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>

drivers/net/ethernet/mellanox/mlx5/core/lib/clock.c

@@ -111,10 +111,10 @@ static void mlx5_pps_out(struct work_struct *work)
 	for (i = 0; i < clock->ptp_info.n_pins; i++) {
 		u64 tstart;
 
-		write_lock_irqsave(&clock->lock, flags);
+		write_seqlock_irqsave(&clock->lock, flags);
 		tstart = clock->pps_info.start[i];
 		clock->pps_info.start[i] = 0;
-		write_unlock_irqrestore(&clock->lock, flags);
+		write_sequnlock_irqrestore(&clock->lock, flags);
 		if (!tstart)
 			continue;
 
@@ -132,10 +132,10 @@ static void mlx5_timestamp_overflow(struct work_struct *work)
 						overflow_work);
 	unsigned long flags;
 
-	write_lock_irqsave(&clock->lock, flags);
+	write_seqlock_irqsave(&clock->lock, flags);
 	timecounter_read(&clock->tc);
 	mlx5_update_clock_info_page(clock->mdev);
-	write_unlock_irqrestore(&clock->lock, flags);
+	write_sequnlock_irqrestore(&clock->lock, flags);
 	schedule_delayed_work(&clock->overflow_work, clock->overflow_period);
 }
 
@@ -147,10 +147,10 @@ static int mlx5_ptp_settime(struct ptp_clock_info *ptp,
 	u64 ns = timespec64_to_ns(ts);
 	unsigned long flags;
 
-	write_lock_irqsave(&clock->lock, flags);
+	write_seqlock_irqsave(&clock->lock, flags);
 	timecounter_init(&clock->tc, &clock->cycles, ns);
 	mlx5_update_clock_info_page(clock->mdev);
-	write_unlock_irqrestore(&clock->lock, flags);
+	write_sequnlock_irqrestore(&clock->lock, flags);
 
 	return 0;
 }
@@ -162,9 +162,9 @@ static int mlx5_ptp_gettime(struct ptp_clock_info *ptp, struct timespec64 *ts)
 	u64 ns;
 	unsigned long flags;
 
-	write_lock_irqsave(&clock->lock, flags);
+	write_seqlock_irqsave(&clock->lock, flags);
 	ns = timecounter_read(&clock->tc);
-	write_unlock_irqrestore(&clock->lock, flags);
+	write_sequnlock_irqrestore(&clock->lock, flags);
 
 	*ts = ns_to_timespec64(ns);
 
@@ -177,10 +177,10 @@ static int mlx5_ptp_adjtime(struct ptp_clock_info *ptp, s64 delta)
 						ptp_info);
 	unsigned long flags;
 
-	write_lock_irqsave(&clock->lock, flags);
+	write_seqlock_irqsave(&clock->lock, flags);
 	timecounter_adjtime(&clock->tc, delta);
 	mlx5_update_clock_info_page(clock->mdev);
-	write_unlock_irqrestore(&clock->lock, flags);
+	write_sequnlock_irqrestore(&clock->lock, flags);
 
 	return 0;
 }
@@ -203,12 +203,12 @@ static int mlx5_ptp_adjfreq(struct ptp_clock_info *ptp, s32 delta)
 	adj *= delta;
 	diff = div_u64(adj, 1000000000ULL);
 
-	write_lock_irqsave(&clock->lock, flags);
+	write_seqlock_irqsave(&clock->lock, flags);
 	timecounter_read(&clock->tc);
 	clock->cycles.mult = neg_adj ? clock->nominal_c_mult - diff :
 				       clock->nominal_c_mult + diff;
 	mlx5_update_clock_info_page(clock->mdev);
-	write_unlock_irqrestore(&clock->lock, flags);
+	write_sequnlock_irqrestore(&clock->lock, flags);
 
 	return 0;
 }
@@ -307,12 +307,12 @@ static int mlx5_perout_configure(struct ptp_clock_info *ptp,
 		ts.tv_nsec = rq->perout.start.nsec;
 		ns = timespec64_to_ns(&ts);
 		cycles_now = mlx5_read_internal_timer(mdev);
-		write_lock_irqsave(&clock->lock, flags);
+		write_seqlock_irqsave(&clock->lock, flags);
 		nsec_now = timecounter_cyc2time(&clock->tc, cycles_now);
 		nsec_delta = ns - nsec_now;
 		cycles_delta = div64_u64(nsec_delta << clock->cycles.shift,
 					 clock->cycles.mult);
-		write_unlock_irqrestore(&clock->lock, flags);
+		write_sequnlock_irqrestore(&clock->lock, flags);
 		time_stamp = cycles_now + cycles_delta;
 		field_select = MLX5_MTPPS_FS_PIN_MODE |
 			       MLX5_MTPPS_FS_PATTERN |
@@ -471,14 +471,14 @@ void mlx5_pps_event(struct mlx5_core_dev *mdev,
 		ts.tv_sec += 1;
 		ts.tv_nsec = 0;
 		ns = timespec64_to_ns(&ts);
-		write_lock_irqsave(&clock->lock, flags);
+		write_seqlock_irqsave(&clock->lock, flags);
 		nsec_now = timecounter_cyc2time(&clock->tc, cycles_now);
 		nsec_delta = ns - nsec_now;
 		cycles_delta = div64_u64(nsec_delta << clock->cycles.shift,
 					 clock->cycles.mult);
 		clock->pps_info.start[pin] = cycles_now + cycles_delta;
 		schedule_work(&clock->pps_info.out_work);
-		write_unlock_irqrestore(&clock->lock, flags);
+		write_sequnlock_irqrestore(&clock->lock, flags);
 		break;
 	default:
 		mlx5_core_err(mdev, " Unhandled event\n");
@@ -498,7 +498,7 @@ void mlx5_init_clock(struct mlx5_core_dev *mdev)
 		mlx5_core_warn(mdev, "invalid device_frequency_khz, aborting HW clock init\n");
 		return;
 	}
-	rwlock_init(&clock->lock);
+	seqlock_init(&clock->lock);
 	clock->cycles.read = read_internal_timer;
 	clock->cycles.shift = MLX5_CYCLES_SHIFT;
 	clock->cycles.mult = clocksource_khz2mult(dev_freq,

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);
 }

include/linux/mlx5/driver.h

@@ -805,7 +805,7 @@ struct mlx5_pps {
 };
 
 struct mlx5_clock {
-	rwlock_t                   lock;
+	seqlock_t                  lock;
 	struct cyclecounter        cycles;
 	struct timecounter         tc;
 	struct hwtstamp_config     hwtstamp_config;