empty(head). Change perf_trace_##call()
to check ->perf_events beforehand and do nothing if it is empty.

This removes the overhead for tasks without events associated
with them. For example, "perf record -e sched:sched_switch -p1"
attaches the counter(s) to the single task, but every task in
system will do perf_trace_buf_prepare/submit() just to realize
that it was not attached to this event.

However, we can only do this if __task == NULL, so we also add
the __builtin_constant_p(__task) check.

With this patch "perf bench sched pipe" shows approximately 4%
improvement when "perf record -p1" runs in parallel, many thanks
to Steven for the testing.

Link: http://lkml.kernel.org/r/20130806160847.GA2746@xxxxxxxxxx

Tested-by: David Ahern <dsahern@xxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
include/trace/ftrace.h | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h
index 4163d93..5c7ab17 100644
--- a/include/trace/ftrace.h
+++ b/include/trace/ftrace.h
@@ -667,6 +667,12 @@ perf_trace_##call(void *__data, proto) \
int rctx; \
\
__data_size = ftrace_get_offsets_##call(&__data_offsets, args); \
+ \
+ head = this_cpu_ptr(event_call->perf_events); \
+ if (__builtin_constant_p(!__task) && !__task && \
+ hlist_empty(head)) \
+ return; \
+ \
__entry_size = ALIGN(__data_size + sizeof(*entry) + sizeof(u32),\
sizeof(u64)); \
__entry_size -= sizeof(u32); \
@@ -681,7 +687,6 @@ perf_trace_##call(void *__data, proto) \
\
{ assign; } \
\
- head = this_cpu_ptr(event_call->perf_events); \
perf_trace_buf_submit(entry, __entry_size, rctx, __addr, \
__count, &__regs, head, __task); \
}
--
1.7.10.4


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