linux-kernel - [PATCH V2] trace: adjust code layout in get_recursion

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 22 Aug 2017 19:22:43 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: [PATCH V2] trace: 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@...hat.com>
---
 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 486fd78eb8d5..e1a7ac7bd686 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;