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Subject[tip:perf/core] tracing, perf: Adjust code layout in get_recursion_context()
Commit-ID:  d0618410eced4eb092295fad10312a4545fcdfaf
Gitweb: http://git.kernel.org/tip/d0618410eced4eb092295fad10312a4545fcdfaf
Author: Jesper Dangaard Brouer <brouer@redhat.com>
AuthorDate: Tue, 22 Aug 2017 19:22:43 +0200
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 25 Aug 2017 11:04:18 +0200

tracing, perf: Adjust code layout in get_recursion_context()

In an XDP redirect applications using tracepoint xdp:xdp_redirect to
diagnose TX overrun, I noticed perf_swevent_get_recursion_context()
was consuming 2% CPU. This was reduced to 1.85% with this simple
change.

Looking at the annotated asm code, it was clear that the unlikely case
in_nmi() test was chosen (by the compiler) as the most likely
event/branch. This small adjustment makes the compiler (GCC version
7.1.1 20170622 (Red Hat 7.1.1-3)) put in_nmi() as an unlikely branch.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/150342256382.16595.986861478681783732.stgit@firesoul
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
kernel/events/internal.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/events/internal.h b/kernel/events/internal.h
index 5377c59..843e970 100644
--- a/kernel/events/internal.h
+++ b/kernel/events/internal.h
@@ -208,7 +208,7 @@ static inline int get_recursion_context(int *recursion)
{
int rctx;

- if (in_nmi())
+ if (unlikely(in_nmi()))
rctx = 3;
else if (in_irq())
rctx = 2;
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