-----Original Message-----
From: Andrew Rybchenko <redacted>
Sent: Saturday, July 24, 2021 3:52 AM
To: Joyce Kong <redacted>; thomas@monjalon.net;
david.marchand@redhat.com; roretzla@linux.microsoft.com;
stephen@networkplumber.org; olivier.matz@6wind.com;
harry.van.haaren@intel.com; Honnappa Nagarahalli
[off-list ref]; Ruifeng Wang
[off-list ref]
Cc: dev@dpdk.org; nd <redacted>
Subject: Re: [PATCH v3 8/8] test/rcu: use compiler atomics for data sync

On 7/20/21 6:51 AM, Joyce Kong wrote:

quoted

Covert rte_atomic usages to compiler atomic built-ins in rcu_perf
testcases.

Signed-off-by: Joyce Kong <redacted>
Reviewed-by: Ruifeng Wang <redacted>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
---
  app/test/test_rcu_qsbr_perf.c | 98 +++++++++++++++++------------------
  1 file changed, 49 insertions(+), 49 deletions(-)

diff --git a/app/test/test_rcu_qsbr_perf.c

b/app/test/test_rcu_qsbr_perf.c index 3017e71120..cf7b158d22 100644

--- a/app/test/test_rcu_qsbr_perf.c
+++ b/app/test/test_rcu_qsbr_perf.c

@@ -30,8 +30,8 @@ static volatile uint32_t thr_id;
  static struct rte_rcu_qsbr *t[RTE_MAX_LCORE];
  static struct rte_hash *h;
  static char hash_name[8];
-static rte_atomic64_t updates, checks; -static rte_atomic64_t

update_cycles, check_cycles;
+static uint64_t updates, checks;
+static uint64_t update_cycles, check_cycles;

  /* Scale down results to 1000 operations to support lower
   * granularity clocks.

@@ -81,8 +81,8 @@ test_rcu_qsbr_reader_perf(void *arg)
  	}

  	cycles = rte_rdtsc_precise() - begin;
-	rte_atomic64_add(&update_cycles, cycles);
-	rte_atomic64_add(&updates, loop_cnt);
+	__atomic_fetch_add(&update_cycles, cycles, __ATOMIC_RELAXED);
+	__atomic_fetch_add(&updates, loop_cnt, __ATOMIC_RELAXED);

Shouldn't __atomic_add_fetch() be used instead since it pseudo-code is a bit
simpler. What is the best option if return value is not actually used?

If the return value is not used, like the situations here, the instructions for __atomic_fetch_add() and __atomic_add_fetch() would be the same on X86 and Arm for gcc and clang that I have tried.
If the return value is used, __atomic_add_fetch() would do two more instructions('mov' 'add') than __atomic_fetch_add() to return the calculation result.
Based on experiments here: https://godbolt.org/ .