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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@linutronix.de>
AuthorDate: Sun, 16 Oct 2016 21:08:02 +0200
Committer: Arnaldo Carvalho de Melo <acme@redhat.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@linutronix.de>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20161016190803.3392-1-bigeasy@linutronix.de
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.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"),
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