net/mlx5e: Replace PTP clock lock from RW lock to seq lock

Changed "priv.clock.lock" lock from 'rw_lock' to 'seq_lock'
in order to improve packet rate performance.

Tested on Intel(R) Xeon(R) CPU E5-2660 v2 @ 2.20GHz.
Sent 64b packets between two peers connected by ConnectX-5,
and measured packet rate for the receiver in three modes:
	no time-stamping (base rate)
	time-stamping using rw_lock (old lock) for critical region
	time-stamping using seq_lock (new lock) for critical region
Only the receiver time stamped its packets.

The measured packet rate improvements are:

	Single flow (multiple TX rings to single RX ring):
		without timestamping:	  4.26 (M packets)/sec
		with rw-lock (old lock):  4.1  (M packets)/sec
		with seq-lock (new lock): 4.16 (M packets)/sec
		1.46% improvement

	Multiple flows (multiple TX rings to six RX rings):
		without timestamping: 	  22   (M packets)/sec
		with rw-lock (old lock):  11.7 (M packets)/sec
		with seq-lock (new lock): 21.3 (M packets)/sec
		82.05% improvement

The packet rate improvement is due to the lack of atomic operations
for the 'readers' by the seq-lock.
Since there are much more 'readers' than 'writers' contention
on this lock, almost all atomic operations are saved.
this results in a dramatic decrease in overall
cache misses.

Signed-off-by: Shay Agroskin <shayag@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/lib/clock.h b/drivers/net/ethernet/mellanox/mlx5/core/lib/clock.h
index 02e2e4575e4f..263cb6e2aeee 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/lib/clock.h
+++ b/drivers/net/ethernet/mellanox/mlx5/core/lib/clock.h
@@ -46,11 +46,13 @@ static inline int mlx5_clock_get_ptp_index(struct mlx5_core_dev *mdev)
 static inline ktime_t mlx5_timecounter_cyc2time(struct mlx5_clock *clock,
 						u64 timestamp)
 {
+	unsigned int seq;
 	u64 nsec;
 
-	read_lock(&clock->lock);
-	nsec = timecounter_cyc2time(&clock->tc, timestamp);
-	read_unlock(&clock->lock);
+	do {
+		seq = read_seqbegin(&clock->lock);
+		nsec = timecounter_cyc2time(&clock->tc, timestamp);
+	} while (read_seqretry(&clock->lock, seq));
 
 	return ns_to_ktime(nsec);
 }