linux-kernel - [tip:perf/core] perf bench futex: Cache align the worker struct

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Message-ID: <tip-34b753007d646482a4125a7095e1d1986d395f95@git.kernel.org>
Date:   Mon, 24 Oct 2016 12:06:24 -0700
From:   tip-bot for Sebastian Andrzej Siewior <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     acme@...hat.com, peterz@...radead.org, tglx@...utronix.de,
        dbueso@...e.de, hpa@...or.com, mingo@...nel.org,
        bigeasy@...utronix.de, linux-kernel@...r.kernel.org
Subject: [tip:perf/core] perf bench futex: Cache align the worker struct

Commit-ID:  34b753007d646482a4125a7095e1d1986d395f95
Gitweb:     http://git.kernel.org/tip/34b753007d646482a4125a7095e1d1986d395f95
Author:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
AuthorDate: Sun, 16 Oct 2016 21:08:02 +0200
Committer:  Arnaldo Carvalho de Melo <acme@...hat.com>
CommitDate: Mon, 24 Oct 2016 11:07:45 -0300

perf bench futex: Cache align the worker struct

It popped up in perf testing that the worker consumes some amount of
CPU. It boils down to the increment of `ops` which causes cache line
bouncing between the individual threads.

This patch aligns the struct by 256 bytes to ensure that not a cache
line is shared among CPUs. 128 byte is the x86 worst case and grep says
that L1_CACHE_SHIFT is set to 8 on s390.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Davidlohr Bueso <dbueso@...e.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Link: http://lkml.kernel.org/r/20161016190803.3392-1-bigeasy@linutronix.de
Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
---
 tools/perf/bench/futex-hash.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/tools/perf/bench/futex-hash.c b/tools/perf/bench/futex-hash.c
index 8024cd5..d9e5e80 100644
--- a/tools/perf/bench/futex-hash.c
+++ b/tools/perf/bench/futex-hash.c
@@ -39,12 +39,15 @@ static unsigned int threads_starting;
 static struct stats throughput_stats;
 static pthread_cond_t thread_parent, thread_worker;
 
+#define SMP_CACHE_BYTES 256
+#define __cacheline_aligned __attribute__ ((aligned (SMP_CACHE_BYTES)))
+
 struct worker {
 	int tid;
 	u_int32_t *futex;
 	pthread_t thread;
 	unsigned long ops;
-};
+} __cacheline_aligned;
 
 static const struct option options[] = {
 	OPT_UINTEGER('t', "threads", &nthreads, "Specify amount of threads"),